Strong light-matter interactions in optical microcavities can form exciton-polaritons – a superposition state of photons and excitons. Above a threshold excitation density, these bosonic quasiparticles can condense into the lowest energy state as a polariton laser. This does not require population inversion and so offers potential to generate coherent light with very low threshold.
Organic semiconductors are attractive materials for microcavity polaritons due to their high exciton binding energy and oscillator strength. It is well reported in inorganic semiconductor microcavities that the threshold for polariton lasing is lower than that for conventional photon lasing. However, polariton lasing thresholds in organic semiconductors are relatively higher than organic photon lasers.
Here, we present a study of strong coupling and polariton lasing in highly soluble fluorene-carbazolylstyryl materials, which exhibit high absorption coefficients, PLQY and radiative rates. We observe polariton lasing under 100 fs optical pumping, with absorbed pump threshold densities as low as 2.2 µJ/cm2. This is the lowest threshold reported to date for polariton lasing in organic microcavities, and comparable to state-of-the-art organic cavity surface-emitting lasers. A step-like blueshift at polariton lasing threshold indicates the interplay between different exciton depletion channels.
Our study also provides insight to the design principles for an optimal polariton lasing threshold. Through a comparison of the photophysics and polariton laser performance of these materials with those of other reported polariton lasers, we identify key materials and cavity properties required for further development of low threshold organic polariton lasers.
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