Electrical and Nonlinear Optical Studies of Specific Organic Molecular 
and Nonconjugated Conductive Polymeric Systems 
 
by 
 
Ananthakrishnan Narayanan 
 
 
 
 
A dissertation submitted to the Graduate Faculty of 
Auburn University 
in partial fulfillment of the 
requirements for the Degree of 
Doctor of Philosophy 
 
Auburn, Alabama 
May 09, 2011 
 
 
 
 
Keywords: Nonlinear optics, nonconjugated conducting polymer, electrical and optical    
characterization, spectroscopic studies, waveguides, photovoltaics 
 
 
Copyright 2011 by Ananthakrishnan Narayanan 
 
 
Approved by 
 
Mrinal Thakur, Chair, Professor of Mechanical Engineering 
Dan B. Marghitu, Professor of Mechanical Engineering 
Lloyd S. Riggs, Professor of Electrical Engineering 
 
 
 
 
 
 
 
ii 
 
Abstract 
 
 In this research, structural, electrical and nonlinear optical characteristics of: (a) single 
crystal films involving a noncentrosymmetric molecule DAST and a laser dye IR125 and (b)  
specific nonconjugated conducting polymers including poly(?-pinene) and polynorbornene have 
been studied. 
 4?-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) is a well known second 
order nonlinear optical material. This material has exceptionally high electro-optic coefficients, 
high thermal stability and ultrafast response time. In this work single crystal films involving a 
combination of DAST and IR125 have been prepared using modified shear method and the films 
have been characterized using polarized optical microscopy, X-ray diffraction, polarization 
dependent optical absorption and photoluminescence spectroscopy. The electro-optic coefficient 
of these films measured at 633nm was found to be 300pm/V. Since IR-125 has a strong 
absorption band from 500nm to 800nm, these films are promising for various applications in 
nonlinear optics at longer wavelength and for light emission. 
Nonconjugated conducting polymers are a class of polymers that have at least one double 
bond in their repeat units.  1,4-cis polyisoprene, polyalloocimene, styrene butadiene rubber, 
poly(ethylenepyrrolediyl) derivatives, and poly(?-pinene) are some of the well known examples 
of nonconjugated conducting polymers. In this work, polynorborne, a new addition to the class 
of nonconjugated conducting polymers is discussed.  Like other polymers in this class, 
polynorbornene exhibits increase in electrical conductivity by many orders of magnitude upon 
doping with iodine. The maximum electrical conductivity of this material is 0.01 S/cm. As 
iii 
 
shown by using FTIR microscopy, the C=C bonds are transformed into cation radicals when 
polynorborne is doped. This is due to the charge-transfer from the double bond to the dopant 
(iodine). These materials like other nonconjugated conducting polymers have significant 
applications in electro-optics and photonics. 
 Electron paramagnetic resonance measurements on poly(?-pinene) before and after 
doping with iodine are  reported in this work. The EPR signal of this polymer increases 
proportionally with the iodine concentration due to the formation of cation radicals upon doping 
and charge-transfer. The results agree well with the doping mechanism of nonconjugated 
conducting polymers discussed earlier in literature. Hyperfine splitting in heavily doped 
polymers is observed due to the reduced distance between the cation radical and the iodine anion. 
 Off-resonant electro-optic measurements in doped poly(?-pinene) at 790nm, 800nm, 
810nm and 1.55?m using field-induced birefringence technique have been studied. The results 
show that this material exhibits the highest cubic nonlinearities of all known materials. The Kerr 
coefficient measured at 1.55?m is 1.6x10-10 m/V2 which is about 30 times higher than that of 
conjugated polymers.  Results of two photon measurements in this doped polymer using pump-
probe technique with a pulsed, mode-locked (150 fs pulses) beam from a Ti-Sapphire laser are 
reported. The measured value of ?2 at 790 nm and 795 nm were found to be 2.28?0.1 cm/MW 
and 2.5?0.1 cm/MW respectively. The data confirms that the nonlinearity in this material is 
ultrafast and electronic in nature. Such large nonlinearities in these materials are attributed the 
charge confinement in these materials in a sub-nanometer domain (upon doping) resulting in a 
metal-like quantum dot structure.  
Photovoltaic measurements in a composite involving poly(?-pinene) and C60 are 
discussed. This is the first time a nonconjugated conducting polymer based photovoltaic cell has 
iv 
 
been fabricated.  A composite involving 4% C60 by weight produced a photovoltage of 280mV 
for an incident light intensity of 6mW/sq.cm. These low cost devices have applications in solar 
cells, photodetectors etc. 
 A nonlinear optical waveguide was prepared by casting a thin film of poly(?-pinene) on 
bare multi-mode optical fiber and doping it with iodine. The doped fibers were of excellent 
optical quality. Two-photon absorption experiments were conducted using these waveguides and 
large changes in transmission upto 28% was observed in 15cm long fiber. More work needs to be 
done to confirm this result. This is a significant step in the direction of making these materials a 
viable choice for ultrafast (femtosecond time-scale) optical devices. 
 To summarize, these works included detailed investigations of structural, electrical and 
nonlinear optical characteristics of specific molecular crystal films and nonconjugated 
conducting polymers. 
 
  
v 
 
Acknowledgments 
 
???? ?? null?????null?? | 
O Lord! Lead me from darkness of ignorance to the light of knowledge
I consider completing this doctoral dissertation, the biggest accomplishment of my life 
thus far and as I write these lines, I am thrilled and full of gratitude. I dedicate this work to my 
family: my late grandmother for her love and affection; my parents for their immense sacrifice, 
support, prayers and their unstinting faith in me; my sister for her love, guidance and 
encouragement; my brother-in-law for being one of my best well-wishers; and to my year and a 
half old niece, for her priceless smiles. 
 The last phase of my graduate life was fraught with challenges. Looking back, I believe, I 
could not have done it without the moral support, encouragement and advice of Sonal. 
 This dissertation, without a doubt, is a culmination of the efforts all my teachers, right 
from the ones in kinder garden who taught all my alphabets, to those at Auburn who taught me 
complex concepts of quantum physics. I take this opportunity to thank them all. In particular, I 
thank my advisor, Dr. Mrinal Thakur, for his technical guidance and financial/infrastructural 
support; my doctoral committee members Dr. Marghitu and Dr. Riggs; Dr. Evert Duin for his 
help with the EPR measurements; Dr. Suhling and Dr. Flowers for helping me through very 
difficult times and Dr. Nels Madsen with whom I had the good fortune of working and learning 
as a Graduate Teaching Assistant. 
vi 
 
 It is a rare privilege to work in a place where every single colleague is a great friend. Jitto 
(you are the dude!), Swamy, Kutty, Harish, Vippa, Veera, Aditya and Sapana thank you for all 
the help. It was a pleasure knowing and working with all of you. 
 I thoroughly enjoyed my role as an instructor in Mechanical Engineering. I thank my 
students for their kindness and cooperation and for inspiring me to be a better teacher. I will also 
take this opportunity to thank my colleagues from Indian Student Association and International 
Student Organization, two student groups at Auburn that I have been closely associated with; I 
will always cherish the fond memories.  
 I want to express my sincere gratitude to a few individuals who have gone out of their 
way to help me: Ms. Nejla Orgen, Ms. Stephanie Bond, Mr. Bret Smith and Dr. James Wohl. I 
will miss you all. 
 My six year lodge at Auburn is a story of some wonderful memories, people and 
incidents. I thank everybody who made it an enriching and enjoyable experience. In particular, 
all my roommates and close confidants over the years: Bharath, Ganesh, Nono, Cheechu, Rahul, 
Tony, Manya, Aditya, Veera and Arjun. I would also like to thank all my friends back home for 
staying in touch and for being there when I needed them. 
 This section cannot accommodate an exhaustive list of all those who have made this 
dream come true.  I will remember all of you. Thank you very much! 
 
  
 
 
 
 
 
 
vii 
 
 
Table of Contents 
Abstract ................................................................................................................................... ii 
Acknowledgments .................................................................................................................... v  
List of Tables .......................................................................................................................... xi  
List of Figures ........................................................................................................................ xii  
Chapter 1: Introduction   .......................................................................................................... 1 
Chapter 2: Background   ........................................................................................................... 5 
 2.1 Introduction to Nonlinear Optics   ........................................................................... 5 
 2.2 History   .................................................................................................................. 6 
 2.3 Propagation of Light in a Medium   ......................................................................... 6 
 2.4 Nonlinear Optical Processes   .................................................................................. 8 
         2.4.1 Second Order Nonlinear Processes   ............................................................. 8 
                    2.4.2 Third Order Nonlinear Processes   .............................................................. 11 
         2.4.3 Resonant Enhancement in Nonlinear Optical Materials   ............................ 13 
         2.4.4 Nonlinear Optics in Confined Systems   ..................................................... 14 
                  2.4.4.1 Quantum Confinement   .................................................................. 14 
                  2.4.4.2 Dielectric Confinement   ................................................................. 16 
                  2.4.4.3 Effect of Quantum Confinement on Third Order Susceptibility  ...... 16 
                  2.4.4.4 Effect of Dielectric Confinement on Third Order Susceptibility  ..... 17 
 2.5 Nonlinear Optical Materials   ................................................................................ 18 
viii 
 
         2.5.1 Second Order Materials   ............................................................................ 18 
                            2.5.1.1 Dispersion of Electro-Optic Coefficient and Its Figure of Merit  ...... 23 
         2.5.2 Third Order Materials   ............................................................................... 25 
         2.5.3 Nonconjugated Conducting Polymers as Third Order Materials   ................ 28 
 2.6 Photovoltaics   ...................................................................................................... 30 
         2.6.1 Organic Materials in Photovoltaic Technology   ......................................... 32 
Chapter 3: Objectives   ........................................................................................................... 35 
Chapter 4: Electro-optic Effect in a Novel Single-Crystal Film Involving DAST and IR125 ... 37 
 4.1 Material and Methods   ......................................................................................... 37 
 4.2 Results and Discussions   ...................................................................................... 39 
         4.2.1 Polarized Optical Microscopy  ................................................................... 39 
         4.2.2 Polarization-Dependent Optical Absorption Spectroscopy   ........................ 40 
         4.2.3 X-Ray Diffraction   .................................................................................... 41 
         4.2.4 Electro-optic Measurements   ..................................................................... 42 
 4.3 Conclusions   ........................................................................................................ 43
Chapter 5: Polynorbornene: Nonconjugated Conducting Polymer   ........................................ 45 
 5.1 Introduction   ........................................................................................................ 45 
 5.2 Experimental  ....................................................................................................... 45 
         5.2.1 Electrical Conductivity   ............................................................................. 45 
         5.2.2 Optical Absorption   ................................................................................... 47 
         5.2.3 FTIR Spectroscopy   ................................................................................... 49 
         5.2.4 Photoluminescence   ................................................................................... 51 
 5.3 Results and Discussions   ...................................................................................... 51 
ix 
 
 5.4 Conclusions   ........................................................................................................ 52 
Chapter 6: EPR Spectroscopic Studies of Radical Cations in a Nonconjugated Conductive 
Polymer, Poly(?-pinene) ....................................................................................... 53 
 
 6.1 Introduction   ........................................................................................................ 53 
 6.2 Experimental  ....................................................................................................... 56 
 6.3 Results and Discussions   ...................................................................................... 57 
 6.4 Conclusions   ........................................................................................................ 59 
Chapter 7: Off-Resonant Electro-optics Measurements in Doped  Poly(?-pinene) .................. 61 
 7.1 Introduction   ........................................................................................................ 61 
 7.2 Experimental  ....................................................................................................... 62 
 7.3 Results and Discussions   ...................................................................................... 64 
 7.4 Conclusions   ........................................................................................................ 67 
Chapter 8: Time Resolved Two-photon Absorption Measurements in Doped Poly(?-pinene) . 69 
 8.1 Introduction   ........................................................................................................ 69 
 8.2 Experimental  ....................................................................................................... 70 
         8.2.1 Sample Preparation   .................................................................................. 70 
         8.2.2 Pump-Probe Setup   .................................................................................... 71 
                  8.2.2.1 Probe Alignment   ........................................................................... 71 
                  8.2.2.2 Pump Alignment   ........................................................................... 72 
         8.2.3 Data Collection   ........................................................................................ 72 
 8.3 Results and Discussions   ...................................................................................... 73 
 8.4 Conclusions   ........................................................................................................ 77 
Chapter 9: Nonlinear Optical Wave-guiding in Doped  Poly(?-pinene) ................................... 78 
 9.1 Introduction   ........................................................................................................ 78 
x 
 
 9.2 Experimental  ....................................................................................................... 79 
 9.3 Results and Discussions   ...................................................................................... 82 
 9.4 Conclusions   ........................................................................................................ 83 
Chapter 10: Photovoltaic Effect in Nonconjugated Conducting Polymers ............................... 84 
 10.1 Introduction   ...................................................................................................... 84 
 10.2 Experimental   ..................................................................................................... 84 
 10.3 Results and Discussions   .................................................................................... 85 
 10.4 Conclusions   ...................................................................................................... 92 
Chapter 11: Conclusions ........................................................................................................ 94 
References ............................................................................................................................. 97 
 
 
 
 
 
 
 
 
 
 
 
xi 
 
List of Tables
Table 2.1: Electro-optic coefficients of DAST  ....................................................................... 22 
Table 2.2:  Electro-optic response of selected organic compounds .......................................... 24 
Table 2.3: Cubic nonlinearities of select metallic nanoparticles  ............................................. 28 
Table 5.1: Conductivity of nonconjugate conducting polymers  .............................................. 46 
Table 7.1: Kerr coefficient of poly(?-pinene) at different wavelength  .................................... 65 
Table 8.1: Comparison of nonlinear absorption coefficients in various nanomaterials  ............ 75 
Table 10.1: Photo-voltages produced for composites   ............................................................ 90 
Table 10.2: Photo-voltages for composites using Nitrogen laser ............................................. 90 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
xii 
 
List of Figures 
 
 
Figure 2.1: Energy level diagrams for different second order nonlinear processes................... 11 
Figure 2.2: Density of states in confined systems ................................................................... 16 
Figure 2.3: Electro-optic figure of merit of selected nonlinear materials ................................. 25 
Figure 2.4: Molecular structure of (a) PPV and (b) electron acceptors  ................................... 33 
Figure 2.5: Schematic of photoinduced electron transfer between PPV and fullerene ............. 34 
Figure 4.1: Molecular structure of (a) DAST  and (b) IR125................................................... 38 
Figure 4.2: Optical micrograph of a single crystal film of DAST and IR125 ........................... 40 
Figure 4.3: Optical absorption spectra of DAST-IR125 single-crystal film ............................. 40 
Figure 4.4: X-ray diffraction (XRD) data for DAST-IR125 single-crystal film. ...................... 41 
Figure 4.5: a) Aluminum electrodes evaporated (with shadow-masking) on a DAST-IR125  
 single-crystal film for use in the electro-optic measurement, b) Schematic of  
 experimental set-up for electro-optic measurement ............................................... 42 
 
Figure 4.6: Oscilloscope trace of electro-optic modulation signal of DAST-IR125 single-     
crystal film. The waveform above is the electro-optic modulation signal and the 
waveform    below is the reference signal. ............................................................. 43 
 
Figure 5.1:  Conductivity of polynorbornene as a function of molar concentration of iodine  .. 47 
Figure 5.2: Cation radicals in doped polynorbornene  ............................................................. 47 
Figure 5.3: Optical absorption spectra of polynorbornene at different iodine doping level  ..... 48 
Figure 5.4: Optical absorption spectra of silver nanocrystals .................................................. 49 
Figure 5.5: FTIR spectra of undoped (a) and doped (b) polynorbornene ................................. 50 
Figure 5.6: Photoluminescence in undoped `polynorbornene .................................................. 51 
xiii 
 
Figure 6.1:  Molecular structure  of undoped (a) and doped (b) poly(?-pinene)  ...................... 54 
Figure 6.2: Optical absorption spectra of poly(?-pinene)for different dopant concentrations ... 55 
Figure 6.3: FTIR spectra undoped (a) and doped(b) poly(?-pinene) ........................................ 56 
Figure 6.4: EPR data of poly(?-pinene) at different dopant concentrations ............................. 57 
Figure 6.5: EPR signal of undoped poly(?-pinene) ................................................................. 58 
Figure 6.6: EPR signal of lightly doped poly(?-pinene) .......................................................... 58 
Figure 6.7: Area of EPR signal per gram of poly-(?-pinene) as a function of molar   
concentration of iodine ......................................................................................... 59 
 
Figure 7.1: (a) Optical micrograph (100x) of  an optimally doped and processed film of     
poly(?-pinene) showing excellent optical quality.(b) Optical micrograph (100x)  
 of typical doped film with poor optical quality. ..................................................... 62 
 
Figure 7.2: Schematic of field induced birefringence experiment............................................ 64 
Figure 7.3: Modulation depths observed at 1.55 ?m as a function of applied electric field for:  
(a) iodine-doped poly(?-pinene) film, and (b) PTS-polydiacetylene single-crystal  
film ....................................................................................................................... 66 
 
Figure 7.4: Organic quantum dots of sub-nanometer dimension formed by doping and       
charge-transfer of poly(?-pinene). ......................................................................... 67 
 
