Engineering Guided Polaritons and Nonlinear Light-Matter Interactions for On-Chip Photonic and Optoelectronic Devices

Abstract

Controlling light at the nanoscale underpins the advancement of next-generation photonic and optoelectronic technologies, particularly for on-chip integration, optical communication, and quantum information processing. However, conventional photonic systems are fundamentally limited by diffraction, weak light-matter interactions, and inefficient nonlinear effects at subwavelength scales. Overcoming these challenges requires platforms that enable strong coupling between electromagnetic fields and material excitations. In this context, polaritonic excitations and nonlinear optical processes provide complementary approaches for confining, guiding, and generating light within compact systems. Polaritons, arising from the strong coupling between photons and material excitations such as phonons and excitons, enable electromagnetic fields to be confined and guided well below the diffraction limit, supporting directional energy transport. Nonlinear optical processes enable functions such as frequency conversion and quantum light generation. However, challenges remain in achieving deterministic control over excitation pathways, maintaining long-range and low-loss propagation, and translating these effects into practical photonic architectures. Low-dimensional and anisotropic materials provide a promising route to overcome these limitations. Anisotropic polar dielectric crystals can support hyperbolic dispersion, enabling directional polariton propagation. Van der Waals materials offer strong optical confinement and tunable light-matter interactions, with TMDCs standing out due to their strong excitonic resonances and symmetry-dependent optical behaviour. The first part of the thesis focuses on the linear regime and the control of polariton propagation. In anisotropic materials, hyperbolic polaritons enable highly directional transport while remaining confined below the diffraction limit. A key example is the ghost hyperbolic phonon polariton (g-HP), which propagates with oblique wavefronts and exhibits long-range, ray-like behaviour. To address the challenge of selective excitation, a mode-selective approach based on structural asymmetry is developed, where micro- and nano-antennas act as momentum-selective sources. This enables directional propagation and extended propagation lengths. This approach is extended to atomically thin materials, where exciton polaritons emerge in TMDC monolayers. Despite strong excitonic responses, guiding light in such ultrathin systems is challenging due to weak confinement and environmental sensitivity. These limitations are addressed using a suspended monolayer design, enabling direct observation of guided exciton polaritons. The thesis further introduces an interaction-driven approach to topological polaritons. In contrast to conventional systems with pre-designed topology, the platforms studied here are initially topologically trivial. Non-trivial topological phases emerge from strong light-matter coupling, resulting in band hybridization and inversion, and giving rise to robust, backscattering-immune modes. These phases can be dynamically tuned through external parameters such as strain or electrostatic gating. The second part of the thesis focuses on nonlinear light-matter interactions. A valley-selective nonlinear meta-waveguide platform based on non-centrosymmetric TMDCs is developed, enabling efficient SHG with polarization selectivity and directional coupling into guided modes. Building on this, a metasurface-integrated waveguide platform is developed for quantum photonics, enabling efficient nonlinear interactions within a compact footprint. A counterpropagating excitation scheme enables phase matching, supporting both SHG and SPDC. Overall, this thesis establishes strategies for controlling light-matter interactions across both linear and nonlinear regimes, connecting fundamental studies with the development of practical photonic and optoelectronic systems.