# Harvesting the Spin–Orbit Interaction of Light to Generate Helicity‐Dependent Complex Rotational Motion in Optically Trapped Mesoscopic Matter

## TL;DR

Scientists use light's spin-orbit interaction to create complex rotational motion in tiny particles trapped by laser beams, enabling new optomechanical applications.

## Contribution

A novel method is introduced to generate indirect spin and orbital motion in particles using spin-orbit coupling and microfluidic effects.

## Key findings

- Spin angular momentum from circularly polarized light is transferred to a central particle, inducing microfluidic flows.
- Extrinsic spin-orbit interaction causes off-axis particles to rotate and revolve simultaneously.
- A Mueller matrix model explains the observed rotational dynamics and helicity coupling.

## Abstract

Spin–orbit interaction (SOI) of tightly focused light in optical tweezers underpins diverse optomechanical applications and the interconversion of spin and orbital angular momentum. Here, we demonstrate that the transfer of the spin angular momentum of a tightly focused circularly polarized beam to an on‐axis birefringent particle can indirectly generate spin in an adjacent particle, leading to exotic rotational motion reminiscent of planetary trajectories. We demonstrate simultaneous rotation and revolution of birefringent liquid crystal (LC) particles by harvesting SOI, such that its two principal governing mechanisms—the momentum‐dependent Pancharatnam–Berry (PB) phase and the anisotropy‐induced PB phase—become coupled. In our experiments, a centrally trapped LC particle in spherically aberrated optical tweezers spins under circularly polarized illumination, generating spin‐induced microfluidic flows that drive surrounding off‐axis LC particles into orbital motion. Simultaneously, interaction of the input helicity with the centrally trapped particle induces spin‐to‐spin conversion through extrinsic SOI. The helicity thereby generated indirectly then couples to the orbiting particles, imparting an additional rotation whose direction is determined by the birefringence of the central particle. A Mueller matrix model that incorporates tight focusing and scattering quantitatively explains these observations. Thus, SOI coupled with microfluidic effects establishes exotic rotational optomechanics and microswitch applications.

Spin–orbit interaction (SOI) of light is exploited to generate helicity‐dependent complex rotational and revolution dynamics in optical tweezers. At the trap center, particle rotation is governed by the direct helicity that follows the input polarization. However, anisotropy of the centrally trapped liquid‐crystal (LC)‐induced Pancharatnam–Berry phase gives rise to extrinsic SOI, generating indirect helicities at off‐axis positions that drive simultaneous rotation and revolution, or counter‐rotation, of secondary LC particles/droplets.