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 optoelectronics1. Although all-optical components such as Raman lasers2, parametric amplifiers3 and electro-optic modulators4, 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 150 fs pulses are detected via the emitted THz radiation. The efficiency of this third-order optical process is surprisingly large for fundamental wavelengths in the 1,420–1,800 nm range.
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