Ultrafast Imaging of Polariton Propagation and Nonlinear Optics in Semiconductor Microcavities

When excitonic systems such as van der Waals semiconductors are embedded in photonic cavities, excitons and photons can hybridize to form part-light part-matter eigenstates known as exciton polaritons. Polaritons are in the vanguard of current interest as observations indicate their propensity for dramatically enhancing energy harvesting, nonlinear optical interactions, and even chemical selectivity by combining strong matter-like interactions with light-like long-range delocalization. We have developed unique ultrafast optical imaging approaches to track polariton transport with femtosecond and few-nanometer spatiotemporal resolution and with momentum selectivity (Fig. 1a). First, I will describe how we have leveraged this approach to provide detailed microscopic insight on polariton transport in the presence of finite disorder (Fig. 1b). Specifically, the degree of polariton coherence depends sensitively on the strength of underlying exciton-phonon interactions and excitonic inter-site coupling. These results reveal that polaritons do not necessarily propagate at the group velocity inferred by their dispersion, due to finite exciton-mediated interactions with the material lattice. Polaritons thus act as sensitive reporters of underlying disorder, and tuning the latter allows controlling polariton properties towards specific applications.