  Strong coupling of excitonic resonances with a cavity gives rise to
exciton-polaritons which possess a modified energy landscape compared to the
uncoupled emitter. However, due to the femtosecond lifetime of the so-called
bright polariton states and transient changes of the cavity reflectivity under
excitation, it is challenging to directly measure the polariton excited state
dynamics. Here, near-infrared pump-probe spectroscopy is used to investigate
the ultrafast dynamics of exciton-polaritons based on strongly-coupled (6,5)
single-walled carbon nanotubes in metal-clad microcavities. We present a
protocol for fitting the reflectivity-associated response of the cavity using
genetic algorithm-assisted transfer matrix simulations. With this approach are
able to identify an absorptive exciton-polariton feature in the transient
transmission data. This feature appears instantaneously under resonant
excitation of the upper polariton but is delayed for off-resonant excitation.
The observed transition energy and detuning dependence point toward a direct
upper polariton to biexciton transition. Our results provide direct evidence
for exciton-polariton intrinsic transitions beyond the bright polariton
lifetime in strongly-coupled microcavities.
