Evidence for a Polariton-Mediated Biexciton Transition in Single-Walled Carbon Nanotubes

Strong coupling of excitonic resonances with acavity gives rise to exciton-polaritons which possess a modifiedenergy landscape compared to the uncoupled emitter. However,due to the femtosecond lifetime of the so-called bright polaritonstates and transient changes of the cavity reflectivity underexcitation, it is challenging to directly measure the polaritonexcited state dynamics. Here, near-infrared pump-probe spectros-copy is used to investigate the ultrafast dynamics of exciton-polaritons based on strongly coupled (6,5) single-walled carbonnanotubes in metal-clad microcavities. We present a protocol forfitting the reflectivity-associated response of the cavity usinggenetic algorithm-assisted transfer-matrix simulations. With thisapproach, we are able to identify an absorptive exciton-polaritonfeature in the transient transmission data. This feature appears instantaneously under resonant excitation of the upper polariton butis delayed for off-resonant excitation. The observed transition energy and detuning dependence point toward a direct upperpolariton-to-biexciton transition. Our results provide direct evidence for exciton-polariton intrinsic transitions beyond the brightpolariton lifetime in strongly coupled microcavities.