  Two-photon lasers require materials with large two-photon gain (2PG)
coefficients and low linear and nonlinear losses. Our previous demonstration of
large enhancement of two-photon absorption in semiconductors for very different
photon energies translates directly into enhancement of 2PG. We experimentally
demonstrate nondegenerate 2PG in optically excited bulk GaAs via femtosecond
pump-probe measurements. 2PG is isolated from other pump induced effects
through the difference between measurements performed with parallel and
perpendicular polarizations of pump and probe. An enhancement in the 2PG
coefficient of nearly two orders-of-magnitude is reported. The results point a
possible way toward two-photon semiconductor lasers.
