Low‐Threshold Exciton‐Polariton Condensation via Fast Polariton Relaxation in Organic Microcavities

Abstract In organic microcavities, a macroscopic condensate of exciton‐polaritons can be formed at high‐exciton polariton densities. The threshold for forming this condensate is proportional to the relaxation rate from initially excited excitons to these polaritons and the lifetime of the lowest energy polariton states. Although the influence of the lower polariton (LP) lifetime on the threshold has been studied, the relationship between the polariton relaxation rate and the threshold has not been fully explored. In this study, a room‐temperature polariton condensate is demonstrated at a threshold pump fluence of 9.7 ± 0.1 µJ cm −2 , in a microcavity containing 4,4″‐bis(( E )‐4‐(3,6‐bis(2‐ethylhexyl)‐(9 H ‐carbazol‐9‐yl))styryl)‐1,1″‐biphenyl (BSBCz‐EH). By using a semiclassical model to describe the polariton kinetics, it is revealed that this low threshold results from the rapid relaxation rate from the dark exciton reservoir to the set of the LP states forming the condensate, with an effective rate W ep ≈ 2.0 × 10 −5 cm 3 s −1 . These results show that accelerating polariton relaxation is possible and is an important factor for realizing low‐threshold polariton condensates.