On orbital angular momentum conservation in Brillouin light scattering within a ferromagnetic sphere

TL;DR

This paper investigates how orbital angular momentum is conserved during Brillouin light scattering in a ferromagnetic sphere, revealing a selection rule that explains nonreciprocal scattering phenomena.

Contribution

It introduces a selection rule governing orbital angular momentum exchange between magnetic and optical vortices in spherical scattering.

Findings

Identification of a selection rule for orbital angular momentum exchange.

Explanation of nonreciprocal Brillouin light scattering.

Observation of topologically non-trivial spin textures in ferromagnetic modes.

Abstract

Magnetostatic modes supported by a ferromagnetic sphere have been known as the Walker modes, each of which possesses an orbital angular momentum as well as a spin angular momentum along a static magnetic field. The Walker modes with non-zero orbital angular momenta exhibit topologically non-trivial spin textures, which we call \textit{magnetic quasi-vortices}. Photons in optical whispering gallery modes supported by a dielectric sphere possess orbital and spin angular momenta forming \textit{optical vortices}. Within a ferromagnetic, as well as dielectric, sphere, two forms of vortices interact in the process of Brillouin light scattering. We argue that in the scattering there is a selection rule that dictates the exchange of orbital angular momenta between the vortices. The selection rule is shown to be responsible for the experimentally observed nonreciprocal Brillouin light scattering.