Figure 8.1: Schematic of the experimental set-up for pump probe technique ........................... 71 
Figure 8.2: Two-photon absorption data measured at 790 nm using pump-probe technique .... 73 
Figure 8.3: Two-photon absorption data measured at 795 nm using pump-probe technique .... 73 
Figure 8.4: Quantum dots (encircled regions) in doped poly(?-pinene) ................................... 75 
Figure 8.5: Size dependent electrical conductivity measurements in indium nanocrystals  ...... 76 
Figure 9.1: Photographs of (a) undoped and (b) doped fiber-optic waveguide  ........................ 80 
Figure 9.2 Optical micrographs of (a) undoped and (b) doped fiber-optic waveguide ............. 81 
Figure 9.3: Experimental setup for two-photon absorption in a fiber-optic waveguide. ........... 81 
xiv 
 
Figure 9.4: Normalized transmission in the optical waveguide at various input power levels .. 82 
Figure 10.1: Optical micrographs (magnification 50x) of the composite films. The composite 
with 4% by weight (a) of C60 is more homogeneous than that with 8% by  
 weight  (b) of C60 ............................................................................................... 85 
 
Figure 10.2: Optical absorption spectra of the composites involving poly(?-pinene) and   
different weight percents of C60. Inset shows the absorption spectrum of C60 . ... 86 
 
Figure 10.3: Quenching of photoluminescence of the composite involving poly(?-pinene)        
and C60 (2% by weight) for excitation at 280 nm. Inset shows the 
photoluminescence spectrum of poly(?-pinene)  for excitation at 280 nm ........... 87 
 
Figure 10.4: Schematic of the photovoltaic experimental setup............................................... 88 
Figure 10.5:  Photovoltage as a function of incident light intensity for a composite with 4%     
C60 by weight ..................................................................................................... 89 
 
Figure 10.6: Schematic of photo-induced electron transfer from poly(?-pinene) to C60 ........... 92 
1 
 
CHAPTER 1 
INTRODUCTION
 The field of photonics has been constantly evolving and adapting to meet the ever-
accelerating demands of the information age. Photonics brings together optics and electronics to 
efficiently handle and process enormous amount of data with exceptional speed. Development 
and characterization of reliable nonlinear optical materials have remained the focus in photonics 
for the past few decades. Organic materials in particular have attracted great research attention. 
Their low dielectric constants ensure that speed is never compromised. Further, the ease of 
fabrication and the freedom they offer to engineer them at molecular level have made them very 
popular. It is not difficult to predict that these materials will soon be the backbone of optical 
communication, information storage and other critical fields. But much waits to be explored and 
experimented in this field. 
      Ever increasing oil prices, global warming, climate change, efficient energy 
production and waste management are issues that have plagued today?s policy makers around the 
globe. Finding a technological breakthrough to tap renewable sources of energy is clearly the 
only solution to all the problems, although, unfortunately not an easy one. Since solar energy is 
the most abundant and obvious of all renewable resources, development of photovoltaic 
materials to efficiently convert solar energy to electrical energy has been a key research initiative 
around the globe.  
 Despite their inherent advantages, organic photovoltaic materials are not widely used 
due to their low efficiency. Understanding the science, materials and methods involved in 
2 
 
organic photovoltaics could well be the solution to a problem that has economic, political, social 
and environmental repercussions. 
 The above-mentioned challenges are the key motivations for this research. The focus 
of this work can be divided into the following three broad categories: 
1. Preparation and characterization of  single crystal films involving a combination of 
DAST and laser dye IR125 
2. Structural, electrical and nonlinear optical studies of nonconjugated conducting polymers: 
poly(?-pinene)and polynorbornene  
3. Development and characterization of novel photovoltaic composites using nonconjugated 
conducting polymers and fullerene C60 
 The contents of this dissertation are organized in 11 chapters. This chapter serves as 
an introduction and also briefly explains the contents and organization of this dissertation. 
Chapter 2 summarizes the background work related to this research. Its scope includes the 
history of nonlinear optics, fundamentals of electromagnetic propagation, nonlinear optical 
effects and processes, a brief overview of second order and third order nonlinear optical 
materials previously reported, a discussion on nonconjugated conducting polymers and the 
materials and methods involved in inorganic and organic photovoltaics.  
 Chapter 3 defines the objectives of this research work.  Single-crystal films involving 
a combination of DAST and laser dye IR 125 for electro-optic applications are discussed in 
chapter 4. This chapter deals with the preparation of the films, their characterization using 
various techniques like polarized optical microscopy, optical absorption and X-ray diffraction. 
Experimental procedure and results of electro-optic measurements are also elaborated in this 
chapter. 
3 
 
 In chapter 5, polynorborne, a nonconjugated conducting polymer, is discussed. The 
contents of this chapter include electrical, optical and structural properties of this polymer when 
doped with iodine. Electrical conductivity measurements, optical absorption, FTIR 
measurements, photoluminescence are discussed. The material?s ability to behave like an organic 
quantum dot upon doping and its promise as a potential electro-optic material is also discussed. 
 Chapters 6 through 9 are dedicated to various experiments conducted on the non-
conjugated conducting polymer, poly(?-pinene).  
 Chapter 6 discusses the structural characterization of doped and undoped poly(?-
pinene) using Electron Paramagnetic Resonance. Relationship between dopant concentration and 
the formation of cation radicals is studied at room tempetarure and at lower temperature (100K). 
Hyperfine splitting observed in heavily doped polymers is also discussed.  
 Chapter 7 deals with the off-resonant electro-optic measurements at wavelengths 
including 1.55?m using field induced birefringence technique. Methods to protect sample life 
and improve the optical quality are discussed. The results are compared with other nonlinear 
optical materials.  
 Chapter 8 discusses the two-photon absorption measurements in doped poly(?-
pinene) using time-resolved pump-probe technique performed with a Ti-Sapphire pulsed tunable 
laser.  
 A nonlinear optical waveguide based on a multi-mode silica fiber with a uniform thin 
film of the doped polymer as cladding layer is discussed in chapter 9.  Experimental techniques 
and the results of two-photon absorption in these waveguides are also discussed. 
 Chapter 10 discusses photovoltaic effect in a composite involving nonconjugated 
polymer and C60. Preparation and characterization of the photovoltaic cell and the results of the 
4 
 
photovoltaic measurements at different wavelengths are discussed. 
 Chapter 11 summarizes the overall conclusions of all works discussed in this 
dissertation 
  
5 
 
CHAPTER 2 
BACKGROUND 
2.1 Introduction to nonlinear Optics 
 Light is an electromagnetic radiation. The theory of electromagnetic propagation of light 
was first proposed by James Clerk Maxwell and was later confirmed by Heinrich Hertz.[1, 2] 
Electromagnetic waves consist of oscillating electric and magnetic field such that the direction of 
propagation of the wave, the electric field and the magnetic field are in mutually perpendicular 
directions. The propagation of light is governed by the following four equations, collectively 
known as the Maxwell's equations.  
 null ? null = null (2.1) 
 null ? null = 0 (2.2) 
 null ? null = ??null ?null?  (2.3) 
 null ? null = null + ?null ?null?  (2.4) 
 where E and H are the electric field and magnetic field vectors, D and B are the electric and 
magnetic displacement vectors, ? and J is the charge and current density respectively. The Gauss 
law in equation (2.1) gives the relationship between the charge in a closed surface and the 
electric field. Gauss's law for magnetism in equation (2.2) states that the net magnetic flux in a 
closed surface is zero, owing to the fact that magnetic monopoles do not exist. Faraday's law of 
induction in equation (2.3) quantitatively defines the electric field caused due to change in 
magnetic field. Equation (2.4) is the Ampere's law which was later modified by Maxwell to its 
current form.  
6 
 
2.2 History 
 Nonlinear optics is a branch of science that deals with the interaction of an intense and 
monochromatic light source or applied electric field with a nonlinear optical (NLO) material. 
Although laser was invented in 1960 by Townes et al.[3], the genesis of nonlinear optics is 
traced to 1961 when Franklin et al. reported second harmonic generation after detecting 
ultraviolet light of wavelength 347.1 nm, when a ruby laser (wavelength 694.2 nm) was passed 
through a quartz crystal[4]. The following year, other optical nonlinear effects like third 
harmonic generation [5] and Raman scattering [6] were reported. Ever since then, this field has 
intrigued many scientists and in less than half a century, nonlinear optics has become one of the 
many promising fields of Physics.  
2.3 Propogation of light in a medium 
 Optical media can be classified into two types:   
    1.  Isotropic medium  
    2.  Anisotropic medium  
In an isotropic medium, the polarization P and the electric field displacement vector D do 
not depend of the direction of the applied electric field. The polarization in an isotropic medium 
is always parallel to the applied field and the linear relationship below holds.  
 null = nullnullnullnull (2.5) 
 The relationship between the displacement vector and the susceptibility null is given by  
 null = nullnullnull + null = nullnull (2.6) 
 where null is the dielectric constant and it depends on the susceptibility as under  
 null = nullnull(1 + null) (2.7) 
 
7 
 
In an isotropic molecular medium that is non-conducting and nonmagnetic, the electrons 
are tightly bound to their nucleus. When light interacts with such a medium, it may be considered 
as a dielectric subjected to an electric field. The polarization due to the dipole induced by the 
field is given by [7]  
 null = ?nullnullnull (2.8) 
 Where P is the polarization, e is the electronic charge, and r is the displacement induced by the 
field. Thus the wave equation for such a medium can be derived from the Maxwell's equations as  
 null ? null ? null = ? nullnullnull nullnullnullnullnull null (2.9) 
 In a nonlinear anisotropic medium, the polarization is not necessarily parallel to the electric 
field. The polarization null is given by  
 nullnull = nullnull(nullnullnullnullnull + nullnullnullnullnull + nullnullnullnullnull) (2.10) 
 nullnull = nullnull(nullnullnullnullnull + nullnullnullnullnull + nullnullnullnullnull) (2.11) 
 nullnull = nullnull(nullnullnullnullnull + nullnullnullnullnull + nullnullnullnullnull) (2.12) 
 Where null, the susceptibility is a tensor of rank 2. The electric displacement vector null is given by  
 nullnull = nullnull(1 + nullnullnull)nullnull = nullnullnullnullnull (2.13) 
 where nullnullnull is the permittivity tensor. If the co-ordinate system is chosen such that it coincides 
with the principal dielectric axes of the medium, we have  
 null
nullnull
nullnull
nullnull
null = null
nullnull 0 0
0 nullnull 0
0 0 nullnull
null ? null
nullnull
nullnull
nullnull
null (2.14) 
 The energy density due to the electric field is given by  
       null = nullnullnull?null                                                                                                     (2.15) 
 Since the x,y,z are chosen along the principal axes, we have  
8 
 
 null = nullnull [nullnullnullnullnullnull + nullnullnullnullnullnull + nullnullnullnullnullnull] (2.16) 
  2null = nullnullnullnull
nullnull
+ nullnullnullnull
nullnull
+ nullnullnullnull
nullnull
 (2.17) 
 nullnullnull = nullnull?2null, null = nullnull?2null, null = nullnull?2null (2.18) 
  nullnullnull
nullnull
+ nullnullnull
nullnull
+ nullnullnull
nullnull
= 1 (2.19) 
 nullnullnull
nullnull
+ nullnullnull
nullnull
+ nullnullnull
nullnull
= 1 (2.20) 
 Equation (2.20) is known as the indicatrix equation, it relates the refractive index and the 
direction of polarization of a monochromatic light wave in a nonlinear anisotropic medium. 
Since the indicatrix equation traces the path of an ellipsoid, it is also called the refractive index 
ellipsoid.  
2.4 Nonlinear optical processes 
 Before the advent of lasers, optical phenomena were considered to be linear. For 
instance, the relationship between polarization and electric field was considered to be linear. 
However, when lasers interacted with certain NLO materials, the linear relationship did not hold 
and a nonlinear dependence of polarization with applied electric field was reported[8, 9, 10, 11, 
12]  
 null = nullnull + nullnullnull = null(null)(null) + null(null)(nullnull) + null(null)(nullnull). .. (2.21) 
where null is the total polarization, nullnull is the linear part of the polarization and nullnullnull is the nonlinear 
part of the polarization caused by the electric field null.  
 nullnull = nullnullnull(null)null (2.22) 
 nullnullnull = nullnullnull(null)nullnull + nullnullnull(null)nullnull. .. (2.23) 
2.4.1 Second order nonlinear processes 
 NLO materials can be classified as centrosymmetric and non-centrosymmetric. Materials 
9 
 
in which the polarization is an odd function of electric field are called centrosymmetric 
materials.(null(?null) = ?null(null)) In a centrosymmetric material  
 null(null)nullnull = ?null(null)nullnull (2.24) 
This is possible only when the second order susceptibility, null(null) = 0. Similarly null(nullnull) = 0 where 
n is a positive integer. In a non-centrosymmetric material the polarization is an even function of 
applied electric field (null(?null) = null(null)) and hence the even terms of polarization can be nonzero. 
Clearly, to exhibit second order nonlinearities, an optical material must be non-centrosymmetric. 
We will now discuss the following second order nonlinearities.   
    1.  Second harmonic generation  
    2.  Sum frequency generation  
    3.  Difference frequency generation  
    4.  Optical rectification  
    5.  Pockels effect  
 When a pump laser beam interacts with certain non-centrosymmetric materials, a laser 
beam with twice the frequency of the pump beam is produced. This phenomenon is called 
second harmonic generation. [13] If the electric field of the pump beam is [8] 
 null = nullnullnullnullnullnull + null. null. (2.25) 
 then the second order polarization P is given by  
 null = nullnullnull(null)nullnull (2.26) 
 = 2nullnullnull(null)nullnull? + (null(null)nullnullnullnullnullnullnull + null.null. ) (2.27) 
 
  If two pump beams of frequency nullnull and nullnull, with electric fields nullnull and nullnull interact with  
 
In the material is non-centrosymmetric material, we have 
10 
 
 null = nullnullnullnullnullnullnullnull + nullnullnullnullnullnullnullnull + nullnull?nullnullnullnullnullnull + nullnull?nullnullnullnullnullnull (2.28) 
 null(null) = nullnullnull(null)[nullnullnullnullnullnullnullnullnullnull + nullnullnullnullnullnullnullnullnullnull 
 +nullnullnullnullnullnullnull(nullnullnullnullnull)null + 2nullnullnullnull?nullnullnull(nullnullnullnullnull)null + null. null. ] 
 +2nullnullnull(null)[nullnullnullnull? + nullnullnullnull?] (2.29) 
 
By carefully tuning the experimental conditions to achieve a particular phase matching 
condition, it is possible to efficiently convert the pump beam to a specific frequency. If the 
output frequency nullnull is such that it is the sum of the two input frequencies nullnull and nullnull then the 
phenomenon is called sum frequency generation (SFG). In SFG the polarization P is given as  
 null(nullnull + nullnull) = 2nullnullnullnullnullnullnull (2.30) 
If ?3 is given by the difference of the pump beam frequencies, then the phenomenon is 
called difference frequency generation (DFG). The polarization P for such a nonlinear process 
is given by  
 null(nullnull ? nullnull) = 2nullnullnullnullnullnullnull?                                                                                         (2.31) 
 
If the two electric fields interacting with the nonlinear crystal results in a static electric field then 
the process is called optical rectification. The polarization in this case is given by  
 null = 2nullnullnull(nullnullnullnull? + nullnullnullnull?  ) (2.32) 
The energy level diagram for the above three processes is shown in the figure below. 
11 
 
 
Figure 2.1 Energy level diagrams for different second order nonlinear processes 
When an optical pump beam and a dc (or low frequency ac) electric field interact with a second 
order material, a change in refractive index proportional to applied dc (or low frequency ac) 
electric field is observed. This process is known as linear electro-optic effect or Pockel's effect. 
This was first observed by a German physicist Friedrich Pockel in 1906.[13] In a linear electro-
optic effect the indicatrix equation shown in (2.20) is changed as shown. It must be noted that the 
x,y and the z axes cease to be the principal axes under the influence of the external field due to 
the presence of cross terms in the equation below.  
 null nullnull
nullnull
+ nullnullnullnullnullnullnullnull+null nullnull
nullnull
+ nullnullnullnullnullnullnullnull+null nullnull
nullnull
+ nullnullnullnullnullnullnullnull + 2nullnullnullnullnullnullnull + 
                    2nullnullnullnullnullnullnull + 2nullnullnullnullnullnullnull =1 (2.33) 
 where r is the electro-optic coefficient. Since the electro-optic tensor is symmetric about the first 
two indices, a shorthand notation rhk is used in the above equation to denote rijk where  
 [14, 8] 
h=1?ij=11;   h=2?ij=22;   h=3?ij=33;    h= 4?ij=23 or 32;        
                                            h=5?ij=13 or 31;   h=6?ij=12 or 21 (2.34) 
2.4.2 Third order nonlinear processes 
            Third order polarization in NLO materials consists of three distinct frequencies and is 
given by the equation below: 
12 
 
