Femtosecond investigation of the hot-phonon effect in GaAs at room temperature

Slowing down of the hot electron-lattice thermalization by generation of nonequilibrium phonons (hot-phonon effect) is investigated in GaAs at room temperature using high-sensitivity femtosecond single- and double-wavelength absorption saturation techniques. Measurements were performed for different amplitudes of the hot-phonon effect by changing either the individual excess energy of the photoexcited carriers or their density in the range 7x10(15)-4x10(17) cm(-3). After an initial regime dominated by nonequilibrium LO-phonon k-space redistribution, the long-time delay (greater than or equal to 3 ps) electron thermalization is found to occur with a characteristic time of similar to 1.9 ps, independent of the total energy initially injected into the carriers and essentially reflecting the LO-phonon energy relaxation. This is in agreement with numerical simulations of the coupled carrier-phonon relaxation dynamics, indicating that energy transfers to holes are responsible for this slight reduction of the thermalization time compared to the LO-phonon lifetime (identified with the dephasing time T-2/2 similar to 2.1 ps).