All-optical coherently controlled terahertz ac charge currents from excitons in semiconductors

We have investigated the influence of excitonic effects on two-color coherently controlled electrical currents in semiconductors. We produce currents in CdSe and CdTe at temperature of 10 or 80 K using quantum interference between single- and two-photon absorption of fundamental and second-harmonic 150 fs optical pulses tuned over a wide energy range. Current injection is monitored via the emitted terahertz generation. For the highest photon energies wherein the injected electron-hole kinetic energy is large compared to the exciton binding energy, "dc" electrical current injection is observed and expected within the independent-particle approximation in which phase control of the current magnitude is governed by the optical phases only. However, as the photon energy decreases to the band-gap energy, features appear in the terahertz emission pattern that increasingly signal the breakdown of this model, in agreement with recent theoretical calculations that incorporate electron-hole interactions. In particular, when the excitation pulse bandwidth spans the 1s-2p exciton states, the terahertz emission characteristics are consistent with a theoretically predicted ac electrical current injection in which the phase of the current-but not its amplitude-is controlled by the relative phase of the optical pulses.