# Spin-Orbit Coupling and the Evolution of Transverse Spin

## TL;DR

This paper explores how spin-orbit coupling affects the evolution of transverse spin in focused circularly polarized light beams, revealing local polarization rotations explained by Gouy-phase shifts, and experimentally demonstrating these effects with nano-probe scattering.

## Contribution

It introduces a new understanding of transverse spin evolution due to spin-orbit coupling in focused light beams and demonstrates experimental observation using a nano-probe reconstruction scheme.

## Key findings

- Local polarization ellipses rotate during propagation.
- Gouy-phase shift explains the spin evolution.
- Experimental verification with nano-probe scattering.

## Abstract

We investigate the evolution of transverse spin in tightly focused circularly polarized beams of light, where spin-orbit coupling causes a local rotation of the polarization ellipses upon propagation through the focal volume. The effect can be explained as a relative Gouy-phase shift between the circularly polarized transverse field and the longitudinal field carrying orbital angular momentum. The corresponding rotation of the electric transverse spin density is observed experimentally by utilizing a recently developed reconstruction scheme, which relies on transverse-spin-dependent directional scattering of a nano-probe.