# Spin Photonics in 3D Whispering Gallery Mode Resonators

## TL;DR

This paper investigates the spin-orbital interaction in whispering gallery modes of spherical resonators, revealing spin-momentum locking and unidirectional mode excitation, with implications for quantum emitters and photonic applications.

## Contribution

It provides an exact Green's function analysis of spin-orbital interactions in WGMs, demonstrating unidirectional excitation linked to spin-momentum locking.

## Key findings

- Spin-momentum locking in whispering gallery modes.
- Unidirectional excitation of modes with specific angular momentum.
- Potential experimental setups with cold atoms or quantum dots.

## Abstract

Whispering gallery modes are known for having orbital angular momentum, however the interplay of local spin density, orbital angular momentum, and the near-field interaction with quantum emitters is much less explored. Here, we study the spin-orbital interaction of a circularly polarized dipole with the whispering gallery modes (WGMs) of a spherical resonator. Using an exact dyadic Green's function approach, we show that the near-field interaction between the photonic spin of a circularly polarized dipole and the local spin density of whispering gallery modes gives rise to unidirectional behaviour where modes with either positive or negative orbital angular momentum are excited. We show that this is a manifestation of spin-momentum locking using the whispering gallery modes of the spherical resonator. We discuss the requirements for possible experimental demonstrations using Zeeman transitions in cold atoms or quantum dots, and outline potential applications of these previously overlooked properties. Our work firmly establishes local spin density, momentum and decay as a universal right-handed electromagnetic triplet for evanescent waves.