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

We experimentally demonstrate unconventional linewidth narrowing and strong exciton-photon coupling in a planar hybrid microcavity (MC) with a low quality factor Q of around 300. As the broad photonic resonance approaches the heavy- and light-hole exciton transitions in GaAs quantum wells, three well-separated exciton-polariton branches emerge, accompanied by a counter-intuitive fourfold narrowing of the total linewidth. This behavior challenges conventional two-oscillator strong-coupling models and is only partially reproduced by transfer-matrix and Green's-function calculations, suggesting the involvement of frequency-dependent self-energies or correlated dissipation. Our findings demonstrate that strong coupling and coherence enhancement can occur even in highly lossy cavities, questioning the conventional requirement for high-Q microcavities and opening new design possibilities for polaritonic and photonic devices.