Experimental Measurement of Transverse Spin Dynamics in the Nonparaxial Focal Region

TL;DR

This paper experimentally measures and maps the transverse spin and phase-polarization dynamics in the nonparaxial focal region of a circularly polarized Gaussian beam, revealing complex polarization singularities and spin behaviors.

Contribution

It introduces a novel measurement method for detailed investigation of nonparaxial focal fields, capturing phase, polarization, and spin dynamics with high spatial resolution.

Findings

Identification of polarization singularities and their behavior

Observation of transverse and longitudinal spin dynamics

Mapping of phase-polarization variations in the focal region

Abstract

The superposition of complex optical fields in three-dimension (3D) is the basis of several non-trivial wave phenomena. Significant among them are the non-uniform polarization distribution and their topological character, leading to the emergence of transverse spin, transverse momentum, and their dynamics. These aspects are experimentally measured in the nonparaxial focal region of a circularly-polarized Gaussian input beam. A dielectric mirror, kept in the focal region, is axially scanned to obtain the phase and polarization variations in the retro-reflected output beam using an interferometer and spatially-resolved Stokes parameter measurements. The identification of phase and polarization singularities in the beam cross-section and their behaviour as a function of the mirror position enabled us to map and study the phase-polarization variations in the nonparaxial focal region. The lemon-monstar type polarization patterns surrounding the C-point singularity in the output beam are identified and tracked to study the transverse spin dynamics for right- and left- circular polarization of the input beam. Direct measurement of the input beam polarization helicity-independent and helicity-dependent aspects of the transverse and longitudinal spin dynamics in the nonparaxial focal region are the significant findings reported here. The proposed and demonstrated measurement method allows us to investigate the nonparaxial focal region in more detail and has the potential to unravel other intricate optical field effects.