                          null(null) = nullnull(null)nullnullnullnullnull + nullnull(null)nullnullnullnullnull + nullnull(null)nullnullnullnullnull + null. null.                         (2.35) 
Different third order nonlinear process can be understood by separating the frequency 
components in the above equation. In this section we will discuss few common third order 
phenomena. 
 When some of these materials interact with an optical field and a slow time varying external 
field, the change in the indicatrix is quadratically related to the external field applied and is given 
by the relationship below: 
                                                            ?null = nullnullnullnull                                                                  (2.36) 
Where ?n is the change in refractive index, K is the kerr co-efficient, null is the wavelength of the 
laser beam and E is the magnitude of the slow time varying electric field. This effect was first 
reported by a Scottish physicist John Kerr in 1875. [15]  
 Kerr effect can also be observed when a highly intense laser beam propagates 
through a third order material. The change in refractive index is given by the equation below: 
                                                         ?null = nullnullnull                                                                         (2.37) 
Where nullnull is the nonlinear refractive index and null is the intensity of the laser source. The nonlinear 
refractive index is related to the third order susceptibility null(null) per the equation below. 
                                                          nullnull = nullnullnullnullnull(null)null
nullnullnull
                                                                  (2.38) 
Where nullnull is the weak field refractive index of the material and c is the speed of light. For glasses 
and some transparent crystals the nonlinear refractive index is about 10-16 - 10-14 cm/W. In 1990 
Sheik Bahae reported [16] that the nonlinear refractive index in semiconductors is related to the 
fourth power of the energy gap and also the proximity of the energy of the laser source to the 
energy gap.  
               When a laser source whose transverse intensity propagates through a nonlinear material, 
13 
 
a phase delay due to Kerr effect deforms the wave front causing it to either focus or defocus 
depending on the sign of the nonlinear refractive index, nullnull.[13] If nullnull is positive the material 
behaves like a positive lens and converges the beam, this is called self-focussing. When nullnull is 
negative an opposite effect is observed, where the material behaves like a negative lens and 
diverges the incident beam and this is known as self-defocusing.  
 Two-photon absorption is a third order nonlinear process where two photons (not 
necessarily of identical frequency) are excited to a higher energy level and the increase in energy is 
given by the sum of the energies of the two photons. The two-photon absorption co-efficient ?2 is 
given by  
                                                                     ?null = nullnullnull                                                             (2.39) 
Where ?null is the change in linear refractive index and I is the intensity of the laser beam. Two-
photon absorption is considered a nonlinear process because the strength of absorption varies as 
the square of the intensity of the source. This effect was first predicted by the Nobel laureate 
Goeppert-Mayerin[17] in her doctoral dissertation in the year 1931. However, the first 
experimental observation of this effect was reported three decades later after the advent of laser by 
two separate research groups who worked on europium doped crystal and cesium vapor.[18, 19] 
2.4.3 Resonant Enhancement in Nonlinear Optical Materials 
 The quantum perturbation theory gives the relationship between the NLO 
susceptibility, transition dipole moment and the energy levels of the material. This is shown in 
equations below[8]  
nullnullnullnull(null)nullnullnull + nullnull;nullnull,nullnullnull = nullnullnull
null?null
? nullnullnullnull nullnullnull
null nullnullnullnull
nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull                 (2.40) 
           nullnullnullnullnull(null) nullnullnull + nullnull + nullnull; nullnull,nullnull, nullnullnull = nullnullnull
null?null
? nullnullnullnull nullnullnull
null nullnullnullnull nullnullnullnull
nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull   (2.41)          
14 
 
Where N is the number density of the atom, F is the permutation operator over the input 
frequencies, nullnullnullnull  denotes the transition dipole moments. When the energy levels involved in the 
NLO process is close to the energy level of the material, the susceptibility increases significantly 
due to the strong coupling between the material and the light source. 
2.4.4 Nonlinear Optics in Confined systems 
Charge confinement is widely used in nonlinear optics to increase the optical 
nonlinearities. Charge confinement is achieved by artificially confining the valence electrons in 
regions smaller than delocalization length which results in discrete optical resonances and the 
characteristics of these resonances are determined by the extent of confinement. This effect has 
been demonstrated in various materials like metals, semiconductors and organic polymers.[20] 
There are two different kinds of confinements namely, quantum confinement and dielectric 
confinement. Both of these would be discussed in this section. 
2.4.4.1Quantum Confinement 
The de Broglie wavelength (?d) of electrons waves is given by[21] 
                                               nullnull = ? nullnull                                                                                   (2.42) 
Where h is the plank?s constant and p is the linear momentum. In a bulk material, free electrons 
can move in any direction and their thermal kinetic energy (ET) is related to their linear 
momentum as 
                               nullnull = nullnullnullnullnull
null?
? nullnullnullnullnull                                                                                  (2.43) 
Where pi is the linear momentum along the i direction, me* is the effective mass of the electron, 
KB is the Boltzmann?s constant and T is the absolute temperature.  From the above two equations 
the de Broglie wavelength at a given temperature T can be expressed as 
15 
 
                                         nullnull = nullnullnull
null?nullnullnull
                                                                                (2.44) 
In bulk materials, the dimensions of the crystals are much larger than the de Broglie wavelength 
shown in equation 2.45. When the dimension of the crystal is reduced in any direction (x,y,or z), 
then the energy level ceases to be continuous and becomes quantized in that direction. This effect 
is called quantum confinement. Quantum confinement can be categorized into three groups 
depending on the number of directions in which the electron is confined. A one dimensional 
confinement can be achieved in semiconductors by sandwiching a thin layer of lower band gap 
material between two higher band gap materials thereby confining the electrons and holes in the 
lower band gap material in a finite potential well. This can be compared to a particle in a one 
dimensional box. Such a system is called a quantum well. Two-dimensional confinement is 
achieved in a quantum wires, were electrons are free to move along the direction of the wire. In a 
quantum wire, energy levels are quantized in the two directions perpendicular to the direction of 
the wire. In a three-dimensional confinement, the electron is confined in all three directions. 
Such a material behaves like an artificial atom. These systems are known as quantum dots. A 
quantum dot is an electron in a three dimensional box. Its energy is discretized in all three 
directions. The energy level of a simple quantum dot is as below[22, 20] 
                                           nullnull = nullnullnullnull
null
nullnullnullnullnull
null
nullnull + nullnullnullnull
null
null
null
+ nullnullnullnull
null
nullnullnull                                              (2.45) 
Where En energy level, h is the planks constant, me is the mass of the electron, ni and li are 
quantum numbers and lengths respectively corresponding to the i direction. The figure below 
shows the density of states of electrons in bulk and confined structures. 
 
 
16 
 
 
Figure 2.2: Density of states in confined systems from [20] 
2.4.4.2 Dielectric confinement 
In a confined system that has non-uniform charge density distribution, the electric field that 
polarizes the charges in the material may be different from the external field. Due to this 
disparity the field strength distribution varies at the interface between the confined region and the 
regions surrounding it. This creates an additional electric field inside the confined region due to 
the surface polarization and hence the dielectric constant of the surrounding area is significantly 
lesser that that on the confined area. When light is shined on such a system there could be an 
enhancement in the local field inside the confined system which would in turn affect the NLO 
properties of the system.  [7] 
 
2.4.4 .3 Effect of quantum confinement on third order susceptibility 
The optical susceptibility in non-absorbing regions depends on the critical regions in the density 
of states. Since the density of states depends on the charge confinement (as can be seen in figure 
2.2) the optical susceptibility is affected by charge confinement at the critical regions. In a one-
dimensionally confined system (quantum well) with critical point Eo the third order 
susceptibility, null(null) can be expressed as [23] 
17 
 
                                                        null(null) = nullnull(null) nullnullnullnull
null
null
null
                                                          (2.46) 
Where nullnull(null) is the third order polarizability of the non-confined structure, EF is the Fermi level. 
The confinement length L dictates the ratio EF/Eo.  
In a quantum dot, where the electron is confined in all three directions with a critical surface E2 
the third order susceptibility is given by[24] 
                                                      null(null) ? nullnullnullnull
nullnull
null nullnull                                                               (2.47) 
Where Pcv is the interband transition dipole moment. The magnitude of Pcv depends on the extent 
of confinement. Equations 2.47 and 2.48 show that the artificial charge confinement has 
significant impact on the third order susceptibility 
2.4.4.4 Effect of dielectric confinement on third order susceptibility 
If spherical quantum dots of radius R (R<?) is dispersed in a dielectric medium of dielectric 
constant ?0, the field (Ei) inside each particle is given by[25] 
                                                     nullnull = nullnullnullnullnullnullnull
null
null ? nullnullnull                                    (2.48) 
Where ?o is the dielectric constant of the bulk (non-confined) material, E is the applied electric 
field and f1 is a factor that approximately equals the ratio of electric field inside each particle to 
the applied electric field. If the polarization inside the quantum dot is of magnitude p, Maxwell 
and Garnett showed that the effective dipole moment of the composite null? can be expressed as 
<agboyd34> 
                                                  null? = nullnull + 3nullnullnull nullnullnullnullnullnullnullnull
null
                                         (2.49) 
If the frequency ?s is such that 
                                                null?(nullnull) + 2nullnull = 0                               (2.50) 
(where ?? is the real part of the bulk dielectric constant ?) the effective dielectric constant 
18 
 
significantly increase, correspondingly, at frequencies near ?s there is a steep increase in the 
local field and such a phenomenon is called surface plasmon resonance(SPR). SPR has 
significant impact on the optical nonlinearities especially the optical Kerr effect. The optically 
induced change in dielectric constant is given by 
                                                nullnull? = 12nullnull(null)|null(null)|null                         (2.51) 
Differentiating equation 2.50 and 2.51 and comparing it with equation 2.52 we have: 
                                             nullnull(null) = null|nullnull|nullnullnullnullnull(null)                   (2.52) 
where  nullnull(null) is the third order susceptibility of the composite. If the Kerr effect is measured at 
frequencies near the SPR frequency there is a substantial increase in   nullnull(null)  in quantum dots due 
to dielectric confinement. This effect is over the above the enhancement caused due to quantum 
confinement. To summarize this discussion, the optical nonlinearities in confined nanostructures 
are significantly higher than their bulk counterparts due to a combination of dielectric and 
quantum confinements. 
2.5 Nonlinear Optical Materials 
 The field of nonlinear optics has witnessed exponential growth since 1960 after 
laser was invented. The quest for materials with high optical nonlinearities has fueled the research 
in the field for about a half a century now. In this section we will discuss several NLO materials 
reported in the past. This section is divided into two subsections. The first subsection will discuss 
second order materials and the second will be dedicated to a discussion on third order materials. 
2.5.1 Second Order Materials 
 During early 1960s when NLO material development and characterization was in 
its primitive stages, the following were taken into consideration while choosing an ideal device 
material. 
19 
 
1. The material must be non-centrosymmetric since the discussions in section 2.4 clearly 
shows that centrosymmetric materials do not have even order terms of susceptibility. 
2. The materials must easily available 
3. The materials must be birefringent to allow phase matching. 
4. The material must have excellent optical quality and high refractive index 
5. The material must have low dielectric constant to enable high bandwidth 
The first reported optical nonlinearity was observed in quartz when Franklin and co-workers 
recorded a very weak second harmonic signal in quartz using a Q-switched ruby laser source [4]. 
The invention of phase matching was the next major breakthrough. In 1962 two research teams 
individually showed that phase matching in ammonium-dihydrogen phosphate (ADP) can result in 
significantly stronger second harmonic signals[26, 27].  Two years later Boyd and co-workers 
reported lithium meta-niobate (LiNbO3) to be ?An efficient phase matchable NLO material.?[28] 
LiNbO3 gained popularity as a photonic device material because of the several advantages it had 
over the then popular NLO materials ADP and KDP. Some of its advantages include its 
nonhygroscopic nature, hardness and ease to polishing [29]. However, the material had a notable 
shortcoming. In 1966 Ashkin and co-workers [30] showed that lithium meta-niobate was prone to 
mechanical damage when exposed to high power continuous wave gas lasers. Despite this 
shortcoming this material is perhaps the most popular inorganic NLO material. Other popular 
inorganic second order materials include potassium titanyl phosphate (KTP), lithium borate (LBO) 
and ?-barium borate (BBO)[31].  
 Although some researchers reported the potential of organic materials in photonic devices, 
[32, 33] much attention was given to inorganic materials in 1960s. This trend changed in 1968 
when Kurtz and his co-workers [34] demonstrated a powder technique to evaluate the optical 
20 
 
nonlinearities in powder samples of materials. Several investigators used Kurtz test to report a 
variety of organic materials with high optical nonlinearities [35, 36]. 
 The shift in focus on organic nonlinear materials was due to the several advantages they 
have over their inorganic counterparts. Since NLO effects are electronic in nature, exceptionally 
high switching speeds can be attained.[37] The upper bound for bandwidth in electro-optic 
switches is determined by the dielectric constant of the material. The dielectric constant of the 
best-known inorganic nonlinear material LiNbO3 is 29[38]. This limits the bandwidth of this 
material to 10GHz.cm. Organic polymers and single crystal films have dielectric constants as low 
as 3 and hence the upper bound for bandwidth for these materials go as high as 120 GHz.cm.[37] 
Organic materials also do not require high temperature processing and can be implemented in 
electrical circuitry[39]. Also, it was shown that organic materials can be optimized for NLO 
applications by tailoring their molecular structure [40, 41]. 
 An ideal organic NLO material should possess a variety of characteristics like good 
physicochemical properties, large optical nonlinearities, wide optical transparency, flexibility in 
molecular design, ease of fabrication and phase matching, and must have the ability to withstand 
high laser intensities [7]. 
 Some of the early organic materials investigated for second order nonlinearities were 
methyl 1-1,4-dinitrophenyl amino propionate  (MAP) and 3-methyl-4-nitroaniline (MNA). In 1977 
Oudar and co-workers reported [42] high optical nonlinearity in MAP. The researchers achieved 
30% SHG conversion efficiency in a sample that was 1mm thick. The material showed promise 
because of its high damage threshold and large optical nonlinearity. More importantly the results 
showed that systematic molecular engineering approach could help achieve high optical 
susceptibilities. In 1979 Levine and co-workers [43] from Bell labs reported high optical 
21 
 
nonlinearities in MNA. The measured value of d12 was 45 times larger than LiNbO3 and d11 was 
about 2000 times larger than LiNbO3. Other early second order materials investigated were N-4-
nitrophenyl-1-prolinol (NPP),[44] 2-cyclooctylamino-5-nitropyridine (COANP),[45]  3-methyl-4-
methoxy-4?-nitrostilbene (MMONS)[46]. Although all the materials exhibited significantly high 
second order nonlinearities compared to organic materials like LiNbO3, development of phase 
matachable high quality crystals with wide optical transparencies remained a challenge [47]. 
 In 1983 Meredith and co-workers found that coulombic interactions in some organic salts 
could aid non-centrosymmetric crystallization in some organic salts [48]. Following this lead, 
Nakanishi [49] and Marder [50, 51] engineered crystalline salts by changing the anions in specific 
stilbazolium salts. Marder found [52] that 4-dimethyl amino-N-methyl-4 stilbazolium tosylate 
(DAST) showed that largest nonlinear coefficient among all the stilbazolium salts examined.  
DAST soon became one of the most widely investigated second order material. It crystallized 
noncentrosymmetrically. It had nearly optimal orientation of the chromophore which yielded a 
very high nonlinearity. The salt was found to be thermally very stable, with a melting point of 
290oC. More importantly its optical properties were found to be unaffected even when heated to 
160oC. DAST has practically no absorption at technologically significant wavelength and scientists 
believed that the material will be highly suited for electro-optic applications due to its ability to 
respond in terahertz range. 
 The initial hiccup with DAST was the non-availability of quality single crystal films. Bulks 
crystals or polycrystalline films of DAST cannot exploit the high optical nonlinearities of the 
molecule because of the following reasons[53] 
1. Application of optical and electrical fields along a specific molecular axis is not feasible. 
2. Loss of phase information due to scattering and destructive interference. 
22 
 
Due to the above reasons the electro-optic coefficient of DAST is about 100 pm/V measured at 
750nm while the electro-optic coefficient of a single crystal film of DAST measured at the same 
wavelength is 445 pm/V[54, 55, 56]. The challenge then ahead was to prepare single crystal films 
of DAST with high optical quality. Several groups proposed different techniques to achieve this 
goal [57, 58, 59, 60, 61]. Thakur?s technique to prepare single crystal films of polydiacetylene was 
revisited [62, 63]. This technique called the ?modified shear method? was successfully 
implemented to prepare single crystal films of DAST with excellent optical quality. His team 
subsequently followed this up by characterizing the electro-optic coefficients of single crystal 
films of DAST at different wavelengths. The results reported are reproduced in the table below 
[55, 64]  
Wavelength (nm) Electro-optic coefficient of DAST - r11 (pm/V) 
633 770 
720 530 
750 445 
1550 200 
Table 2.1: Electro-optic coefficients of DAST as reported by Thakur et al. 
The magnitude of imaginary part of imaginary part of the electro-optic coefficient was reported to 
be 280pm/V and 519 pm/V at 633nm and 488nm respectively [65]. DAST has absorption maxima 
at 550nm, [66] the high electro-optic coefficient at 633nm is attributed to the resonance 
enhancement due to the single photon absorption. However, as the wavelength increases, the 
electro-optic coefficient decreases due to the decrease in the resonance enhancement effect. The 
electro-optic coefficient of DAST at 1550nm is about 200pm/V. 1550nm is a technologically 
significant wavelength and very high electro-optic coefficients are desirable at this wavelength.  
23 
 
