Materials have always defined the trajectory of human technology and civilization — from the Stone and Bronze Ages to the Silicon Age — and today’s challenges demand materials engineered at the atomic scale. Ultrathin materials offer unique advantages: large oscillator strengths and tunable electro-optical properties by seamless stacking into hybrid platforms. In this cumulative thesis, we develop a unified microscopic framework to explore light–matter interactions in such hybrids when electric fields approach the nanoscale, where classical and local electrodynamics give way to quantum and nonlocal material responses. We begin by deriving the optical response from the Heisenberg equations of motion, self‐consistently coupled to Maxwell’s equations, for three prototypical materials: 1. 2D plasmonic crystals of metal nanoparticles, whose confined intraband dynamics support localized surface‐plasmon resonances and spectrally sharp, spatially extended lattice modes. 2. Landau‐quantized 2D electron gases in GaAs quantum wells under a few tesla perpendicular magnetic fields where the free‐electron Drude response transforms into discrete Landau‐levels, the quantum analogue of the classical cyclotron motion. 3. Transition‐metal dichalcogenide (TMDC) monolayers, hosting tightly bound excitons with binding energies of several hundred meV, due to reduced screening in the surrounding dielectric. Building on these foundations, we develop a fully self‐consistent Maxwell–Bloch formalism to describe electromagnetic energy transfer between a TMDC monolayer and various neighbors — molecules, graphene, other TMDCs, individual metal nanoparticles and plasmonic crystals. The results are detailed for TMDC–plasmonic crystal hybrids, showing that only momentum‐dark excitons couple strongly to confined near fields. Numerical diagonalization reveals a triplet plexcitonic spectrum, two hybrid modes with Rabi splittings beyond 100 meV in the strong coupling regime flanking a weakly coupled bright exciton peak. We proceed by deriving an analytical, simplified version of the momentum-resolved Maxwell-Bloch equations: a three‐coupled‐oscillator model in which all coupling constants are expressed directly in terms of material parameters and geometry. This reduced model agrees quantitatively with both the full numerical solution and available experimental data. Finally, we turn to the prospects for ultrastrong coupling of Landau‐quantized 2D electron gases in nanocavities. The delocalized Landau‐level orbitals introduce inherently nonlocal responses, which we propose to probe through elastic scattering using XUV light with wavelengths similar to the magnetic length, i.e.~the minimal cyclotron radius and, hence, the electron localization length. Although a full ultrastrong‐coupling analysis is reserved for future work, our treatment lays the microscopic groundwork by analyzing the interplay of Landau-quantized electron delocalization and electric fields on the nanoscale. In summary, across the three distinct materials, this thesis quantifies how light–matter coupling strength and characteristic length scales interrelate and provides a unified microscopic theory of nanoscale light-matter interaction in ultrathin materials.
Greten, Lara (Wed,) studied this question.