Direct Measurement of Hot-Electron Relaxation by Picosecond Spectroscopy

The time dependence of the optical absorption and the gain of a hot electron-hole plasma in GaAs is measured by means of tunable picosecond light pulses. The electrons and holes (\ensuremath{\sim}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) are photoexcited via two-photon absorption. Our measurements provide a detailed picture of the temperature evolution, showing that in 250 ps the plasma cools from over 100 K down to \ensuremath{\sim} 40 K and does not reach the lattice temperature. The energy relaxation process is shown to be polar LO-phonon emission of hot electrons and holes.