# Optical responses induced by spin gauge field at the second order

## TL;DR

This paper theoretically investigates how spin gauge fields in ferromagnetic materials influence optical responses, revealing anisotropic effects that could aid in experimental detection of magnetization structures.

## Contribution

It introduces a second-order linear response theory to analyze non-linear optical effects induced by spin gauge fields in ferromagnets.

## Key findings

- Spin gauge fields cause anisotropic optical responses.
- Second-order effects are significant for uniform spin gauge fields.
- Potential application in detecting magnetization structures experimentally.

## Abstract

Optical responses of ferromagnetic materials with spin gauge field that drives intrinsic spin curren is theoretically studied. The conductivity tensor is calculated based on a linear response theory to the applied electric field taking account of the non-linear effects of the spin gauge field to the second order. We consider the case where the spin gauge field is uniform, realized for spiral magnetization structure or uniform spin-orbit interaction. The spin gauge field, or an intrinsic spin current, turns out to give rise to anisotropic optical responses, which is expected to be useful to experimental detection of magnetization structures.