DIRECT AND NON-DEMOLITION OPTICAL MEASUREMENT OF PURE SPIN CURRENTS IN SEMICONDUCTORS

The photon helicity may be mapped to a spin-1/2, whereby we put forward an effective interaction (a scalar) between a light beam and an electron spin current through virtual optical transitions in a direct-gap semiconductor such as GaAs. Such an effective interaction is possible since the pure spin current and the photon spin current, both keeping the time-reversal symmetry but breaking the space-inversion symmetry of the system, are of the same tensor type, namely, the rank-2 pseudo-tensor. The optical effects due to the effective coupling induces the circular birefringence, which is similar to the Faraday rotation in magneto-optics but nevertheless involve no net magnetization. Such optical birefringence effect of a pure spin current originate from the intrinsic spin-orbit coupling in valence bands but involves neither the Rashba effect from structure inversion asymmetry nor the Dresselhaus effect due to bulk inversion asymmetry of the material. This novel optical birefringence effect may be exploited for direct, non-demolition measurement of a pure spin current.