2.5.1.1 Dispersion of electro-optic coefficient and its figure of merit 
 The electro-optic coefficient is related to the macroscopic hyperpolarizabilities of the 
organic molecules and hence the wavelength dependence of electro-optic coefficients can be 
related to certain molecular parameters of the organic crystal. The electro-optic figure of merit is 
given by 
                                                nullnullnull = nullnullnullnullnull ? nullnullnullnullnullnull
null (nullnullnullnullnull nullnullnull)
(nullnullnullnullnull nullnullnull)null                                                        2.53 
Where null(null) = nullnullnull
null(null)
nullnullnullnull(null)nullnullnull null
null
 accounts for the local field correction, nullnullnull is the resonance 
frequency of optical transition,  nullnull a parameter that depends on: the dielectric constant ?, number 
of molecules per unit volume N, the difference in dipole between the ground state and excited state 
?, and the square of the transition dipole between the ground state and the excited state. The value 
for nullnullnull can be obtained from the absorption measurements or the Sellmeier parameters [67], 
nullnull and null can be computed from  nullnullnull and the experimental value of n3r at any wavelength. Thus, 
the figure of merit n3r for organic materials can be computed as a function of the wavelength. 
Figure 2.4 shows the wavelength dependence of various NLO materials. Table 2.2 gives the 
parameters used to calculate the wavelength dependence of the organic materials shown in figure 
2.5. Table 2.2 and figure 2.3 are reproduced from reference [37] 
 
 
 
 
 
 
24 
 
Material r (pm/V), n nullnullnull = nullnullnull/nullnullnull  
of bulk (nm) 
DAST r11= 400  n1=2.19 at 820nm 600 
NMBA 
4-nitro-4?-methylbenzylidene  
aniline 
r11= 37.2 n1=2.283 at 488nm 
r11= 36.4  n1=2.216 at 514.5nm 
r11= 25.2  n1=2.078 at 632.8nm 
 
353 
PNP 
2-(N-prolinol)-5-nitropyridine 
r22= 28.3  n2=1.873 at 514.5nm 
r12= 20.2  n1=2.614 at 514.5nm 
r22= 12.8  n2=1.788 at 632.8nm 
 
404 
DAN 
4-(N,N-dimethylamino)-3- 
acetamidonitrobenzene 
 
r32= 13  n3=1.949  at 632.8nm 
 
419 
metanitroaniline r33= 16.7  n3=1.875  at 632.8nm 400 
COANP r33= 16.7  n3=1.875  at 632.8nm 410 
MMONS r33= 39.9  n3=2.129  at 632.8nm 421 
MNA r11= 67  n1=2.00  at 632.8nm 414 
Table 2.2: Electro-optic response of selected organic compounds 
 
 
25 
 
 
Figure 2.3: Electro-optic figure of merit of selected nonlinear materials as a function of wavelength 
2.5.2 Third Order Materials 
 Since third order nonlinearities do not require symmetry conditions, a wide array of 
materials have been investigated for third order properties.  Different techniques such as third 
harmonic generation (THG), degenerate four-wave mixing(DFWM), optical Kerr gate (OKG), and 
self-focusing techniques may be used to determine third order susceptibility and it is difficult to 
compare the results directly because different optical processes may use different experimental 
conditions. The magnitude of third order nonlinearity is reported to be dependent on the 
conjugation length of the material. Hence in 1970s several scientists conducted elaborate studies to 
26 
 
empirically determine the dependence.[68] In 1977 Ducuing and co-workers found the relationship 
between the delocalization length L and the number of electrons N. [69] Rustugi and co-workers 
deduced a relationship between microscopic polarizibility and delocalization length for one 
dimensional free electron gas model. In 1978 Agarwal and co-workers suggested third order 
susceptibility?s dependence on the sixth power of ? electron delocalization length for one 
dimensional ? conjugated polymers. They further deduced that the third order susceptibility is 
inversely proportional to the sixth power of the energy gap. Other factors that affect the magnitude 
of third order susceptibility include donor-acceptor functionalities orientation of the polymer chain 
and packing density, conformation and its dimensionality. Several research groups worked on 
developing materials that have high third order nonlinearities and they can be broadly classified 
into the following categories: [68] 
1. Inorganic materials 
2. Organic liquids  
3. Organic molecular solids  
4. Organic charge transfer complexes  
5. Organic ? conjugated polymers 
6. Organometallic compounds  
7. Liquid crystals 
8. Organic composites 
9. Nano-metallic clusters and semiconductor nano-composites 
10. Dye grafted polymers  
11. Non-conjugated conducting polymers 
 Among the above organic ? conjugated polymers and semiconductor and metallic nano-
27 
 
particles have attracted keen attention in the recent past. Stauteret and co-workers reported 
exceptionally high third order nonlinearity in polydiacetylene p-toluene sulfonate (PDA-PTS). [70] 
This finding provided a major breakthrough for research in organic ? conjugated polymers. The 
third order susceptibility of a single crystal film of PDA-PTS at 1890nm was reported to be as high 
as 8.5?10-12 esu. At resonance the imaginary part of ?(3) was found to be 2.5?10-5 esu, which is one 
of the largest reported resonant values. [71] Other ? conjugated polymers that were studied include 
(and are not limited to) poly (p-phenylenevinylene) (PPV), polyacetylenes, polythiophenes etc.  
 The high optical nonlinearity and ultra-fast response at non-resonant frequencies of ?-
conjugated polymers is attributed to the delocalization of ? electrons along the conjugated chain or 
the quantum wire. 
 Metallic nanoparticles and semiconductor nano-clusters have also been pursued for third 
order properties. The cubic nonlinearities in synthetic materials can be increased by confining the 
valence electrons in a very small region typically within a few nanometers. These confinements 
fall into two categories namely quantum confinement and dielectric confinement. [72] 
 The high cubic nonlinearities in metallic clusters embedded in glass matrices are due to the 
surface Plasmon resonance effect (SPR). In such materials there exists a difference in dielectric 
constant between the metal clusters and the surrounding glass matrix which leads to a dielectric 
confinement in these materials. Large resonant cubic nonlinearities around SPR frequency are 
exhibited by these materials since the dielectric difference changes the field distribution and hence 
the optical properties. Optical nonlinearities of some metallic nanoparticles is shown in table 
below. [73] 
 
 
28 
 
Nanocluster  Size  
(nm)  
Host  ?  
(cm-1)  
?  
(m2/W)  
?  
(nm)  
Gold (Au)  5-30  SiO2 104 4 x 10-14 570-600  
Copper Cu)  < 10  SiO2 104 2 x 10-11 532  
Silver (Ag) < 10 SLG 1 10-14 591 
 Table 2.3: Cubic nonlinearities of select metallic nanoparticles 
 In semiconductor nanostructures the high cubic nonlinearity is due to the reduced particle 
size which increases the inter-particle spacing and hence the valence electron is confined in sub-
delocalization dimensions. This changes the electron quantum states and the way they interact with 
applied optical fields. This effect is called the quantum confinement effect. Several materials like 
CdS, CdSe, ZnS, ZnSe have been reported to possess high third order nonlinearities due to 
quantum confinement effect. 
 Recently Thakur and co-workers have shown that a new class of conducting polymers 
called nonconjugated conducting polymers (NCP) exhibit exceptionally high cubic nonlinearities 
upon doping. These materials also have interesting electrical properties. The following subsection 
is discusses the electrical properties and their potential in the field of nonlinear optics. 
2.5.3 Nonconjugated conducting polymers as third order materials 
 Conjugated polymers were the first known conducting polymers. In 1977 Shirakawa and 
co-workers reported that the conductivity of conjugated polymer polydiacetylene increased about a 
billion times when doped with iodine. [74] The electrical properties of several doped conjugated 
polymers were reported thereafter. In 1988 Thakur provided a breakthrough in understanding the 
conductivity theory of conducting polymers. He showed that specific polymers with at least one 
double bond in their repeat unit could be electrically conducting upon doping with electron donors 
or acceptors. Conducting polymers do not necessarily need to have a conjugated backbone and 
hence a new class of conducting polymers called nonconjugated conducting polymers (NCP) came 
29 
 
into existence. The first reported NCP by Thakur and co-workers was 1-4, cis(polyisoprene). [75] 
Its conductivity increased by about ten orders when doped with iodine. Thakur?s team and others 
subsequently reported other NCPs that showed similar behavior such as poly(?-pinene) [76], 
styrene butadiene rubber (SBR), [77]  poly(ethylenepyrroledyl) (PEP) [78] and polyaniline furfural 
[79]. 
 When NCPs are doped, the dopants interact with the double bond to form a radical cat ion 
or a polaronic state since electron transfer from the double bond to dopant occurs creating a hole or 
an electron vacancy. When the concentration of the dopant increases the number of double bonds 
transformed into cat ion radicals increase proportionally. The electrical conductivity saturates 
when almost all available double bonds are transformed into cat ion radicals. It must be noted that 
in doped NCPs the electrical conductivity is due to the holes that are loosely bound to the dopant. 
Doping causes charges to be confined in sub-nanometer domains. Further, the loosely bound hole 
can be modeled as a spring mass arrangement that has a quartic term in its potential energy. Such a 
system shows a great promise as a third order material. These materials  unlike conjugated 
polymers do not have extended delocalizations, their optical nonlinearity is solely attributed to the 
charge confinement.  Thakur and co-workers first reported third order nonlinearities in the NCP 
1,4-cispolyisoprene. A change in refractive index of 4?10-4 was reported in iodine doped1,4-
cispolyisoprene at 633nm for an applied voltage of 2V/?m. The corresponding Kerr coefficient 
was reported to be 1.6?10-10 m/V2 which is about 66.67 times higher than that of nitrobenzene 
measured at 589nm.[80] A modulation depth of 0.12% was reported in iodine Poly(?-pinene) at 
633nm. The Kerr coefficient at this wavelength was about 1.2?10-10 m/V2 and the measured ?(3) 
was reported to 0.65?10-8 esu. [81]. Susequently two-photon measurements were performed in this 
polymer using a 150fs Ti-Sapphire laser and maximum two-photon absorption coefficient of 
30 
 
2.6cm/MW was reported at 810nm [82]. The Kerr co-efficient of another doped NCP, 
poly(ethylenepyrroleiyl) (PEP) was reported to be 1.2?10-10 m/V2 at 633 nm [78]. 
 These reports prove without doubt that NCPs have a great potential in the field of nonlinear 
optics due to their exceptionally high cubic nonlinearities. These materials are cheap and thin films 
of these materials with exceptional optical quality can be prepared easily. 
2.6 Photovoltaics 
 Photovoltaics is the branch of science that deals with the conversion of light energy to 
electrical energy. Modern day photovoltaic (PV) cells can be divided into two broad categories: 
inorganic and organic. An inorganic PV cell normally acts like a PN junction. When light strikes 
the junction, electron hole pairs are created; electrons flow to the positive side and holes flow to 
the negative side and this constitutes a flow of current. The driving forces in this process are the 
built-in potential and the work functions of the contacts. In organic PV cells a different mechanism 
is used. They consist of composite involving a polymer and an electron acceptor. Light of 
appropriate wavelength excites the electrons in the polymer and these electrons are accepted by the 
electron acceptor and are carried through a low work function electrode while the holes are carried 
through an electrode that has a higher work function. PV cells have distinct advantages over the 
conventional power generation methods. They are [83]: 
1. Photovoltaic cells are driven by solar power, an abundant, renewable resource. This 
reduces the pressure on our depleting fossil fuel reserves. 
2. PV cells do not produce any harmful radiations; it is the safest means of energy production.  
3. PV technology does not leave any traces. No by products are wastes. Hence waste 
management is not an issue. 
4. PV technology is the most environmentally friendly technology 
31 
 
5. The PV cells can be used both in centralized and decentralized power plants 
6. PV cells have no moving parts, hence no wear, no replacement. This results in reduced 
maintenance cost 
However, the current PV technology also suffers from the following shortcomings: 
1. The PV grids and arrays are very expensive. Their installation is extremely capital 
intensive. Although the unit does not have any recurring cost, it takes an extremely long 
time to break even. The general perception is that solar cells are extremely capital 
intensive. 
2. The electricity produced is dc and it needs to be converted to ac. This again adds to the 
cost. 
3. Solar cells bring along an element of uncertainty. It depends on the climatic and 
geographical conditions. Hence it can only be used to compliment a conventional power 
system. 
In the early days of PV technology, the cells were necessarily inorganic. The first PV cells was 
demonstrated using Selenium by Fritts in 1883 [84]. Gold electrodes served as the higher work 
function electrodes and the iron electrodes served as the lower work function electrodes. When 
photons strike Fritts? Selenium based cell, the electron hole pairs are generated. The golds 
electrode accepts the holes while the lower work function iron electrodes accept the electrons to 
constitute a photocurrent. [85] The PN junction based PV cell was a chance discovery at Bell labs 
in 1954 that revolutionized the field of inorganic photovoltaics. The first demonstrated junction 
based solar cell had 6% efficiency. [86] This lead to a great research attention in this field. 
Subsequently several other semiconductor materials were investigated for PV properties. Some of 
them include Cu2S, CdS, CdTe, GaAs etc. [87, 88, 89] 
32 
 
Some of the milestones in inorganic PV technology are listed below 
1. In 1973 a violet cell was demonstrated that had improved efficiency in shorted wavelength 
region. [90] 
2. In 1980 thin film solar cells were successfully demonstrated. [83] 
3. In 1985 efficiency as high as 20% was achieved under normal sunlight condition and over 
25% using 200X concentration[91, 92] 
4. In 1990 PV technology reported a growth rate of 33%. [83] 
5. In 2004 solar power accounted for 0.04% of total power generated and in 2010 this is 
tipped to go up to 0.4%. [93] 
2.6.1 Organic materials in Photovoltaic Technology 
 Although the work in organic photovoltaics began as early as in 1950, commercially viable 
organic cells still seem to be in their nascent stages. The first organic PV cell was reported in 1958. 
The issue with organic PV technology has been the efficiency. The high efficiency inorganic PV 
cells have reached an efficiency of 40%. Much work needs to be done in organic materials to attain 
such high efficiencies. But this technology has significant advantages over the inorganic PV 
technology. They are: 
1. Organic PV composites are easy to process and fabricate. They do not require sophisticated 
high temperature fabrication facilities 
2. PV composites are extremely inexpensive.  
3. Large area PV composites can be easily produced 
4. Organic materials extend the possibility of extremely high optical absorption coefficients. 
In 1980s organic materials like mercocyanine and phthalocyanine based PV cells were reported. A 
maximum efficiency of 1% was achieved for small area cells. [94, 95] Organic conjugated 
33 
 
polymers and electron acceptor based configuration was then tested for PV cells. Buckminster 
fullerene, C60 has been widely used as an electron acceptor in organic PVs [96]. Tang and 
coworkers achieved a breakthrough in organic PV technology by developing a cell using copper 
phthalocyanine and perylene tetracarboxylic derivative.  [97] Several groups studied a 
polyphenylene vinylene (PPV) based PV cells. [98, 99, 100] Phthalocyanine has also generated 
wide research interest as a potential PV material. [101] Many other conjugate polymer, electron 
acceptor based PV composites were studied. [102] The structures of PPV and specific electron 
acceptors are shown below. 
 
(a) 
 
(b) 
Figure 2.4: Molecular structure of (a) PPV and (b) electron acceptors [102] 
 
 
34 
 
In 1992 Saricifti and coworkers reported  a strong photoinduced charge transfer between MEH-
PPV and C60 [103]. This result was further bolstered by Morita and coworkers who observed a 
similar effect in other fullerene composities [104, 105] The mechanism of photoinduced charge 
transfer is pictorially illustrated below. 
 
Figure 2.5 Schematic of photoinduced electron transfer between PPV and fullerene [85] 
 
 In summary the field of organic PVs has experienced a great progress in the past few 
decades. Organic PV technology due to its low cost is the only viable route to accomplish 
widespread solar cell penetration. It is believed that a 5% efficiency is organic solar cells is 
attainable [106]. Clearly the tradeoff between inorganic and organic solar cells boils down to a cost 
versus efficiency. If the efficiency of organic solar cells can be increased, it can change the way 
world perceives the energy crisis. 
 
