Measuring the Transverse Spin Density of Light

TL;DR

This paper develops a method to measure the transverse electric spin density of tightly focused optical vector beams using near-field interference and far-field detection, revealing the elusive spin momentum component.

Contribution

It introduces an experimental approach combining near-field interference and dielectric interface conversion to measure transverse spin density with nanoscopic resolution.

Findings

Successful measurement of transverse electric spin density.

Detection of the longitudinal electric component of Belinfante's spin momentum.

Enhanced understanding of optical spin and momentum fields.

Abstract

We generate tightly focused optical vector beams whose electric fields spin around an axis transverse to the beams' propagation direction. We experimentally investigate these fields by exploiting the directional near-field interference of a dipole-like plasmonic field probe, placed adjacent to a dielectric interface, which depends on the transverse electric spin density of the excitation field. Near- to far-field conversion mediated by the dielectric interface enables us to detect the directionality of the emitted light in the far-field and, therefore, to measure the transverse electric spin density with nanoscopic resolution. Finally, we determine the longitudinal electric component of Belinfante's elusive spin momentum density, a solenoidal field quantity often referred to as 'virtual'.