All-optical coherent control of electrical currents in centrosymmetric semiconductors

The absorption of phase-related near-infrared fundamental (omega,0.7 eV <= h omega <= 0.9 eV) and second harmonic (2 omega) pulses of 150 fs duration results in ballistic electrical currents in clean bulk germanium and silicon at room temperature. The ultrafast charge motion is directly monitored via a time-resolved analysis of the emitted bursts of terahertz radiation. The current generation process relies on a third-order optical nonlinearity with a current injection efficiency only slightly reduced compared to the established current injection in direct-gap semiconductors such as GaAs. In the present case, current injection takes place across the direct band gap of germanium, whereas it involves indirect optical transitions in silicon. The vector direction of the current is defined by the polarization of the two-color pump field and the relative phase Delta Phi=2 Phi(omega)-Phi(2 omega). Microscopically, current injection can be understood as arising from the quantum interference of one- and two-photon absorption processes. In the case of silicon, these indirect optical transitions may involve different types of phonons and can occur via numerous pathways. We therefore propose a model based on third-order perturbation theory which qualitatively explains why a current injection can occur across an indirect band gap.