All-optical injection of ballistic electrical currents in unbiased silicon

Silicon has been the dominant material in electronics since the invention of the integrated transistor. In contrast, silicon's indirect bandgap and vanishing second-order optical nonlinearity limit its applications in optoelectronics(1). Although all-optical components such as Raman lasers(2), parametric amplifiers(3) and electro-optic modulators(4,5) have recently been reported, control over charge motion in silicon has only ever been achieved electronically. Here, we report all-optical generation of ultrafast ballistic electrical currents in clean, unbiased, bulk silicon at room temperature. This current injection, which provides new insights into optical processes in silicon, results from quantum interference between one- and two-photon absorption pathways across the indirect bandgap despite phonon participation and the multi-valley conduction band. The transient currents induced by 150fs pulses are detected via the emitted THz radiation. The effciency of this third-order optical process is surprisingly large for fundamental wavelengths in the 1,420-1,800 nm range.