ULTRAFAST OPTICAL-GENERATION OF CARRIERS IN A DC ELECTRIC-FIELD - TRANSIENT LOCALIZATION AND PHOTOCURRENT

Recent experimental observations of terahertz radiation produced by the ultrafast optical excitation of surface depletion layers of GaAs and InP have generated a lot of interest but its physical origins are still not fully understood. The source of the radiation is believed to be either the time-dependent transport current produced by the optically generated carriers in the depletion field, or the displacement current due to creation of polarized electron-hole pairs. We show that, in general, both mechanisms must be included. The ultrafast optical generation in a dc field is shown to result in the creation of carriers in transiently localized states that evolve into delocalized states, causing transport current in the process. By calculating the dipole moments of these localized states, we are able to determine the time-dependent polarization and photocurrent including both transport and displacement contributions. We find that the displacement contribution comes mainly from the virtual carriers while the real carriers are responsible for the transport current. The competition between transport and displacement current leads to a non-trivial dependence of the overall signal on the dc field strength, the excitation duration, and the detuning. In particular, we predict a sign reversal of the signal at sufficiently high detunings in agreement with recent experimental findings.