  We show that when a monopolar current is passed through an n-i-n structure,
superlinear photocurrent response occurs when there is a polariton condensate.
This is in sharp contrast to the previously observed behavior for a standard
semiconductor laser. Theoretical modeling shows that this is due to a
stimulated exciton-exciton scattering process in which one exciton relaxes into
the condensate, while another one dissociates into an electron-hole pair.
