2025 Theses Doctoral

Spatiotemporal imaging of exciton-polariton transport and nonlinear optics in van der Waals semiconductors under strong light-matter coupling

Xu, Ding

The development of quantum and nonlinear photonic technologies requires material systems that support both efficient light propagation and strong optical interactions—ideally under ambient conditions. Exciton-polaritons, formed by the strong coupling between excitons and cavity photons, represent a promising platform that combines the coherence and speed of photons with the interaction strength of matter. However, achieving robust, coherent transport and efficient nonlinear conversion at room temperature remains a major challenge due to phonon-induced dephasing and limited nonlinear phase-matching in solid-state systems. Additionally, controlling pseudospin light transport for spintronic applications has been largely constrained to photonic engineering and remains underexplored in van der Waals semiconductors.

This dissertation addresses these challenges by exploring coherent exciton-polariton transport dynamics through strong light-matter interaction in van der Waals semiconductor microcavities, including polariton-phonon scattering, nonlinear optics, and spin-orbit coupling. To investigate polariton transport, we developed momentum-resolved ultrafast imaging microspectroscopy capable of tracking polariton propagation in real space with sub-micrometer and femtosecond resolution. Using halide perovskites BA₂MAPb₂I₇ microcavities, we quantified the influence of exciton–phonon interactions on transport and coherence properties. For nonlinear optics, we applied ultrafast far-field imaging to rhombohedral-stacked MoS₂ waveguides, enabling direct measurement of phase-matching conditions, mode profiles, and optical losses. To study intrinsic spin-orbit coupling in polariton transport, we investigated the optical spin hall effect of waveguided photon currents in highly anisotropic NbOI₂ waveguides.

Together, these results demonstrate that strong light–matter coupling in van der Waals semiconductors enables coherent polariton transport up to a critical excitonic fraction, beyond which phonon scattering induces diffusive behavior. Efficient second-harmonic generation is achieved through birefringent phase matching and modal phase-matching, confirming the potential of van der Waals materials for integrated nonlinear photonic devices. Furthermore, the discovery of pronounced optical spin hall effect in NbOI₂ waveguides indicates the generality of spin-orbit coupled polariton transport in birefringent microcavities, allowing remarkably efficient on-chip beam steering for next-generation photonic computing and sensing architectures. This work lays the foundation for scalable, multifunctional optoelectronic and optical spintronic devices based on van der Waals semiconductors.