Subpicosecond time-resolved Raman spectroscopy of LO phonons in GaAs

Time-dependent spontaneous Raman spectra of optically pumped GaAs have been measured with subpicosecond time resolution. When the optically injected carrier density is less than 1017 cm-3, we temporally resolve the growth of the optically induced nonequilibrium LO-phonon population and deduce an average electron-phonon scattering time of about 165 fs. For higher injected carrier densities, substantial time-dependent changes are observed in the spectra which result from screening of the LO phonons by the free carriers and the relaxation of the free carriers to the band edges. © 1985 The American Physical Society.