ULTRAFAST RELAXATION OF PHOTOEXCITED ELECTRONS IN UNDOPED GAAS MEASURED BY ABSORPTION SATURATION OF SPIN-ORBIT-SPLIT TRANSITIONS

An ultrafast redistribution time of less than 40 fs for nonequilibrium photoexcited electron densities as low as 9 x 10(16) cm(-3) is report ed. The electron populations thermalize within 100 to 200 fs after the end of the pump pulse and the cooling of the thermalized electrons to the lattice is observed. These results are obtained from femtosecond pump-probe measurements yielding differential absorption signals direc tly proportional to the electron distribution function and independent of the hole distribution. This is achieved by using transitions from the unoccupied split-off valence band to test the electron populations in the conduction band.