  We show that recently observed DC currents produced by below-the-bandgap
femtosecond pulses [1] can be explained as nonlinear optical effects based on
multi-photon quantum interference and 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 observed DC currents and charges and the odd-order nonlinear optical
susceptibilities of the material. 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 [1] without resorting to extensive
numerical modeling. A clear physical picture of the origin of the photo-induced
currents and charges shows that the versatility of ultrafast (virtual)
nonlinear optical phenomena extends even further than had been previously
thought.