  
35 
 
CHAPTER 3 
OBJECTIVES
 
 The objective of the proposed research work is to study the structrural, electrical and 
nonlinear optical properties of specific organic materials that have applications in the field of 
photonics. The second order optical material discussed in this work is a novel single-crystal film 
involving a combination DAST and IR125. This material is expected to have a high electro-optic 
coefficient at technologically significant wavelengths like 1550nm. Third order optical materials 
discussed fall under the class of conducting materials called nonconjugated conducting polymers. 
In particular, poly(?-pinene), polynorbornene and trans-polyisoprene will be discussed. 
Photovoltaic composites of nonconjugated conducting polymers and C60 will also be discussed. 
The specific objectives of this research are: 
1. to develop a single crystal film of DAST and IR125 using modified shear method; 
characterize the single crystal film using polarized optical microscopy, x-ray diffraction 
and optical absorption;  determine the electro-optic coefficients of the film at 633nm 
using the field induced birefringence method  and compare the results obtained with that 
of single crystal films of DAST 
2. to measure the electrical conductivities of iodine doped polynorbornene and trans-
polyisoprene as a function of dopant concentration; characterize polynorbornene and 
trans-polyisoprene using optical spectroscopy, Infrared spectroscopy (FTIR) and optical 
microscopy 
36 
 
3. to perform electron paramagnetic resonance studies on poly(?-pinene) at different 
temperatures and dopant concentrations of iodine and explain the effect of doping in this 
nonconjugated conducting polymer. 
4. prepare doped poly(?-pinene)  samples to be used for electro-optic measurements; 
optimize doping duration and ambience to enhance optical quality of the doped polymer 
samples; find a technique to preserve the doped polymer samples for longer durations; 
monitor the protected sample to determine the shelf life 
5. to perform electro-optic measurements in poly(?-pinene)   at off-resonant wavelengths 
including 1.55?m; find the electro-optic Kerr coefficients at these wavelengths, compare 
the results so obtained with other third order material and explain the behavior 
6. to perform time resolved interferometry measurements on doped poly(?-pinene) to  
determine the two-photon absorption coefficient at 790nm and 795nm using pump probe 
technique. 
7. to improve the interaction length of the polymer with light  using nonlinear optical wave-
guiding  and demonstrate large modulation. 
8. to prepare thin film composites of nonconjugated conducting polymers poly(?-pinene) 
and buckminsterfullerene, C60; measure the optical absorption and photoluminescence 
characteristics of these composites;  fabricate photovoltaic cells using thin film 
composites of poly(?-pinene) and buckminsterfullerene, C60 
 
 
 
 
37 
 
CHAPTER 4 
ELECTRO-OPTIC EFFECT IN A NOVEL SINGLE CRYSTAL FILM INVOLVING DAST 
AND IR125
Organic single crystal films have attracted significant research attention due to a wide 
range of applications in electronics and photonics. Exceptionally large optical nonlinearities have 
been observed in specific organic single-crystal films. Extensive research has been performed on 
organic films for electronics, owing to their lower cost and easier processibility. Field-effect- 
transistors based on several organic single crystalline materials such as rubrene, pentacene, 
tetrazene, BP2T etc[107, 108] have been reported. Organic single-crystal materials are known 
have significantly higher electronic mobility compared to solution-cast and evaporated 
organic/polymeric films.  
 
4.1 Material and Methods 
 
Single-crystal films of DAST were prepared using modified shear method. [62] The 
electro-optic coefficients of single-crystal films of DAST at different wavelengths have already 
been reported[55, 65, 66].  
The values are exceptionally large due to the optical uniformity of the films. The unit cell 
[109] of DAST crystal is monoclinic with the lattice parameters: a=10.365A, b= 11.322A, 
c=17.892A, ?=92.24o. Single-crystal films of DAST were prepared by the modified shear 
method at a higher temperature to obtain larger areas (~ 1 cm2 and larger).[55] The film has a 
[001] surface orientation. The dipole axis of DAST molecules or the overall orientation of DAST 
38 
 
molecules is along the a-axis. In the present report, we discuss growth and characterization of 
single-crystal film of a combination of materials including DAST and a laser dye, IR-125 
(molecular structure shown in Fig.4.1b). The DAST single-crystal film exhibits the largest 
electro-optic coefficient among all materials reported so far. At 633nm its electro-optic co-
efficient is 770pm/V. At 1.55 ?m the electro-optic coefficient of DAST film is about 200pm/V.  
 
Figure 4.1: Molecular structure of DAST (a) and IR125(b) 
The optical absorption spectra of DAST single-crystal film ([001] orientation) for 
different incident polarizations have been reported. DAST has strong optical absorption (inset of 
Fig.4.3) over the wavelength range of 400 to 600nm with a peak at around 550nm. [66] The 
electro-optic coefficient is largest at 633nm due to resonance enhancement. The magnitude of the 
electro-optic coefficient reduces at off-resonant wavelengths. Despite this reduction the value is 
still higher compared to other organic and inorganic materials (lithium niobate, r33 = 30pm/V). In 
this work, a single-crystal film involving a combination of two organic materials: DAST and IR-
125, is reported. This material is expected to exhibit novel electronic and optical characteristics 
that DAST by itself may not have. 
39 
 
Single-crystal films of a combination of DAST and the laser dye, IR-125 (molecular 
structure shown Fig.4.1b) were grown using the modified shear method [62, 63] at a higher 
temperature. Solid DAST and IR-125 samples were dissolved in methanol, with DAST in super-
saturation. The amount of IR-125 was about 15% of the weight of DAST. A small amount of this 
solution was introduced between two opposing flat substrates and the substrates were moved 
with respect to each other to impart a shear. The shear method leads to molecular organization 
induced by polar forces (strong capillary effect) at the interface and shear between two opposing 
substrates. The capillary effect between two opposing substrates is critical for such controlled 
orientation crystal growth. As the solvent is slowly evaporated in a controlled environment, 
single-crystal films of predictable orientation are formed on the substrate. The single-crystal 
films obtained by this method have excellent optical qualities. This is the first report on single-
crystal film of a combination of two organic materials.  
The single-crystal film of a combination of DAST and IR-125 (we will call this DAST-
IR125 film) was characterized using: 1. polarized optical microscopy, 2. polarization-dependent 
optical absorption spectroscopy, and 3. x-ray diffraction. Electro-optic measurement of the films 
was made using field-induced birefringence technique. 
4.2 Results and Discussion 
4.2.1 Polarized optical microscopy:  
The DAST-IR125 single-crystal films were characterized using polarized optical 
microscopy. An optical micrograph of a single-crystal film prepared using the modified shear 
method is shown in Fig.4.2. These crystal-films are bright red in color. They have excellent 
optical quality and uniform surfaces. The single-crystal films grown have a thickness in the 
range of 0.5 to 3?m and areas of about 1cm2 (micrograph below does not show the whole film).  
40 
 
 
Figure 4.2: Optical micrograph of a single crystal film of DAST and IR125 
4.2.2 Polarization-dependent optical absorption spectroscopy: 
  Optical absorption spectroscopy was performed on these films at different polarizations 
of the incident beam. The spectra are shown in Fig.4.3. Two distinct bands are observed: one  
 
Figure 4.3: Optical absorption spectra of DAST-IR125 single-crystal film. Figure in the inset: 
Optical absorption spectrum of DAST single-crystal film 
 
corresponding to DAST from 400 to 600nm and the other corresponding to IR-125 from 600nm 
to 900nm. The absorption spectrum along the dipole axis of DAST shows a strong DAST peak 
and a weaker IR-125 peak. When measured with polarization perpendicular to the dipole of 
41 
 
DAST, the IR-125 absorption (600 to 900 nm) becomes prominent (optical gap ~ 1.4eV). For 
polarization at 45o with respect to the dipole-axis, both DAST and IR-125 peaks are observed. 
The dipole axis of DAST is along a-axis. The optical absorption spectra indicate that the IR-125 
molecule is oriented perpendicular to the a-axis. Since these single-crystal films have high 
absorption at longer wavelengths, high electro-optic coefficients at longer wavelengths can be 
expected. In addition, this may be the first example of an organic single-crystal film with a low 
band-gap. 
4.2.3. X-ray diffraction  
X-ray diffraction was performed on these films. Fig.4.4 shows the X-ray diffraction 
(XRD) data for DAST-IR125 single-crystal film. The amorphous background in the XRD data of 
single-crystal film of DAST IR-125 is due to the quartz substrate. Comparing with known XRD 
data of DAST single-crystal film, the DAST-IR125 film appears to have a [001] surface 
orientation. 
 
Figure 4.4: X-ray diffraction (XRD) data for DAST-IR125 single-crystal film. 
4.2.4. Electro-optic Measurement 
Electro-optic measurements were performed on the DAST-IR125 single-crystal films at 
633 nm wavelength. The samples were prepared using a shadow mask technique. The dipole axis 
of DAST was determined and Al electrodes were deposited on the sample using vacuum 
42 
 
deposition technique. The electrodes were deposited perpendicular to the dipolar axis and a slit 
(50-100?m) was formed. A sample prepared by this technique is shown in Fig.4.5a. The electro-
optic measurements were performed using field-induced birefringence technique at 633nm. A 
cross-polarized geometry was achieved using a polarizer oriented at 45? from the incident laser 
beam polarization. An analyzer was oriented orthogonally to the polarizer and was placed after 
the sample. Electric field was applied along the dipole axis of DAST using the electrodes 
deposited on the sample. A photo detector was used to measure the output beam. The signal was 
analyzed using an oscilloscope or a lock-in amplifier. A schematic of the experimental setup is 
shown in Fig.4.5b. 
 
Figure 4.5 a) Aluminum electrodes evaporated (with shadow-masking) on a DAST-
IR125 single-crystal film for use in the electro-optic measurement, b) Schematic of experimental 
set-up for electro-optic measurement 
 
An exceptionally high modulation depth of about 14% was observed at 633 nm for a 
2.7?m thick film when an ac field of 1V/?m was applied. An oscilloscope trace of the 
43 
 
modulation signal is shown in Fig.4.6. Calculation of the electro-optic coefficient requires values 
of refractive indices and absorption coefficients at these wavelengths. In the absence of those 
data at the present, we can determine an approximate value using known refractive indices of 
DAST by itself. The magnitude of the electro-optic coefficient (r11) of DAST-IR125 film as 
obtained is about 300 pm/V at 633 nm.  
 
Figure 4.6 Oscilloscope trace of electro-optic modulation signal of DAST-IR125 single-
crystal film. The waveform above is the electro-optic modulation signal and the waveform below 
is the reference signal. 
 
4.3. Conclusions 
In conclusion, single-crystal film involving a combination of two organic materials: 
DAST and IR-125, is reported for the first time. This film has been characterized using polarized 
optical microscopy, polarization-dependent optical absorption and x-ray diffraction. The film has 
a [001] surface orientation. The IR-125 molecule is oriented close to perpendicular to the DAST 
dipole-axis. The film has a low optical gap of about 1.4 eV for polarization perpendicular to the 
DAST dipole-axis. Along the dipole-axis, the optical gap is about 2 eV. The optical absorption 
44 
 
characteristics of DAST-IR125 film is significantly different than that of DAST film. The 
electro-optic coefficient of DAST-IR125 film as measured is about 300 pm/V at 633 nm. These 
novel materials have a wide variety of applications in organic electronics and photonics. 
 
 
45 
 
CHAPTER 5 
POLYNORBORNENE: NONCONJUGATED CONDUCTING POLYMER
5.1 Introduction 
In this discussion electrical and optical properties of a novel nonconjugated conducting 
polymer polynorbornene is discussed. Polynorbornene is synthesized from the monomer 
norbornene using ring opening metathesis reaction. Like other nonconjugated conducting 
polymers, the conductivity of this polymer increases by several orders upon doping with iodine. 
this material is also expected to exhibit high cubic nonlinearities due to the charge confinement 
in sub-nanometer domains. 
5.2 Experimental 
5.2.1 Electrical Conductivity 
As discussed previously, the maximum conductivity of a nonconjugated conducting 
polymer depends on the number fraction of its double bonds per repeat unit.  The maximum 
conductivity of polyisoprene[80] which has a double-bond number fraction of 1/4, is 0.1 S/cm. 
Poly(alloocimene)[110], which has a double bond number fraction of 1/3 reaches a maximum 
conductivity of 1S/cm, while poly(?-pinene)[76] with double bond number fraction of 1/6, has a 
maximum electrical conductivity of 0.008 S/cm.   
In this report, we discuss electrical conductivity of polynorbornene which has a double-
bond number fraction of 1/5. For this polymer, we expect a maximum conductivity between 
those of polyisoprene and poy(?-pinene).  
Table 5.1 shows the maximum electrical conductivity and the corresponding double bond 
46 
 
number fraction of specific conducting polymers 
 
 
 
 
 
 
 
 
Table 5.1: Conductivity of nonconjugate conducting polymers 
Polynorbornene samples obtained from Aldrich (average Mw> 2,000,000) was powdered and 
dissolved in benzene. The polymer solution was then used to cast a thin film on a glass slide. 
Initial weight of the polymer and the conductivity of the film were measured. The film was then 
doped with iodine; the weight uptake of iodine and the corresponding conductivity of the film 
were noted at regular intervals. The conductivity of the polymer was observed to increase with 
increase in dopant concentration. A maximum conductivity of 0.01S/cm was observed 
corresponding to an iodine concentration of about 1.5 molar. This value is in between those of 
poly(?-pinene) and polyisoprene. The conductivity of polynorbornene as a function of molar 
concentration of dopant is shown in Fig.5.1. A comparison of the conductivities of some of these 
nonconjugated conductive polymers are shown in Table 5.1.  It should be noted that the double 
bond number fraction of polynorbornene is 1/5 and its conductivity lies between that of cis-
poly(isoprene) and poly(?-pinene) as one might have expected.   
Polymer Double bond number 
fraction 
Electrical 
Conductivity 
(S/cm) 
Poly(?-pinene)[76] 1/6 0.008 
Polynorbornene 1/5 0.01 
Cis-poly(isoprene)[80] 1/4 0.1 
Poly(alloocimene)[110] 1/3 1 
Polyacetylene 
(Conjugated polymer) 
1/2 100 
47 
 
 
Figure 5.1:  Conductivity of polynorbornene as a function of molar concentration of iodine 
When the polymer is doped with iodine, cation radicals are formed. The number of cation 
radicals formed is directly proportional to the molar concentration of iodine. The holes formed 
when the polymer is doped are responsible for the increased conductivity. Molecular structure of 
doped polynorbornene with the cation radicals formed due to iodine doping is shown in figure 
5.2. 
 
Figure 5.2: Cation radicals in doped polynorbornene 
5.2.2 Optical absorption 
The undoped film of polynorbornene is transparent. The film turns brownish green upon doping 
with iodine. Optical absorption measurements were performed on polynorbornene film at 
1.00E-16
1.00E-14
1.00E-12
1.00E-10
1.00E-08
1.00E-06
1.00E-04
1.00E-02
1.00E+00
0.0 0.3 0.5 0.8 1.0 1.3 1.5
Molar concentration of Iodine
Co
nd
uc
tiv
ity
 (S
/cm
)
10  00
10 -02
10 -08
10 -06
10 -04
10 -10
10 -12
10 -14
10 -16
 C    C C     C  C    C 
   I3    I3 
+ + 
48 
 
different dopant concentrations using thin films cast on quartz substrates. The spectra are shown 
in Fig. 5.3.  
 
Figure 5.3: Optical absorption spectra of polynorbornene at different iodine doping level 
The lightly doped polymer has two peaks at 295nm (5.3eV) and 396nm (3.13eV). The peak at 
5.3eV corresponds to the cation radicals formed due to doping and the peak at 3.13eV 
corresponds to the charge transfer between the double bond and the dopant. When the dopant 
concentration is increased, the intensity of the peak at 5.3eV increases and the peak 
corresponding to charge transfer broadens and undergoes a red shift. This behavior is observed in 
other nonconjugated conducting polymers such as poly(?-pinene)[76]  and cis-poly(isoprene). 
[80] 
The optical absorption of doped polynorbornene is comparable to that of metallic nanoclusters. 
Figure 5.4 shows the absorption spectra of silver nanocrystals.[111] 
0.000
0.200
0.400
0.600
0.800
1.000
1.200
280.0 380.0 480.0 580.0 680.0 780.0 880.0
Wavelength (nm)
Ab
so
rb
an
ce
  (
a.u
)  
   
   
  .
Undoped
Lightly doped Medium doped
Heavily doped
49 
 
 
Figure 5.4: Optical absorption spectra of silver nanocrystals 
The absorption peak of the nanocrystal is a function of the particle size. With increase in particle 
size the absorption peak broadens and red shifts. A similar effect can be found is the absorption 
peak corresponding to charge transfer in polynorbornene. In doped nonconjugate polymers, 
charges are confined in subnanometer domains and hence doped polymers behave like 
nanometals or quantum dots. These nano-optical materials exhibit exceptionally high optical 
nonlinearities. 
5.2.3 FTIR Spectroscopy 
FTIR spectroscopic studies of polynorbornene as a function of dopant concentration (iodine) 
were done using a Nicolet 5PC FT-IR spectrometer.  Pellets of doped and undoped 
polynorbornene were made using potassium bromide. FTIR studies show a decrease in the peak 
intensities at 1718 cm-1 and 967 cm-1corresponding to C=C stretching and =C-H bending 
vibration bands respectively, upon doping (Fig.5.5). The C=C transforms into cation radicals 
upon doping and charge-transfer. The decrease in the peak at 1718 cm-1 is directly related to the 
dopant concentration.  
50 
 
 
(a) 
 
(b) 
Figure 5.5: FTIR spectra of undoped (a) and doped (b) polynorbornene 
5.2.4 Photoluminescence  
Photoluminescence measurements were performed on polynorbornene using a Perkin Elmer LS-
55 spectrometer. When the undoped polymer was excited at 280nm an emission band from 
400nm to 480nm was observed with a peak at 425nm. The spectrum is shown in figure 5.6. The 
0.0
0.2
0.4
0.6
0.8
1.0
1.2
425142524253425
Ab
ro
rb
an
ce
 (a
.u
.)
Wavenumber (cm-1)
C = C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
425142524253425
Ab
so
rb
an
ce
 (a
.u
.)
Wavenumber (cm-1)
C = C 
51 
 
undoped polymer has a weak absorption at 300nm. In doped polynorbornene the emission was 
quenched due to iodine.   
 
