Direct optical detection of pure spin current in semiconductors

We suggest a new practical scheme for the direct detection of pure spin current by using the two-color Faraday rotation of optical quantum interference process (QUIP) in a semiconductor system. We demonstrate theoretically that the Faraday rotation of QUIP depends sensitively on the spin orientation and wave vector of the carriers, and can be tuned by the relative phase and the polarization direction of the $omega$ and $2omega$ laser beams. By adjusting these parameters, the magnitude and direction of the spin current can be detected.