  We study exciton-polariton nonlinear optical fluids in a high momentum regime
for the first time. Defects in the fluid develop into dark solitons whose
healing length decreases with increasing density. We deduce interaction
constants for continuous wave polaritons an order of magnitude larger than with
picosecond pulses. Time dependent measurements show a 100ps time for the
buildup of the interaction strength suggesting a self-generated excitonic
reservoir as the source of the extra nonlinearity. The experimental results
agree well with a model of coupled photons, excitons and the reservoir.
