Picosecond spectroscopy of hot anti-Stokes luminescence in GaAs

We use 0.5-psec laser pulses to excite broad-band optical emission up to 0.9 eV above the band gap of GaAs. This emission is characterized by a pump-probe luminescence correlation time of as small as 1.5 psec (full width at half maximum) and allows us to directly study the energy relaxation and thermalization of hot carriers with a temporal resolution of about 1 psec. The variation with carrier density of both the spectral shape and the emission intensity at a given energy show that at densities less than 3×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$, the carriers can undergo appreciable energy loss via LO phonon emission before attaining a thermal distribution by carrier-carrier scattering.