The role of stimulated scattering in the picosecond formation dynamics of a Bose-Einstein condensate of molecular polaritons

Molecular polaritons have emerged as systems of intriguing research due to their ability to form non-equilibrium Bose-Einstein condensates at relatively low energy thresholds and at ambient temperatures. While many-body interactions are known as the key to driving such transitions, the nature and extent of these interactions, and as importantly, their timescales, are poorly understood. This is primarily due to the lack of unambiguous measurement of the buildup and decay dynamics of the condensate in these systems. Here, we investigate the time-resolved dynamics of a condensate of molecular polaritons in a prototypical chromophore system. We use Excitation Correlation Photoluminescence spectroscopy (ECPL): a nonlinear, lock-in based method to estimate ultrafast population evolution through measurement of time-integrated emission. Intriguingly, we observe the condensate populating in hundreds of femtoseconds, a timespan longer than the polariton lifetime. Through complementary transient absorption measurements, we associate the early dynamics with the radiative pumping mechanism that populates the lower polariton state through the exciton reservoir. The condensate then decays over several picoseconds, but 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.