Nanophotonics of Thin-film Materials: a Many-body Approach

This thesis introduces a comprehensive approach based on a first-principles quantum electrodynamic method, alongside a variety of two-dimensional models, for calculating the optical properties of van der Waals (vdW) heterostructures consisting of stacked two-dimensional (2D) crystals. In particular, excitation spectra were calculated and Scanning Near-field Optical Microscopy (SNOM) measurements were simulated for both longitudinal and transversal electromagnetic modes in free-standing as well as layered thin-films composed of weakly bound sheets of graphene (GR) and transition metal dichalcogenides (TMDs) such as WS2 spaced apart by dielectric hexagonal boron nitride (hBN) layers. The strong coupling between an inherent electromagnetic field and the electronic subsytem was modelled with the help of an electric field propagator which mediates current fluctuations between electrons. The electric field propagator was perturbatively expanded to include both RPA self-energy as well as ladder vertex corrections to correctly describe Coulomb screening and electron-hole binding, respectively. The Dyson equation was solved to obtain dynamic response properties of the composite system at various levels of approximation. The introduced theoretical models were thus applied to large heterostructures with both semiconducting/insulating as well as metallic layers, depending on the type of quasiparticle modes involved. We investigated plasmon-polariton modes in heterostructures of repeating GR/hBN layers on an Al2O3 substrate while truncating terms beyond RPA due to diminishing returns for zero bandgap systems (in the long-wavelength limit). In this system, Dirac and layer-dependent multiples of acoustic 2D plasmon-polariton (2D-PP) branches were identified. Their respective dispersions were again found to be highly dependent on the number of layers in the heterostructure. To bridge the momentum gap between the evanescent 2D-PP modes and infrared photons, the topmost GR layer was patterned into graphene nanoribbons (GNR). This introduced symmetry breaking and lead to the formation of Bloch plasmon-polariton modes. The emerging Bloch band structure is in many ways similar to photonic crystals and the emerging modes exhibit both a radiative and evanescent character, allowing for a wide range of possible applications. The effect on the scattered electric field was examined when a spherical silver nanoparticle (Ag-NP) was positioned above the GR/hBN heterostructure, with the system being driven at a fixed frequency. Ag-NP can be used to model a SNOM tip driving the conversion of an incident electromagnetic field into 2D-PPs. Moreover, the conversion efficiency may be optimized using specific tailored combinations of driving frequency and Ag-NP dimensions as well as the number of layers and GR doping. Additionally, exciton-polaritons in semiconducting heterostructures composed of WS2 layers, spaced apart by hBN subunits, were investigated, culminating in the discovery of an exotic transversal plasmon mode that behaves akin to a trapped photon. In this case, vertex corrections were included to account for electron-hole interactions. In particular, the strength of exciton-photon binding as well as electron-hole-photon binding was quantified as a function of the number of TMD layers and a linear increase in the coupling strength was observed, eventually leading to a slowdown of the trapped photon to half the speed of light in the semi-bulk limit. This thesis presents significant advancements in the study of photonic properties of thin-film materials, marked by three key contributions: 1) elucidation of the mechanism of manipulation of dispersion, hybridization, and intensity of Dirac, acoustic, and Bloch plasmon-polaritons in pristine and patterned GR/hBN heterostructures; 2) analysis of the mechanism and efficiency of excitation of various multipole plasmons in GR/hBN structures of different thicknesses, by monochromatic radiation through illuminating a silver nanosphere; 3) the discovery of exotic transversal plasmons in semiconducting heterostructures.