Engineering phonon polaritons in van der Waals materials

Abstract

Light trapped at the nanoscale, deep below the optical wavelength, exhibits an increase in the associated electric field strength, which results in enhanced light-matter interaction. Such hybrid light-matter modes involve collective oscillations of polarization charges in the matter, hence the term polaritons. In recent years, enhanced light-matter interactions through a plethora of dipole-type polaritonic excitations have been observed in van der Waals (vdW) materials: layered systems in which individual atomic planes are bonded by weak vdW attraction This class of quantum materials include graphene and other two-dimensional crystals provide idea platform to study light-matter interactions at the nanoscale. There are many types of polaritons due to the different excitations of materials, such as plasmon polariton (PPs) in graphene, phonon polaritons (PhPs) in hBN, exciton polaritons (EPs) in transition-metal dichalcogenides. Polaritons in vdW materials have aroused wide interest in nano-optics because of their novel properties which cannot be found on bulk materials, such as relative low loss, tunability, hyperbolicity, heterostructures as well as various potential applications, such as subdiffraction focusing, emission engineering, light steering, and detectors, etc. Polaritons associated with different constituents can interact to produce unique optical effects by design, study the basic properties of polaritons and new methods to engineer polaritons is of great importance in the nano-optics field. In this dissertation, we developed four novel methods for engineering phonon polaritons in van der Waals materials: First, we report configurable phonon polaritons by twisting stacked α-phase molybdenum trioxide (α-MoO3) slabs in a broad range from 0° to 90°; Second, we report altering the polariton reflection phase by varying the geometry of polaritonic microstructures for the case study of hyperbolic surface polaritons (HSPs) in hexagonal boron nitride (hBN). Third, we developed a new materials engineering method—van der Waals (vdW) isotope heterostructuring—to configure material properties by repositioning isotopes in engineered isotopic heterostructures. Finally, in Chapter 5 we proposed a new approach to achieve topological transitions of phonon polaritons in hBN/calcite heterostructure. All those projects developed and proposed in this dissertation will provide more degree of freedom to engineer polaritons in materials and will inspire the field of nano-optics and related research areas.