Mapping the Strong-to-Weak Coupling Crossover in Polymer-Film Microcavity Lasers.

Organic semiconductors are particularly attractive for polaritonics due to their large exciton binding energies and oscillator strengths. Among them, the ladder-type conjugated polymer poly(paraphenylene) is distinguished by its rigid backbone, narrow exciton linewidth, high photoluminescence quantum yield, and enhanced photostability, making it an excellent candidate for organic polariton devices. Although polariton lasing has been reported in various organic systems, systematic studies of the transition from polariton lasing to conventional photon lasing within a single, well-controlled material platform remain limited. Here, we present planar organic microcavities incorporating MeLPPP as the active medium, in which continuous tuning of the effective cavity length within a single device enables us to map the strong-to-weak coupling transition across five distinct cavity-mode orders. We demonstrate an approximately eighteen-fold increase in the lasing threshold when crossing from polariton to photon lasing. We further establish a quantitative framework in which the spectral dependence of the threshold governs a universal V-shaped blueshift of the emission energy across both coupling regimes. Finally, we show that vibron-mediated exciton relaxation, previously identified in the strong-coupling limit, persists across the crossover: lasing-threshold minima track the vibron resonances throughout the coupling transition.