Efficient Radiative Pumping of Polaritons in a Strongly Coupled Microcavity by a Fluorescent Molecular Dye

The optical properties of a series of strongly coupled microcavities containing the fluorescent molecular dye BODIPY-Br (bromine-substituted boron-dipyrromethene) dispersed into a transparent dielectric matrix are explored, with each cavity having a different exciton-photon detuning. Using temperature dependent emission, time-resolved spectroscopy, white-light reflectivity, and measurements of fluorescence quantum yield, the population of polaritons is explored along the lower polariton branch. It is found that both the cavity fluorescence quantum efficiency and the distribution of polariton states along the lower polariton branch is a function of exciton-photon detuning. Importantly, it is shown that in the most negatively detuned cavities, the emission quantum efficiency approaches that of a control (noncavity) film. A simple fitting model is developed, which is based upon direct radiative pumping of polariton states along the lower polariton branch and used it to obtain an excellent agreement with measured photoluminescence as a function of temperature and exciton-photon detuning, and qualitative agreement with the measured photoluminescence quantum efficiency. The radiative pumping mechanism indicates that to facilitate the formation of a nonequilibrium polariton condensate in strongly-coupled microcavities containing dispersed molecular dyes, it is important to utilize materials having high fluorescent quantum efficiency and fast radiative rates.