Imaging the polariton relaxation bottleneck in strongly coupled organic semiconductor microcavities

Abstract

We use angular-resolved photoluminescence excitation spectroscopy to probe energy relaxation towards the lowest lying states (${k}_{\ensuremath{\parallel}}=0$) on the lower polariton branch (LPB) of two different strong-coupled $J$-aggregate organic semiconductor microcavities. We find that states along the upper polariton branch (UPB) are able to undergo efficient relaxation to populate states around ${k}_{\ensuremath{\parallel}}=0$ on the LPB; a process that occurs through the rapid relaxation of UPB polaritons to the exciton reservoir. In contrast, LPB states having a photon fraction $>$0.5 are not apparently able to undergo further relaxation towards ${k}_{\ensuremath{\parallel}}=0$, constituting an effective relaxation bottleneck.