Generally applicable holographic torque measurement for optically trapped particles

TL;DR

This paper introduces a universal optical torque measurement method for particles of any shape in optical traps, using interference patterns and phase retrieval to analyze light's angular momentum changes.

Contribution

It provides a novel, comprehensive technique to measure all torque components on arbitrarily shaped particles from a single interference pattern.

Findings

Accurately measures all torque components using a single camera image.

Can distinguish between spin and orbital torque contributions.

Applicable to particles of any shape in optical trapping experiments.

Abstract

We present a method to measure the optical torque applied to particles of arbitrary shape held in an optical trap, inferred from the change of angular momentum of light induced by the particle. All torque components can be determined from a single interference pattern recorded by a camera in the back focal plane of a high-NA condenser collecting all forward scattered light. We derive explicit expressions mapping the measured complex field in this plane to the torque components. The required phase is retrieved by an iterative algorithm, using the known position of the optical traps as constraints. The torque pertaining to individual particles is accessible, as well as separate spin or orbital parts of the total torque.