Tunable Optical Torque by Asymmetry-Induced Spin-Hall Effect in Tightly Focused Spinless Gaussian Beams

TL;DR

This paper demonstrates how asymmetric illumination of tightly focused Gaussian beams can induce and control optical torque on microparticles via spin-orbit interactions, enabling tunable rotation without intrinsic light angular momentum.

Contribution

It introduces a simple, experimentally accessible method to generate and manipulate optical torque using symmetry-breaking illumination to exploit the spin Hall effect of light.

Findings

Asymmetric illumination causes measurable optical torque on microparticles.

Rotation direction can be reversed by rotating the incident polarization.

Tunable rotational frequencies are achieved at constant input power.

Abstract

A linearly polarized Gaussian beam, carrying zero net spin angular momentum, is conventionally not expected to exert optical torque or induce rotational motion in birefringent microparticles. When such a beam is tightly focused, the constituent left- and right-circular polarization components separate spatially due to spin-orbit interaction, commonly known as the spin Hall effect of light. However, this separation is at wavelength scales and is also axially symmetric, resulting in zero net spin angular momentum, and concomitantly no optical torque near the focal plane. Here, we demonstrate that this limitation can be overcome using several commonly encountered asymmetric illumination modalities that break the axial symmetry of the focusing system, thereby disrupting the symmetric separation of the spin components for the same linearly polarized Gaussian beam. As a consequence, trapped microparticles experience a tunable optical torque and exhibit rotational motion with distinct rotational frequencies at the same input power. The particles also undergo controlled reversal of the rotation direction simply by rotating the incident plane of polarization using a half-wave plate. Despite their apparent diversity, all these methods share the same physical origin rooted in asymmetric illumination. These results establish an experimentally accessible and minimal strategy for realizing controllable optical rotation devices exploiting spin-orbit optomechanics without requiring intrinsic angular momentum in the light.