Abstract

We have used femtosecond optical spectroscopy to probe semiconductor carrier transport on a time scale comparable to the carrier scattering time, where ballistic effects on the transport can be observed.¹ Carriers are photoexcited with a short (100-fs) pump pulse into a quantum-well structure in which there is a strong (up to 16 kV/cm) electric field in the plane of the wells. The pump is tuned to resonantly excite heavy-hole excitons, so the initial carrier kinetic energy is zero. A 100-fs white-light continuum pulse then probes the non-equilibrium distribution functions of the electrons as they accelerate under the electric field. Differential transmission spectra (modulating the electric field) are shown in Fig. 1. During the first 150 fs after the pump pulse, a high-energy tail appears on the spectra, which is due to the acceleration of electrons to high energies by the field; the time scale manifests the acceleration as ballistic. The average energy per electron reaches a maximum in about 150 fs, after which it rapidly relaxes, reading equilibrium slightly above the zero-field average energy of 25 meV. We understand this as being the result of the rapid collapse of the electric field in the gap region, coupled to the strong energy relaxation of the electrons (through LO-phonon emission).

© 1992 Optical Society of America