Optically induced currents in dielectrics and semiconductors as a nonlinear optical effect

We demonstrate theoretically that when well-below-the-bandgap femtosecond optical pulses propagate through a dielectric or semiconductor, DC current and charges are produced even though no real carriers are excited in the bands. The photoinduced current is a new ultrafast nonlinear optical effect based on multiphoton quantum interference and the creation of an asymmetric distribution of virtual carriers in the conduction and valence bands. We establish an unambiguous connection between the nonlinear optical conductivity responsible for the photoinduced DC currents and charges and the odd-order nonlinear optical susceptibilities of the material. We then apply our results to the recent experiments [Nature 493, 70 (2013)] in which photoinduced charges were observed in SiO2 irradiated by below-the-gap ultrashort optical pulses. Using a single well-known measured value of the third-order susceptibility (nonlinear index) of SiO2, we obtain excellent agreement with all the experimental data of [Nature 493, 70 (2013)]. A clear physical picture of the origin of the photoinduced currents and charges shows that the versatility of ultrafast (virtual) nonlinear optical phenomena extends even further than had been previously thought. (C) 2015 Optical Society of America