Experimental study of light emission from strongly coupled organic semiconductor microcavities following nonresonant laser excitation

We present a study of the photon emission from strongly coupled organic microcavities, following both cw and ultrafast nonresonant excitation. A thin film of molecular J aggregates is used as the active semiconductor material in the cavity. Due to the very large oscillator strength of the organic semiconductor, a Rabi splitting of 80 meV is observed in emission between the upper and lower polariton branches. We find that at all exciton-photon detunings, the emission from the cavity comes mainly from the lower polariton branch. We show that the photon emission rate from the cavity is not significantly different to that from J aggregates outside the cavity, indicating that the overall exciton decay dynamics are not significantly modified by strong coupling. We compare our cw emission measurements with the predictions of a two-level polariton model, in which we allow the transfer of polaritons between the upper and lower polariton branches. It is found that this model provides a very satisfactory description of the observed emission. Our model indicates that at resonance, population transfer between the branches occurs over time scales of approximately 30 fs, and is significantly faster than normal radiative decay of polaritons. We argue that such interbranch transitions might be used in advanced optoelectronic devices, to transfer energy via a cavity photon between different coherently coupled excitonic states.