Optical effects of spin currents in semiconductors

TL;DR

This paper explores how spin currents in semiconductors cause unique optical effects, such as birefringence and nonlinear processes, enabling optical detection of pure spin currents without magnetic fields.

Contribution

It provides a systematic analysis of optical effects induced by spin currents in semiconductors, highlighting symmetry-based effects and their potential for spin current detection.

Findings

Spin currents induce optical birefringence effects.

Circular birefringence occurs without net magnetization.

Spin currents enable second-order nonlinear optical processes.

Abstract

A spin current has novel linear and second-order nonlinear optical effects due to its symmetry properties. With the symmetry analysis and the eight-band microscopic calculation we have systematically investigated the interaction between a spin current and a polarized light beam (or the "photon spin current") in direct-gap semiconductors. This interaction is rooted in the intrinsic spin-orbit coupling in valence bands and does not rely on the Rashba or Dresselhaus effect. The light-spin current interaction results in an optical birefringence effect of the spin current. The symmetry analysis indicates that in a semiconductor with inversion symmetry, the linear birefringence effect vanishes and only the circular birefringence effect exists. The circular birefringence effect is similar to the Faraday rotation in magneto-optics but involves no net magnetization nor breaking the time-reversal symmetry. Moreover, a spin current can induce the second-order nonlinear optical processes due to the inversion-symmetry breaking. These findings form a basis of measuring a pure spin current where and when it flows with the standard optical spectroscopy, which may provide a toolbox to explore a wealth of physics connecting the spintronics and photonics.