Time-resolved observation of ballistic acceleration of electrons in GaAs quantum wells

The authors have studied high-field parallel transport of photoinjected carriers in GaAs quantum wells using femtosecond optical spectroscopy. They have directly observed the transient nonequilibrium distribution functions that occur during the field-induced acceleration of the electrons. At a high field (16 kV cm-1), a nonthermal high-energy tail in the distribution function is apparent during the first 150 fs, which is due to electrons ballistically accelerated from the band edge. At later times, the authors observe the relaxation of the applied electric field due both to radiation from the accelerating carriers and from the build-up of space charge as the electron and hole gases separate.