Two-photon four-wave mixing in III-V semiconductors: Evidence for coherent phonons

We present midgap femtosecond four-wave mixing measurements on single-crystal GaAs and InP that show a surprising nondegenerate emission, shifted to higher energy relative to the laser, by an amount comparable to the energy of an optical phonon. The polarization dependence of the signal rules out ordinary Raman scattering. We discuss the possibility of impulsive excitation of symmetric lattice vibrations, around localized midgap states that may play a significant role in the two-photon transition amplitude.