  We propose a time-resolved optical measurement scheme for sampling transient
charge and spin currents in a bulk centrosymmetric semiconductor. The technique
relies on emission of second harmonic light triggered by a pulsed below-gap
optical excitation and a spontaneous intraband polarization arising from spin
or charge motion, mediated by a $\chi^(3)$-based nonlinear optical process. Our
proposal uses homodyne amplification to boost the weak second harmonic signal,
making it detectable with conventional electronics, calculated for charge
current in a room temperature GaAs semiconductor. This all-optical technique
requires neither electrical contact nor bias fields and the signal is estimated
at a few percent relative to the shot noise of the probe. This proposal
motivates a novel method for exploring thermal and quantum fluctuations in the
solid state in a non-invasive manner.
