Ultrafast electron-phonon scattering in semiconductors studied by nondegenerate four-wave mixing

Nondegenerate four-wave-mixing allows one to monitor the femtosecond relaxation of free carriers in GaAs and CdTe at densities as low as $2\times {10}^{15}$ ${\mathrm{cm}}^{-3}.$ The energy relaxation time of electrons via LO-phonon emission is determined to be $240\pm 20$ fs in GaAs. In more polar CdTe, this time constant is found to be as short as $70\pm 15$ fs which is even shorter than the phonon oscillation period of 200 fs.