Strong Coupling beyond the High-Q Limit and Linewidth Narrowing in a Multi-Exciton Planar Microcavity

We systematically study the linewidths of multilevel exciton-polariton modes as a function of the detuning in a planar hybrid microcavity (MC) with low quality factor (Q~300) operating in the linear optical response regime. Using optical reflectivity, we observe that, counterintuitively, the linewidths of the polariton modes undergo a pronounced spectral narrowing as detuning is reduced. Using optical reflectivity, we observe that, counterintuitively, the linewidths of the polariton modes undergo a pronounced spectral narrowing as detuning is reduced. By benchmarking the experimental results against three commonly used constant-loss theoretical descriptions, we find that this behavior is only partially reproduced, highlighting limitations of conventional strong-coupling models when applied to non-standard MC architectures. Our results suggest that frequency-dependent self-energy effects or correlated dissipation mechanisms, typically neglected in simplified treatments, may play an important role.