Figure 5.6 Photoluminescence in undoped polynorbornene 
5.3.Results and Discussions 
Polynorbornene is a novel polymer in the class of nonconjugated conducting polymer. 
Undoped polynorbornene is non-conducting. When doped with iodine, its electrical conductivity 
increases by several orders of magnitude. The maximum conductivity of this polymer was found 
to be 0.09 S/cm corresponding to iodine molar concentration of 1.5. Optical absorption spectrum 
of lightly doped polymer has two distinct peaks. One at 5.4eV corresponding to cation radicals 
and the other at 3.13 eV corresponding to charge transfer between double bond and dopant. 
When the dopant concentration is increased, the intensity of the peak due to cation radicals 
increases and the peak due to charge transfer broadens and undergoes a red shift.  FTIR 
measurements show a decrease in C=C peak at 1718 cm-1 when the polymer is doped. 
Photoluminescence measurements show an emission peak at 425nm when the undoped polymer 
is excited at 300nm. Electrical and optical properties of polynorbornene are comparable with 
52 
 
other nonconjugate conducting polymers. 
5.4 Conclusions 
Electrical and optical properties of a novel nonconjugated conducting polymer polynorbornene 
are discussed. Upon doping the polymer behaves like a nano-metal. Like other nonconjugated 
conducting polymers, polynorbornene seems to be a promising nonlinear optical  material. 
 
 
 
 
53 
 
CHAPTER 6 
EPR SPECTROSCOPIC STUDIES OF RADICAL CATIONS IN A NONCONJUGATED 
CONDUCTIVE POLYMER, POLY(? -PINENE)
 
6.1 Introduction 
Electronic and optical polymers have attracted significant research attention because of 
the fundamentally interesting characteristics and a wide range of potential applications. 
Nonlinear optical properties of organic and polymeric materials have various applications in 
photonics. The research on nonconjugated conductive polymers was initiated in 1988. [75] The 
field has attracted significant research interest since then [79, 78, 76] . Detailed studies of 
nonconjugated conductive polymers have been made using various spectroscopic methods 
including optical absorption, FTIR, Raman, 13C-NMR, Mossbauer and EPR[112]. Various 
applications of these polymers have been demonstrated. Nonconjugated conductive polymers 
such as doped polyisoprene are important materials having confined electronic or nano-optical 
structures [80]. Exceptionally large quadratic electro-optic effect (third order optical 
susceptibility) has been reported for iodine-doped polyisoprene.[80] Electrical conductivity, 
FTIR spectroscopic and photoluminescence characteristics of a novel nonconjugated conductive 
polymer, poly(?-pinene) have been recently reported [76]. Large quadratic electro-optic effect in 
iodine-doped poly(?-pinene) due to formation of nano-optical domains has been recently 
reported. The molecular structures of poly(?-pinene) before and after doping with iodine are 
shown in Figs.6.1(a) and 1(b) respectively. The doping leads to a charge-transfer from the 
isolated double bond to iodine forming radical cations. In this discussion, the results of electron 
54 
 
paramagnetic resonance (EPR) measurements of poly(?-pinene) for different doping levels to 
confirm formation of radicals are discussed. 
 
   
 
(a) 
 
 
Figure 6.1(b) 
(b) 
Figure 6.1:  Molecular structure  of undoped (a) and doped (b) poly(?-pinene) 
The electrical conductivity of poly(?-pinene) increases by more than ten orders of 
magnitude upon doping with iodine. Undoped poly(?-pinene) has conductivity less than 10-12 
S/cm. Upon doping with iodine the conductivity increases rapidly. When the molar concentration 
of iodine reaches about 0.85 the conductivity saturates [76] at ~ 8x10-3 S/cm.  
 Optical absorption and FTIR spectroscopic results of poly(?-pinene) have been reported. 
The optical absorption spectra for different dopant concentrations are shown in Fig.6.2. Two 
major optical absorption peaks have been observed for lightly doped samples: one at 4 eV 
corresponding to cation radicals and the other at 3.1eV due to the charge transfer between the 
donor and the acceptor. The lower energy peak undergoes a red shift when the dopant 
concentration increases. This is due to the reduction of the average distance separating the 
radical cation and the iodine anion [76, 81]. 
55 
 
 
Figure 6.2: The optical absorption spectra of poly(?-pinene)for different dopant concentrations 
FTIR spectra of the polymer before and after doping are shown in Fig. 6.3. Upon doping, there is 
a decrease in the FTIR peaks at 1610 cm-1   and 728 cm-1 corresponding to C=C stretching 
vibration and =C-H bending vibration respectively. This decrease is due to charge-transfer from 
the isolated double-bond to the dopant and formation of radical cations (Fig.1b). In the present 
report, we discuss results of EPR measurements to show that cation radicals are formed upon 
doping of poly(?-pinene).  
56 
 
 
Figure 6.3: FTIR spectra undoped (a) and doped(b) poly(?-pinene)[76] 
6.2 Experimental 
The EPR measurements were performed on poly(?-pinene) at different doping levels of 
iodine. The poly(?-pinene) samples used in this work were purchased from Aldrich Chemicals 
Corp. Films of poly(?-pinene) were prepared on a glass substrate from a toluene solution. The 
57 
 
films were then doped with iodine at different concentrations. The molar concentrations were 
calculated by measuring the weight uptake of iodine in the film. The powder samples obtained 
from the doped films were used to perform the EPR measurements. Undoped poly(?-pinene) 
samples were also prepared as a powder and weighed to perform the EPR measurement. The 
EPR experiment was conducted at X-band (9 GHz) using a Bruker EMX spectrometer at room 
temperature as well as at a lower temperature (100 oK). Cooling of the sample was performed 
with an Oxford Instruments ESR 900 flow cryostat with an ITC4 temperature controller. 
6.3 Results and Discussion 
The undoped poly(?-pinene) sample showed a very weak EPR signal due to the methyl 
radicals. Significant EPR signals increasing in proportion to the dopant concentration have been 
observed and the results are shown in Fig. 6.4.  
 
Figure 6.4: EPR data of poly(?-pinene) at different dopant concentrations 
Clearly, the EPR signal of the heavily doped sample is large. As these results have shown, g = 
3400 3450 3500 3550 3600
Int
en
sit
y (
a.u
)
[G]
Heavily doped
lightly doped
Undoped
58 
 
2.0042 and the EPR line-width (?Hpp) of the doped samples is about 13 G. These values compare 
well with those of doped cis-polyisoprene reported earlier [112]. The line-width is larger 
compared to that in conjugated conductive polymers[113] because the radical is less mobile 
(confined) in the case of the nonconjugated conductive polymer. The EPR signals of a lightly 
doped and a heavily doped sample are shown in Figs.6.5 and 6.6 respectively.  
 
Figure 6.5: EPR signal of undoped poly(?-pinene) 
 
Figure 6.6: EPR signal of lightly doped poly(?-pinene) 
Hyperfine splitting was observed in the EPR signal particularly in the case of heavily doped 
samples (Fig.6.6). Six peaks due to hyperfine splitting were observed. If there are ?n? nuclear 
spin halves, n+1 distinct peaks due to hyperfine splitting are expected in the EPR signal. Iodine 
has a nuclear spin of 5/2. Therefore, six peaks as a result of hyperfine splitting can be expected 
59 
 
and were observed. At the high dopant concentration, there is a reduction in the distance between 
the cation radical and the iodine anion leading to the hyperfine splitting as observed. 
The area under the curve in the EPR signal (at 100oK) was plotted as a function of molar 
concentration of iodine, and the correlation is shown in Fig.6.7. The area per gram varies linearly 
with the molar concentration of iodine. Since the area is proportional to the spin concentration, 
the results show that the concentration of radicals increases proportionally with the dopant 
concentration. These results clearly show that cation radicals are formed upon doping and 
consequent charge-transfer from the isolated double bonds to the dopants. 
 
Figure 6.7: Area of EPR signal per gram of poly-(?-pinene) as a function of molar concentration 
of iodine 
6.4 Conclusions 
In summary, EPR measurements have been performed on poly(?-pinene) at different 
concentrations of iodine. The signal was found to increase proportionally with the iodine 
concentration due to the formation cation radicals upon doping and charge-transfer. The results 
are consistent with the FTIR and optical absorption data. The g-value and the EPR line-width 
(?Hpp) compares well with that of another nonconjugated conductive polymer, doped 1,4-cis-
60 
 
polyisoprene.[112] Hyperfine splitting in the EPR signal has been observed for heavily doped 
samples because of reduced distance between the cation radical and the iodine anion. 
 
  
61 
 
CHAPTER 7 
OFF- RESONANT ELECTRO-OPTIC MEASUREMENTS IN DOPED                   
POLY(?-PINENE)
7.1 Introduction 
Third-order optical materials have attracted significant research attention due to their 
applications in ultrafast switching, electro-optic modulation, optical phase conjugation, optical 
limiting, etc.[7, 8]Various classes of third-order materials have been investigated. High optical 
nonlinearities have been reported on conjugated polymers such as polydiacetylenes.[14] The 
optical nonlinearities in these polymers are attributed to the delocalized electrons along their 
backbones forming quantum wires. Metallic nanoparticles (quantum dots) have been widely 
investigated for their large third-order nonlinearities.[114] In metallic quantum dots, the high 
optical nonlinearity is due to the surface plasmon resonance and associated characteristics of the 
confined systems. Recently, exceptionally high third-order optical nonlinearities have been 
reported in doped nonconjugated conductive polymers.[80, 81, 78] Nonconjugated conducting 
polymers have been discussed in detail.[76, 110] Optical nonlinearities in nonconjugated 
conducting polymers have been attributed to the charge confinement in subnanometer domains 
(subnanometer metallic quantum dots) formed upon doping. 
The magnitude of the Kerr coefficient of iodine-doped poly(?-pinene) as measured at 
633nm is 50 times that of the standard Kerr-electro-optic material, nitrobenzene [81]. The two-
photon absorption coefficient of this iodine doped polymer measured using an open aperture z-
scan technique at 810 nm was reported [82] to be 2.6 cm/MW. Here, we discuss the electro-optic 
62 
 
measurements performed at 790 - 810 nm and 1.55 ?m.  
7.2 Experimental 
Poly(?-pinene) samples purchased from Sigma Aldrich was used in this work. A measured 
quantity of the polymer was dissolved in toluene at room temperature. A thin (~ 1 ?m thick) 
transparent film of poly(?-pinene) was cast on a glass slide from this solution. The film was then 
doped with iodine by evaporation at room temperature. The duration of doping determined the 
dopant concentration in the films. Heavily doped films looked dark and had high electrical 
conductivity of about 10-2 S/cm. Electro-optic measurements were performed on medium doped 
samples with electrical conductivity of ~ 10-3 S/cm. The doping time was found to be critical to 
produce better quality films. Figure 7.1 shows the optical micrographs of an over-doped and an 
optimally doped film. The over-doped film developed cracks producing poor optical quality.  
 
(a) 
 
(b) 
 
Figure 7.1: (a) Optical micrograph (100x) of  an optimally doped and processed film of poly(?-
pinene) showing excellent optical quality. 
     (b) Optical micrograph (100x) of typical doped film with poor optical quality. 
 
63 
 
Therefore, emphasis was placed on optimizing doping time. The sample was then covered with a 
film of optical glue to preserve it for longer duration. Two copper electrodes were then applied to 
the sample to form a narrow slit of about 100 ?m.  
 Besides iodine-doped poly(?-pinene) a number of other polymers were investigated for 
quadratic electro-optic effect at 1.55 ?m using the same sample configuration and measurement 
techniques. This was to provide sufficient background check-up and eliminate the possibilities of 
measuring slow processes including: electrostriction, molecular movements, capacitive effect 
and other potential mechanical effects resulting from application of electric fields. The polymers 
used include: undoped polyisoprene (natural rubber), undoped polybutadiene, undoped 
polystyrene, dye-doped polystyrene, undoped poly(?-pinene), PTS-polydiacetylene and also 
silicon. 
Field-induced birefringence technique was used to make the electro-optic measurements. 
A schematic of the setup is shown in Fig.7.2 The sample was placed between a polarizer and an 
analyzer which were oriented orthogonal to each other and at an angle of 45? to the polarization 
of the incident laser beam. An ac voltage at 4 kHz was applied between the two copper 
electrodes to create an electric field on the sample. The modulation in optical intensity due to 
field induced birefringence in the sample was measured using a photo-diode and a lock-in 
amplifier latched to twice the frequency of the applied field. A Ti-Sapphire laser in cw mode was 
used to make measurements at 790, 800 and 810nm. Measurements at 1.55 ?m were performed 
using a semiconductor laser (BW-Tek). The applied ac electric field (at 4kHz) was varied from 
0.2 - 2.2 V/?m. 
64 
 
 
Figure 7.2: Schematic of field induced birefringence experiment 
7.3 Results and Discussion 
An exceptionally large quadratic electro-optic effect was recorded in doped poly(?-pinene). The 
electro-optic modulation depth at 1.55 ?m for different applied electric fields are shown in Fig. 
7.3a. A modulation depth of ~ 0.1% was observed in a 1 ?m thick sample for an applied field of 
1V/?m at 1.55 ?m. Therefore, the Kerr-coefficient as determined is 1.6x10-10 m/V2. The 
Modulation depths at 790, 800 and 810 nm were also very large leading to exceptionally large 
Kerr coefficients (Table 7.1). The Kerr coefficient at 810 nm is slightly smaller since this 
wavelength coincides with the two-photon resonance peak [82]. Two-photon absorption 
coefficient is the imaginary part of nonlinear refractive index, while the measurement reported 
here corresponds to the real part of nonlinear refractive index. 
 
 
 
 
65 
 
Wavelength 
(nm) 
Kerr Coefficient 
(m/V2) 
790 1.3?10-10 
800 1.8?10-10 
810 1.5?10-10 
1550 1.6?10-10 
Table 7.1: Kerr coefficient of poly(?-pinene) at different wavelength 
 The background materials including: undoped polyisoprene (natural rubber), undoped 
polybutadiene, undoped polystyrene, dye-doped polystyrene, undoped poly(?-pinene), 
polyethylene and silicon did not show any appreciable quadratic electro-optic effect. As 
expected, only PTS-polydiacetylene showed significant quadratic electro-optic signal since that 
is known to have large and ultrafast optical nonlinearity. Therefore, the observed nonlinearity in 
doped poly(?-pinene) is not due to slow effects. The modulation signal observed for PTS-
polydiacetylene film showed a quadratic dependence on the applied field (Fig.7.2 b). The Kerr-
coefficient as determined for PTS-polydiacetylene is ~ 3x10-12 m/V2 at 1.55 ?m. 
The studies as discussed in this report have been performed under ambient conditions. 
The results were repeatable even seven months after the sample preparation partly because of the 
coating with the optical glue. 
The nonlinear index of refraction, n2 and the third order nonlinear susceptibility, ?3 can be 
approximately estimated from the magnitude of Kerr coefficient utilizing known correlations. 
The estimated ?3 is about 3x10-8 esu which is exceptionally large, larger than that of known 
metallic quantum dots. 
66 
 
The exceptionally large nonlinearity as observed has been attributed to the metallic (a) 
(b) 
Figure 7.3: Modulation depths observed at 1.55 ?m as a function of applied electric 
 field for: (a) iodine-doped poly(?-pinene) film, and (b) PTS-polydiacetylene single-crystal film. 
 
quantum dots of subnanometer dimension that are formed upon doping of a nonconjugated 
conductive polymer such as poly(?-pinene). The quantum dot structures are schematically 
presented in Fig.7.4(encircled regions).  
0.000
0.003
0.006
0.009
0.012
0.015
0.018
0.021
0.6 0.9 1.2 1.5 1.8
Mo
du
lat
ion
 De
pth
 (%
)
Applied Electric Field (V/?m)
0.00
0.10
0.20
0.30
0.40
0.50
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Mo
du
lat
ion
 De
pth
 (%
)
Applied Electric Field (V/?m) 
67 
 
 
Figure 7.4: Organic quantum dots of sub-nanometer dimension formed by doping and charge-
transfer of poly(?-pinene). 
 
