  We take advantage of the polariton bistability in semiconductor microcavities
to estimate the interaction strength between lower exciton-polariton and dark
exciton states. We combine the quasiresonant excitation of polaritons and the
nominally forbidden two-photon excitation (TPE) of dark excitons in a GaAs
microcavity. To this end, we use an ultranarrow linewidth cw laser for the TPE
process that allows us to determine the energy of dark excitons with high
spectral resolution. Our results evidence a sharp drop in the polariton
transmission intensity and width of the hysteresis cycle when the TPE process
is resonant with the dark exciton energy, highly compromising the bistability
of the polariton condensate. This behavior demonstrates the existence of a
small symmetry breaking such as that produced by an effective in-plane magnetic
field, allowing us to directly excite the dark reservoir. We numerically
reproduce the collapse of the hysteresis cycle with the increasing dark exciton
population, treating the evolution of a polariton condensate in a one-mode
approximation, coupled to the exciton reservoir via polariton-exciton
scattering processes.
