Negative optical spin torque wrench of a nondiffracting non-paraxial fractional Bessel vortex beam

TL;DR

This paper investigates how a non-diffracting fractional Bessel vortex beam can induce negative axial spin torque on an absorptive dielectric sphere, enabling control over particle rotation and manipulation.

Contribution

It introduces the concept of negative optical spin torque using non-paraxial fractional Bessel vortex beams and analyzes the effects of beam parameters on torque direction.

Findings

Negative axial spin torque occurs for positive beam order.

Spin reversal can be achieved with off-axial sphere positioning.

The beam's topological charge influences torque direction.

Abstract

An absorptive Rayleigh dielectric sphere in a non-diffracting non-paraxial fractional Bessel vortex beam experiences a spin torque. The axial and transverse radiation spin torque components are evaluated in the dipole approximation using the radiative correction of the electric field. Particular emphasis is given on the polarization as well as changing the topological charge and the beam's half-cone angle. When the beam order is zero, the axial spin torque component vanishes. However, when the beam order becomes a real positive number, the vortex beam induces left-handed (negative) axial spin torque as the sphere shifts off-axially from the center of the beam. The results show that a non-diffracting non-paraxial fractional Bessel vortex beam is capable to induce a spin reversal of an absorptive Rayleigh sphere placed arbitrarily in its path. Potential applications are yet to be explored in particle manipulation, rotation in optical tweezers, optical tractor beams, the design of optically-engineered metamaterials to name a few areas.