The wavelength of maximum absorption in the optical absorption spectrum of doped poly(b-
pinene) is shorter compared to that of 5 nm size metallic quantum dots as would be expected.  
The encircled regions in Fig.7.4 are less than one nanometer in dimension. In addition doped 
poly(?-pinene) is known to have a broad two-photon absorption peak at ~ 810 nm. The optical 
nonlinearity (Kerr coefficient) at 1.55 ?m is enhanced partly because of the position of the two-
photon absorption peak. The improved film quality as achieved by optimized doping and 
processing is another reason for the enhanced nonlinearity. 
 The results at 1.55 ?m are significant considering applications in ultrafast electro-optic, 
all-optical switching and modulation for telecommunication. Such high third order optical 
nonlinearities at technologically significant wavelengths are being reported for the first time. The 
Kerr coefficient at 1.55 ?m as measured is the largest known at this wavelength considering all 
materials including metallic and semiconductor quantum dots and conjugated polymers [115, 7]. 
7.4 Conclusions 
 Quadratic electro-optic effect in iodine-doped poly(?-pinene) at off resonant wavelengths 
have been measured using field-induced birefringence technique. The magnitude of the Kerr 
coefficient at 1.55 ?m is 1.6x10-10 m/V2, which is the largest known for any material. At 790 - 
68 
 
810 nm the magnitudes are also very large. The magnitude is slightly smaller at 810 nm since 
this wavelength coincides with the two-photon resonance peak. These exceptionally large 
nonlinearities are attributed to the sub-nanometer size metallic quantum dots formed upon 
doping and charge-transfer involving poly(?-pinene). The large nonlinearity at 1.55 ?m is 
important for ultrafast device applications in telecommunication.  
  
69 
 
CHAPTER 8 
TIME RESOLVED TWO-PHOTON ABSORPTION MEASUREMENTS IN DOPED 
POLY(?-PINENE)
8.1 Introduction 
Organic and inorganic quantum dots have attracted wide research attention due to their 
potential applications in photonics. Significant advances have been made in the development and 
characterization of inorganic quantum dots. Optical nonlinearities in inorganic quantum dots are 
due to a combination of quantum and dielectric confinements.[20] A variety of inorganic systems 
like CdS, As2S3, ZnS etc. have shown high cubic nonlinearities, however their response is slow 
when compared to organic third order materials. [14] Organic-inorganic hybrid materials have 
also been reported to possess high third order nonlinearities.[116] In the recent years the focus 
has shifted to organic polymers because of their ultrafast response, high damage threshold and 
the freedom they provide in engineering their structures to increase optical nonlinearities.   
Conjugated polymers; a class of conducting polymers with alternating single and double bonds, 
have been widely studied by various groups for their cubic nonlinearities. Optical nonlinearities 
in these materials are due to the presence of delocalized electrons in their sp2 hybridized carbon 
atoms.[7] These materials behave like organic quantum wires.  Nonconjugated conducting 
polymers based quantum dots are the first known metal-like organic quantum dots. These 
polymers do not have alternating single and double bonds. The electrical and nonlinear optical 
properties of various doped nonconjugated conducting polymers like 1-4 cis-polyisoprene[80], 
polyalloocimene[110], poly(ethylenepyrrolediyl) derivative[78], polynorbornene[117], and 
70 
 
poly(?-pinene)[81] have been previously reported. Electro-optic coefficients of iodine doped 
poly(?-pinene) at 1.55?m is higher than that of the best known conjugated polymer 
polydiacetylene[118]. When a nonconjugated polymer is doped with iodine, the double bonds 
(C=C) are transformed into cation radicals which are confined in a subnanometer domain 
resulting in a quantum dot like structure. Optical nonlinearities in these systems are attributed to 
the charge confinement in these systems. 
Recently, Titus and co-workers reported an exceptionally high two-photon absorption 
coefficient (~2.6 cm/MW at 810 nm) in iodine doped poly(?-pinene) using open aperture Z-scan 
technique.[82] The two-photon absorption in this material is higher than or comparable to many 
semiconductor and metallic quantum dots. It is critical to confirm the origin of optical 
nonlinearities in these materials. In this chapter time resolved interferometry measurements of 
iodine doped poly(?-pinene) are discussed. This experiment measures both the magnitude and 
response time of two-photon absorption in doped poly(?-pinene). 
8.2 Experimental 
8.2.1 Sample preparation 
Poly(?-pinene) pellets purchased from Sigma-Aldrich were used in this experiment. 
Known mass of these pellets was dissolved in measured quantity of toluene at room temperature. 
A thin and uniform film of poly(?-pinene) was cast on a micro cover glass slides (average 
thickness 150?m) using this solution. The average thickness of the film varied from 1?m to 
4?m. The thickness of the film was controlled by changing the solute to solvent ratio.  These 
films were then doped in iodine for 9 to 11 hours. The duration of doping was found to be critical 
because over doped samples resulted in surface cracks that adversely affected the optical quality 
of the films. After doping the sample was left in fume hood for about 30 minutes to get rid of the 
71 
 
surface iodine. The resistance of the doped samples used in this experiment was about 35M?. A 
coating of optical glue on the doped sample may be used to preserve it for a longer duration.  
8.2.2 Pump-probe setup 
 The experiment setup for time resolved interferometry technique using the pump probe 
method is shown in figure 8.1. A tunable (710nm ? 940nm) Ti-Sapphire laser (Tsunami model 
3950-L1S) producing 150fs pulses was used in this experiment. 
 
Figure 8.1 Schematic of the experimental set-up for pump probe technique 
The beam from the laser was split into two using a beam splitter. One of the beams was used as 
the probe and the other as the pump. 
8.2.2.1 Probe alignment 
In figure 8.1 the path of the probe beam is traced by the green line. An optical attenuator was 
used to reduce the intensity of the probe beam to a desired level. A prism based retro-reflector as 
shown in the figure was used to change the path length of the beam. The motion of the prism was 
controlled using a precision motion controller. The beam then passed through a polarizer 
oriented parallel to the polarization of the incident laser beam.  A focusing lens (5X) was used to 
focus the beam on the doped poly(?-pinene) sample. The beam from the sample passed through 
72 
 
an analyzer and was then detected using a photodiode. The orientation of the analyzer was also 
parallel to the polarization of the incident laser beam. The photodiode was connected to a lock-in 
amplifier that recorded the probe signal. The amplifier also received a reference signal from an 
optical chopper placed in the path of the probe beam. The lock-in amplifier only recorded signal 
from the photodiode that matched the frequency of the chopper, thereby eliminating spurious 
signals or noise. 
8.2.2.2 Pump alignment 
 The pump beam is traced by a red line in figure 8.1. After the beam splitter the pump 
beam passed through multiple mirrors before being focused on the sample using a 5X lens. The 
pump beam and probe beam were made to overlap on the sample. The pump beam was not 
detected by the photodiode. 
8.2.3 Data collection 
 The retro-reflector was aligned such that the path length of the two beams matched 
exactly which resulted in a spatial and temporal overlap of the beams right at the sample. The 
intensity of the probe beam was reduced to about 5% of the pump beam using the attenuator. The 
signal in the lock-in amplifier was recorded when the pump beam was incident on the sample 
and when the pump beam was blocked from the sample. The path length of the probe was then 
changed using the retro-reflector to produce a delay between the pump and the probe, and the 
signal from the lock-in was recorded. The ratio of change in transmission to normalized 
transmission was plotted as a function of the path difference. 
8.3 Results and discussion 
The two-photon absorption data measured at mode-locked condition (150 fs) at 790nm 
and 795nm are shown in figure 8.2 and 8.3 respectively. 
73 
 
 
Figure 8.2: Two-photon absorption data measured at 790 nm using pump-probe technique 
 
Figure 8.3: Two-photon absorption data measured at 795 nm using pump-probe technique 
When the intense pump beam interacts with the sample it induces a change in the local linear 
absorption. This causes a change in transmission which is detected by the probe beam and 
measured in the lock-in. The plots in figures 8.2 through 8.3 clearly show that this effect is 
ultrafast since the response time is pulse width (150fs) limited. Thus the optical nonlinearity in 
this material is clearly electronic in nature and is not caused due to excited state absorption or 
0.986
0.988
0.990
0.992
0.994
0.996
0.998
1.000
1.002
1.004
-300 -200 -100 0 100 200 300
No
rm
ali
ze
d T
ran
sm
iss
ion
Delay (fs)
gaussian fit Data set for 790 nm
0.984
0.986
0.988
0.990
0.992
0.994
0.996
0.998
1.000
1.002
1.004
-300 -200 -100 0 100 200 300
No
rm
ali
ze
d T
ran
sm
iss
ion
Delay (fs)
Gaussian fit Data set for 795 nm
74 
 
thermal effects. In metallic nanoparticles as a consequence of dielectric confinement, surface 
plasmon resonance enhances the optical nonlineartities at specific wavelengths. Time resolved 
experiments in these systems have been reported by other research groups.[119, 120] Their 
results show that the response is longer (~2ps) than the pulse-width used (30fs) and that the 
change in transmission is positive at 532nm.  In doped poly(?-pinene) we find that the response 
is less than the pulse width used (150fs) and hence much faster than the effects observed in 
nanometals. This makes doped poly(?-pinene) well suited for high speed all-optical switching 
applications.   
The magnitude of two-photon absorption coefficient (?2) can be calculated from the 
change in transmission (?null nullnull ) as below. 
                                                                   ?null nullnull = ?null?null                                                          8.1                         
where ?T/T is ratio of change in transmission to normalized transmission, ?null is the change in 
linear absorption coefficient and l is the thickness of the sample. The two-photon absorption 
coefficient ?2 is related to ?null through the equation below 
                                                           ?null = nullnullnull                                                                           8.2 
Using the above equations the value of two-photon absorption coefficients found at 
790nm and 795nm were found to be 2.28?0.1 cm/MW and 2.50?0.1 cm/MW respectively. The 
magnitude of ?2 of doped poly(?-pinene) shown above is exceptionally high and also consistent 
with the previously published results obtained using Z-scan.[82] Table 8.1 lists the two-photon 
absorption reported in metallic and semiconductor nanosystems.   
 
 
 
75 
 
Material Wavelength (nm) ?2 (cm/MW) 
GaAs nanocrystals [121] 527 
1064 
0.03 
0.05 
Cu nanoclusters in fused silica [122] 532 0.01 
ZnO beaded MWNT[123] 780 0.02 
Iodine doped poly(?-pinene)  (this work) 790 
795 
2.28 
2.50 
Table 8.1: Comparison of nonlinear absorption coefficients in various nanomaterials 
Clearly the two-photon absorption coefficient in doped poly(?-pinene) is significantly 
higher than the these systems. This can be explained by examining the transformation that occurs 
in the polymer upon doping. Doping changes the electronic, structural and optical properties of 
conducting polymers. In case of conjugated polymers there is a significant decrease in optical 
nonlinearities upon doping because conjugated polymers transition to metallic state upon doping. 
In nonconjugated conducting polymers, doping leads to a quantum dot like structure. When 
nonconjugated conducting polymers are doped, the C=C bonds are transformed into cation 
radicals and there is a charge transfer between the double bond and the dopant. Optical 
absorption spectra, EPR and FTIR measurements in doped poly(?-pinene) and other 
nonconjugated systems have confirmed this phenomenon. The structure of doped poly(?-pinene)  
is shown in the figure below 
 
Figure 8.4: Quantum dots (encircled regions) in doped poly(?-pinene) 
From the above figure and from the bond length calculations it is evident that the charge transfer 
76 
 
sites are confined in sub-nanometer domains resulting in an organic quantum dot structure.  The 
work by Nimtz and coworkers on three dimensional size induced metal insulator transition 
(SIMIT) offers an interesting comparison. Nimtz et. al measured the electrical conductivity of 
indium nanocrystals as a function of the particle diameter. [124] They observed that when the 
diameter of the nanoparticle is comparable to the electron wavelength (~1?m) the electrical 
conductivity is inversely proportionally to the third power of the particle diameter. The bulk 
indium material is highly electrically conductive but as the particle size is reduced to 10-8 m, the 
indium quantum dots are far less conducting. This is shown in figure 8.5 (reproduced from 
reference [126]). In nonconjugated conducting polymers the electrical conductivity is at a similar 
level as in indium quantum dots. The charge transfer sites formed upon doping are of 
subnanometer dimension resulting in the organic quantum dot structure. 
 
Figure 8.5: Size dependent electrical conductivity measurements in indium nanocrystals [124] 
 
77 
 
8.4 Conclusions: 
Exceptionally high two-photon absorption coefficient, ?2 was measured in doped poly(?-pinene) 
at 790nm and 795 nm using pump probe technique using a mode locked Ti-Sapphire laser with 
150 fs pulses. The response time of ?2 was ultrafast clearly indicating that the nonlinearities in 
this material are of electronic origin. Such high nonlinearities are attributed to the quantum dot 
like structure formed upon doping. This material has a wide variety of applications in photonics. 
  
 
78 
 
CHAPTER 9 
NONLINEAR OPTICAL WAVEGUIDING IN DOPED POLY(?-PINENE)
9.1 Introduction 
There has been increasing interests in nonlinear optical waveguides due to their ability to 
provide exceptional beam confinement for large propagation lengths.[125] Nonlinear optical 
waveguides in several materials like polymeric hosts, glass substrate, crystal cored fibers and 
Langmuir Blodgett films have been reported.[37] A nonlinear optical waveguide increases the 
interaction length of the optically active material and the lights source. In the previous chapters 
the exceptionally high optical nonlinearities in doped poly(?-pinene) were discussed using 
various techniques like field induced birefringence and time resolved interferometry. These 
experiments were conducted on thin films (1?m -6?m) of doped poly(?-pinene). Increasing their 
interaction lengths and demonstrating higher modulation depths is desired to ensure that these 
materials have applications in various nonlinear optical devices. However, the conventional 
sample preparation discussed previously cannot be used to prepare thick film suitable for 
nonlinear optical applications because of the inability of the dopants to penetrate deep into thick 
films. Since the optical nonlinearities in nonconjugated systems heavily depend on the dopant 
interaction, uniform and optimal concentration of the dopant through the entire thickness of the 
film is essential. Also, thick films often have poor optical quality and large scattering losses. 
Alternatively, a thin and uniform layer of doped poly(?-pinene) on a bare glass fiber can provide 
long interaction between the sample and the laser without compromising on the dopant 
homogeneity or the optical quality. In this chapter a nonlinear optical wave-guide based on 
79 
 
doped poly (?-pinene) is discussed.  
9.2 Experimental 
 Several techniques were tried to increase the interaction length. A poly(?-pinene) pellet 
was one of the first approaches. The undoped polymer was finely ground and spread evenly on a 
glass slide. The powder was then doped with iodine for 9 to 12 hours. The doped polymer 
powder was then placed on a dye and pressed using a hydraulic press at 1500 psi. The resulting 
pellet was about 15 mm thick. However, the pellet yielded poor light throughput and accounted 
for large scattering losses and hence could not be used for the nonlinear optical experiments. 
Another technique was to draw fibers of desried length using the undoped poly(?-pinene). Pellets 
of undoped fiber were heated above the melting point. Thin uniform fibers were drawn out of the 
molten polymer using a sharp needle. The diameter of the fiber could be altered by the 
temperature of the polymer. Thin fibers of about 30?m could be drawn at about 75? C. Although 
these fibers were uniform, had no pores and were of exceptional optical quality (before and after 
doping) they could not be used to nonlinear optical measurements for the following reasons: 
1. The polymer fiber became very brittle after doping and handling doped fibers was 
extremely difficult.  
2. Their thermal damage threshold was extremely poor and could not withstand high intensities 
of laser beam. 
To overcome these shortcomings, a fiber-optic waveguide was prepared. Multi-mode optical 
fiber with a numerical aperture of 0.37 and core diameter 200?m purchased from Thorlabs was 
used to prepare the waveguide. The fiber had a silica core, an acetone soluble TEQS cladding 
and Tefzel outer jacket. A 15 cm long fiber was sliced with flat edges on both sides.  The outer 
jacket and the cladding was burned using a flame torch. The bare fiber was then left in acetone 
80 
 
for about an hour to dissolve any remnants of the cladding and to purge other contaminants. The 
dry fiber was then dipped in a thick solution of poly(?-pinene) and toluene. The fiber was then 
taken out of the solution and dried in room temperature. Thus, a uniform ?cladding? of the 
polymer was cast on a bare multi-mode silica fiber. The fiber was then doped with iodine for 
required amount to time. Pictures of poly(?-pinene) cladded fibers before and after doping is 
shown in figure 9.1. The poly(?-pinene) cladded fibers were of excellent optical quality making 
them ideal candidates for nonlinear optical measurements. The optical micrographs of doped 
and undoped fibers are shown in figure 9.2. 
 
 
Figure 9.1  photographs of (a) undoped and (b) doped fiber-optic waveguide  
(a)
(b)
81 
 
 
Figure 9.2: Optical micrographs of (a) undoped and (b) doped fiber-optic waveguides 
Figure 9.3 shows the schematic of the experimental setup for measuring two-photon absorption 
in doped optical fiber. 
 
 
Figure 9.3: Experimental setup for two-photon absorption in a fiber-optic waveguide. 
 
