Second-order nonlinear optical effects of spin currents

TL;DR

This paper explores the second-order nonlinear optical effects caused by spin currents in semiconductors, revealing their symmetry properties and potential for non-destructive detection in spintronics.

Contribution

It provides a symmetry-based analysis and microscopic calculations demonstrating measurable second-order optical susceptibilities induced by spin currents.

Findings

Spin currents induce second-order nonlinear optical effects.

Chiral sum-frequency effects occur with longitudinal spin currents.

Susceptibilities are measurable under realistic experimental conditions.

Abstract

A pure spin current formed by opposite spins moving in opposite directions is a rank-2 axial tensor which breaks the inversion symmetry. Thus a spin current has a second-order optical susceptibility, with unique polarization-dependence determined by the symmetry properties of the current. In particular, a longitudinal spin current, in which the spin polarization directions are parallel or anti-parallel to the moving directions, being a chiral quantity, leads to a chiral sum-frequency effect. Microscopic calculations based on the eight-band model of a III-V compound semiconductor confirm the symmetry analysis and show that the susceptibility is quite measurable under realistic conditions. The second-order nonlinear optical effects may be used for in-situ and non-destructive detection of spin currents, as a standard spectroscopy tool in research of spintronics.