  We demonstrate a new, nonlinear optical effect of electric currents. First, a
steady current is generated by applying a voltage on a doped GaAs crystal. We
demonstrate that this current induces second-harmonic generation of a probe
laser pulse. Second, we optically inject a transient current in an undoped GaAs
crystal by using a pair of ultrafast laser pulses, and demonstrate that it
induces the same second-harmonic generation. In both cases, the induced
second-order nonlinear susceptibility is proportional to the current density.
This effect can be used for nondestructive, noninvasive, and ultrafast imaging
of currents. These advantages are illustrated by the real-time observations of
a coherent plasma oscillation and spatial resolution of current distribution in
a device. This new effect also provides a mechanism for electrical control of
the optical response of materials.
