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

Abstract

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 $1s\text{\ensuremath{-}}2p$ 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.