Unraveling the Many-Body Physics of Polariton Condensation Through Nonlinear Spectroscopy

Non-equilibrium Bose-Einstein condensates of exciton-polaritons offer a promising platform for both fundamental and applied research. The transition to spontaneous coherence is widely believed to be driven by complex many-body interactions between polaritons and the dark exciton reservoir. To elucidate the role of these interactions, we employ novel optical spectroscopies to investigate the nonlinear dynamics of polariton condensates. Our aim is to identify the critical factors, including many-body interactions and material properties, that govern the threshold density for condensation.
First, we examine the polariton dynamics in a two-dimentional metal-halide semiconductor to pinpoint the many-body processes hindering polariton condensation. We find enhanced nonlinear exciton-exciton interactions in the microcavity versus the bare semiconductor and ultrafast polariton scattering depopulating the lowest-energy polariton state. We posit that the complex scattering landscape between the exciton reservoir and polaritons limits the formation of polariton condensates in these semiconductors, and we discuss the generality of our conclusions for highly polar materials in which the lattice mediates multi-particle correlations.
Second, we also present a photophysical study of a prototypical molecular system that hosts room-temperature polariton condensate. We measure the time-resolved buildup and decay dynamics of the condensate using excitation correlation photoluminescence (ECPL) spectroscopy. We observe the condensate populating in hundreds of femtoseconds, a timespan longer than the polariton lifetime. Through complementary transient absorption measurements, we attribute the formation dynamics to the radiative pumping mechanism that populates the lower polariton state through the exciton reservoir. The condensate then decays over several picoseconds, but is shorter than the reservoir lifetime. Furthermore, we employ a theoretical model based on the semiclassical Boltzmann equations to explain the ECPL dynamics in terms of phenomenological parameters and explore how these parameters can be used to tune the threshold of the condensate.