Coherence and Interaction in Confined Room-Temperature Polariton Condensates with Frenkel Excitons

Strong light-matter coupling of a photon mode to tightly bound Frenkel excitons in organic materials has emerged as a versatile, room-temperature platform to study nonlinear many-particle physics and bosonic condensation. However, various aspects of the optical response of Frenkel excitons in this regime remained largely unexplored. Here, a hemispheric optical cavity filled with the fluorescent protein mCherry is utilized to address two important questions. First, combining the high quality factor of the microcavity with a well-defined mode structure allows to address whether temporal coherence in such systems can be competitive with their low-temperature counterparts. To this end, a coherence time greater than 150 ps is evidenced via interferometry, which exceeds the polariton lifetime by 2 orders of magnitude. Second, the narrow line width of the device allows to reliably trace the emission energy of the condensate with increasing particle density and thus to establish a fundamental picture that quantitatively explains the core nonlinear processes. It is found that the blue-shift of the Frenkel exciton-polaritons is largely dominated by the reduction of the Rabi splitting due to phase space filling effects, which is influenced by the redistribution of polaritons in the system. The highly coherent emission at ambient conditions establishes organic materials as a promising active medium in room-temperature polariton lasers, and the detailed insights on the nonlinearity are of great benefit toward implementing nonlinear polaritonic devices, optical switches, and lattices based on exciton-polaritons at room temperature.