All measurements in this report were performed using a mode-locked, pulsed Ti-Sapphire 
Tsunami laser with 150 fs pulse at 790nm. The optical power of the laser was adjusted using an 
attenuator, the beam was then focused on the doped fiber using a 40x lens. The fiber was 
mounted on a precision stage whose motion could be controlled along all the three axes.  The 
fiber was carefully aligned to achieve maximum throughput. The beam from the other end of the 
fiber was focused using a 20X lens on to a photodiode. An optical chopper placed in the path of 
the beam chopped the beam at a set frequency. The photodiode and the optical chopper were 
connected to a lock-in amplifier. The lock-in amplifier recorded the signal from the photodiode 
(a (b
82 
 
that matched the frequency of the chopper. The input power was measured using a power meter. 
The input power was varied from 10 mW to 60 mW in cw and mode-locked conditions; the 
corresponding output power recorded by the lock-in amplifier was noted.  
9.3 Results and discussions 
 A light throughput of about 1.3% is observed in a 15cm long doped fiber at an input 
power level of 10mW. The normalized transmission of the optical waveguide plotted as a 
function of input power is shown in figure 9.4.  
 
Figure 9.4: Normalized transmission in the optical waveguide at different input power levels 
The data shows a clear decrease in the transmission at higher power levels. No change in 
transmission was recorded in undoped fibers. The experiment was also repeated in the cw mode 
and little or no change in transmission was observed.  This decrease in transmission in doped 
fibers under mode-locked conditions is due to the two-photon absorption in doped poly(?-
pinene). A maximum decrease of 28 % is observed at 60 mW input power. The peak intensity 
within the guide was about 2.5MW/cm2. The observed change in transmission in the waveguide 
is more than 200 times that observed in doped thin films of poly(?-pinene). More work needs to 
be done to confirm this result. 
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
0 20 40 60 80Nor
mal
ize
d t
ran
sm
iss
ion
Input power (mW)
83 
 
The magnitude of two-photon absorption in this material can be computer from the change in 
transmission.  
                                              nullnullnull = ?null?null                                                                          9.1 
where ?T/T is ratio of change in transmission to normalized transmission, ?null is the change in 
linear absorption coefficient, l is the thickness of the sample. Equation 9.1 gives the change in 
linear absorption. This was discussed in chapter 8 for thin films. The two-photon absorption 
coefficient null2 is given by 
                                                 ?null = null2null                                                                              9.2 
9.4 Conclusions 
In conclusion, a nonlinear optical waveguide was prepared by casting a uniform cladding of 
poly(?-pinene) on a bare multi-mode fiber and doping it. The waveguide was of excellent optical 
quality and could withstand a peak intensity of 2.5 MW/cm2 without damage. Two photon 
absorption measurements were made using this fiber at 795 nm. A maximum decrease in 
transmission of 28% was observed at an input power level of 60mW in mode-locked condition. 
The observed change is higher compared to thin films by a factor of over 200.  The result is 
significant considering its potential applications in high speed (femtosecond timescale), optical 
limiting and others. More work needs to be done to confirm this result. 
  
84 
 
CHAPTER 10 
PHOTOVOLTAIC EFFECT IN NONCONJUGATED CONDUCTING POLYMER
10.1 Introduction 
 Organic polymers have attracted a great deal of interest in the area of photovoltaics. 
These materials show promising results and can serve as lower cost alternatives to inorganic 
photovoltaic materials [85]. The first two-layer organic photovoltaic system appears to have been 
reported in 1958 [94]. Poly(p-phenylene vinylene) (PPV) has been widely studied in the research 
area of organic photovoltaic devices [98, 99, 100]. Reports have been made on photovoltaic 
devices based on polythiopene [96].  
Composites involving conjugated organic polymers and electron acceptors have been 
widely studied for photovoltaics. Buckminsterfullerene, C60 and its derivatives have been widely 
used in the area of photovoltaics since its discovery [103]. Photo-induced electron-transfer 
between conjugated polymer and C60 leading to photovoltaic effects have been investigated in 
detail [104, 105]. 
10.2 Experimental 
In this discussion, the photo-induced electron-transfer and resulting photovoltaic effect in 
a composite involving a nonconjugated conductive polymer and fullerene will be discussed. 
Such effect involving a nonconjugated conducting polymer is being reported for the first time. 
The solutions of composites were prepared by dissolving weighed quantities of poly(?-pinene) 
and C60 in toluene. Composites having different concentrations of C60 ranging from 0% to 8% by 
weight were prepared. The films of these composites were cast on a glass slide under normal 
85 
 
environmental conditions. Fig. 10.1 shows the optical micrographs of these films.  
 
                                             (a)       (b) 
Figure 10.1: Optical micrographs (magnification 50x) of the composite films. The composite 
with 4% by weight (a) of C60 is more homogeneous than that with 8% by weight (b) of C60 
 
It was observed that the composite films having about 4% C60 by weight or less were more 
homogeneous compared to films having a higher weight percent of C60. This is because the 
solution is saturated when the C60 added is more than 4% by weight. As a consequence, the film 
with 8% by weight of C60 is less homogeneous. The optical absorption, photoluminescence 
spectra and photovoltaic effects in these composite films were measured for different 
concentrations of C60. 
10.3 Results and Discussions: 
 Thin films of the composites were cast on glass substrates and the absorption spectra of 
the films were studied. Pristine poly(?-pinene) has an absorption peak at about 270 nm. The 
optical absorption spectra of the composites formed by poly(?-pinene) and C60  at different 
concentrations is shown in Fig.10.2. The composites show two distinct peaks, one at ~ 270nm 
and the other at ~ 335 nm. The inset in Fig.10.2 shows the absorption spectrum of C60. The peak 
at 270 nm observed in the composite is due to poly(?-pinene) and C60, while the peak at 335nm 
is due to C60 alone. The intensity of the absorption peak for the composite film increases with 
increase in the concentration of C60 (by % weight). As shown in the Fig.10.2, the bottom most 
86 
 
spectrum corresponds to the composite with 0% C60 by weight (pristine poly(?-pinene)). The 
spectrum above it corresponds to the composite having 1% C60 by weight. The top-most 
spectrum corresponds to the composite having 8% C60 by weight. As these spectra show, the 
absorption characteristics of poly(?-pinene) does not change in association with C60 since no new 
peak appears in the composite spectra other than those of the components. Therefore, no charge-
transfer between the polymer and C60 occurs at the ground state. 
 
Figure 10.2: Optical absorption spectra of the composites involving poly(?-pinene) and different 
weight percents of C60. The inset shows the absorption spectrum of C60 alone. 
 
The photoluminescence of the composite films has been studied (Fig.10.3) using Perkin Elmer 
LS-55 spectrometer. As shown in the inset of Fig.10.3, pristine poly(?-pinene) has an emission 
peak at 360 nm for an excitation wavelength of 280 nm =[76]==[76]==[76]=[76]. The 
photoluminescence is quenched in the composite involving poly(?-pinene) and C60. This is 
87 
 
because the photo-excited electron in poly(?-pinene) is transferred to C60 leading to no 
luminescence. These results confirm photo-induced charge-transfer in the composite involving 
poly(?-pinene) and C60. 
 
Figure 10.3  Quenching of photoluminescence of the composite involving poly(?-pinene) and 
C60 (2% by weight) for excitation at 280 nm. The inset shows the photoluminescence spectrum 
of poly(?-pinene) by itself for excitation at 280 nm 
 
 Photovoltaic cells used in the report were formed by sandwiching the composite film 
between two electrodes. Aluminum coated glass slide was used as one electrode and indium-tin-
oxide coated glass slide was used as the other electrode. The film of the composite involving 
poly(?-pinene) and C60 was cast on the aluminum electrode and the indium-tin-oxide electrode 
was placed on it. Pressure was applied to keep the composite and the electrodes in contact. The 
film thickness was about 1 ?m. Films thinner than that were not used in these experiments to 
avoid short-circuit between the electrodes. Aluminum electrode was connected to the negative 
terminal of a high impedance electrometer (Keithley 617 Programmable Electrometer) and the 
88 
 
indium-tin-oxide electrode was connected to the positive end of the electrometer to measure the 
photovoltage. The indium-tin-oxide electrode was placed face-up to ensure that the light is 
incident on the polymer through this electrode.  The experimental setup is shown in Fig. 10.4. 
The inset in the Fig.10.4 shows the spectrum of the white light source used in these 
measurements. In addition, a nitrogen laser (325 nm and 425 nm) was used as a light source for 
these measurements. Besides the composite samples, poly(?-pinene) by itself and C60 by itself 
were also used in the sandwiched structure for photovoltaic measurement as control experiments. 
 
 
Figure 10.4: Schematic of the photovoltaic experimental setup 
 
The photovoltage produced for a composite involving poly(?-pinene) and 4% C60 by weight was 
recorded for different intensities of the incident light. White light source with a wavelength range 
of 300-700nm was used. It was observed that the photovoltage produced increased with increase 
in the intensity of the incident light as shown in the Figure 10.5. A light intensity of 6mW/cm2 
89 
 
yielded a photovoltage of 280mV. For poly(?-pinene) by itself and C60 by itself no appreciable 
photovoltage was observed. This shows that the measured photovoltage is due to the interaction 
between poly(?-pinene) and C60.  
 
 
Figure 10.5:  Photovoltage as a function of incident light intensity for a composite containing 4% 
C60 by weight 
 
Photovoltages were recorded for the composites having different concentrations of C60 at 
different intensities of incident light. Table 10.1 shows the photovoltage produced for composites 
having different concentrations of C60 at different intensities of the incident light.  
 
 
 
90 
 
Intensity Of 
Incident light 
(mW/cm2) 
Photo-voltage (mV) 
Composite 
with 
8% C60 
Composite 
with 
6% C60 
Composite 
with 
4% C60 
Composite 
with 
2% C60 
3.5 110 190 162 80 
6 200 290 280 130 
9 210 380 430 170 
12 255 420 550 210 
Table 10.1: Photo-voltages produced for composites  
Measurements were also performed for composites having different concentrations of C60 using 
Nitrogen laser operating at 325 nm with a fixed intensity of 2mW/sq.cm. The results are shown 
in the Table 10.2. In addition, another line from the nitrogen laser with wavelength of 425 nm 
was used for such experiments but no measurable photovoltage was observed. This is because 
pristine poly(?-pinene) does not absorb significantly at 425 nm.   
 
 
 
 
 
 
Table 10.2: Photo-voltages produced for composites with different concentrations of C60 using 
Nitrogen laser as light source (intensity 2 mW/cm2) 
 
It was observed that the composite involving poly(?-pinene) and 4% C60 by weight showed 
better performance when compared to the composites involving poly(?-pinene) with other 
concentrations of C60. This is due to the better homogeneity of the composite involving poly(?-
Sample Photo-voltage (mV) 
Composite with 2% C60 20 
Composite with 4% C60 80 
Composite with 6% C60 39 
Composite with 8% C60 18 
91 
 
pinene) and 4% C60 by weight when compared to other composites. For composite films with 
higher weight percentages of C60, clusters of C60 were formed leading to lower interaction with 
the polymer double bonds. As a result, less photo-induced charge-transfer and smaller photo-
voltages were observed (Table 10.1 and Table 10.2). The photovoltaic device under the present 
configuration can be used in photo-detector application in the ultraviolet regime. Photovoltaic 
cells for power generation will need a more refined configuration involving thinner composite 
films that will allow larger currents for the devices. 
 In the composite involving poly(?-pinene) and C60, the polymer, poly(?-pinene) acts as 
an electron donor and the C60 acts as an electron acceptor. When light is incident and absorbed 
by the polymer, electron hole pairs are produced. From this excited state of the polymer, electron 
is transferred to C60 as shown in the Fig.10.6. The electrons are then transported to the aluminum 
electrode by the C60 and the holes are transported to indium tin oxide electrode. This 
transportation of electrons and holes produces the observed photovoltage. The observation that 
the nitrogen laser at 325nm produces photovoltage while the same laser at 425nm does not 
produce significant photovoltage implies that electron-transfer occurs only from the polymer to 
C60 but not vice-versa. Further studies on efficiency and current ratings of the cells are under 
progress. 
92 
 
 
Figure 10.6: Schematic of photo-induced electron transfer from poly(?-pinene) to C60 
10.4 Conclusions 
 Photo-induced charge-transfer and resulting photovoltaic effect in composite films 
involving a nonconjugated conductive polymer and C60 have been observed for the first time. It 
was observed that the composite 4% C60 by weight produced a photovoltage of 280mV for an 
incident light intensity of 6mW/cm2. The photovoltage produced increases with increase in the 
intensity of the light. It was also observed that the composite with 4% C60 showed better 
performance when compared to the composites having other concentrations of C60. This 
photovoltaic cell has many applications at a lower cost in the areas of solar cells, photo-
detectors, photo-sensors, etc. While pristine poly(?-pinene) has significant photoluminescence 
(peak at 360 nm) for excitation at 280 nm, the photoluminescence is quenched when C60 is added 
to poly(?-pinene) to form the composite. This shows that the observed photovoltaic effect is due 
93 
 
to photo-induced charge-transfer from the isolated double-bond of the polymer to C60. 
 
 
  
94 
 
CHAPTER 11 
CONCLUSIONS
Single crystal films involving a combination of DAST and laser dye IR125 of excellent optical 
quality were prepared using the modified shear method. Polarized optical absorption spectra 
show that the IR 125 molecules are aligned perpendicular to the DAST dipole axis. The optical 
absorption characteristics of DAST-IR125 film are significantly different than that of DAST 
film. The electro-optic coefficient of DAST-IR125 film as measured is about 300 pm/V at 633 
nm. These novel materials have a wide variety of applications in organic electronics and 
photonics. 
Electrical and optical properties of nonconjugated conducting polymer, polynorbornene 
were studied. When doped with iodine, its electrical conductivity increases by several orders of 
magnitude. The maximum conductivity of this polymer was found to be 0.01 S/cm 
corresponding to iodine molar concentration of 1.5. Optical absorption spectrum of lightly doped 
polymer has two distinct peaks. One at 4.21eV corresponding to cation radicals and the other at 
3.13 eV corresponding to charge transfer between double bond and dopant. When the dopant 
concentration is increased, the intensity of the peak due to cation radicals increases and the peak 
due to charge transfer broadens and undergoes a red shift.  FTIR measurements show a decrease 
in C=C peak at 1718 cm-1 when the polymer is doped. Photoluminescence measurements show 
an emission peak at 425nm when the undoped polymer is excited at 300nm. Electrical and 
optical properties of polynorbornene are comparable with other nonconjugate conducting 
polymers. 
95 
 
EPR measurements were performed on poly(?-pinene) at different concentrations of 
iodine. The EPR signal was found to increase proportionally with the iodine concentration due to 
the formation cation radicals upon doping and charge-transfer. The results are consistent with the 
FTIR and optical absorption data. The g-value and the EPR line-width (?Hpp) compare well with 
that of another nonconjugated conductive polymer, doped 1,4-cis-polyisoprene. Hyperfine 
splitting in the EPR signal was observed for heavily doped samples due to reduced distance 
between the cation radical and the iodine anion. 
Quadratic electro-optic effect in iodine-doped poly(?-pinene) at off resonant wavelengths 
was measured using field-induced birefringence technique. The magnitude of the Kerr 
coefficient at 1.55 ?m was found to be 1.6x10-10 m/V2, which is the largest known for any 
material. At 790 - 810 nm the magnitudes are also very large. The magnitude is slightly smaller 
at 810 nm since this wavelength coincides with the two-photon resonance peak. These 
exceptionally large nonlinearities are attributed to the sub-nanometer size metallic quantum dots 
formed upon doping and charge-transfer involving poly(?-pinene). The large nonlinearity at 1.55 
?m is important for ultrafast device applications in telecommunication.  
Exceptionally high two-photon absorption coefficient, ?2 was measured in doped poly(?-
pinene) at 790nm and 795 nm using pump probe technique using a mode locked Ti-Sapphire 
laser with 150 fs pulses. The response time of ?2 was ultrafast clearly indicating that the 
nonlinearities in this material are of electronic origin. Such high nonlinearities are attributed to 
the quantum dot like structure formed upon doping. This material has a wide variety of 
applications in photonics. 
A nonlinear optical waveguide was prepared by casting a uniform cladding of poly(? 
pinene) on a bare multi-mode fiber and doping it. The waveguide was of excellent optical quality 
96 
 
and could withstand a peak intensity of 2.5MW/cm2 without damage. Two photon absorption 
measurements were made using this fiber at 795 nm. A maximum decrease of 28% in 
transmission was observed at an input power level of 60mW in mode-locked condition. The 
observed change is higher than that observed in thin films by a factor of over 200. More work 
needs to be done to confirm this important result. Nonlinear waveguiding has significant 
applications in photonics including high speed switching and optical limiting. 
Photo-induced charge-transfer and resulting photovoltaic effect in composite films 
involving a nonconjugated conductive polymer and C60 were studied. It was observed that the 
composite involving 4% C60 by weight produced a photovoltage of 280mV for an incident light 
intensity of 6mW/sq.cm. The photovoltage produced was found to increase with increase in the 
intensity of the light. This photovoltaic cell has many applications at a lower cost in the areas of 
solar cells, photo-detectors, photo-sensors, etc. While pristine poly(?-pinene) has significant 
photoluminescence (peak at 360 nm) for excitation at 280 nm, the photoluminescence is 
quenched when C60 is added to poly(?-pinene) to form the composite. This shows that the 
observed photovoltaic effect is due to photo-induced charge-transfer from the isolated double-
bond of the polymer to C60. 
In conclusion, detailed structural, electrical and nonlinear optical properties of specific 
important organic molecular and nonconjugated conductive polymeric systems have been 
studied. 
 
97 
 
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