Generally applicable holographic torque measurement for optically trapped particles

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.

V2, with updated figures

References in corpus (5)

• Optical alignment and spinning of laser-trapped microscopic particles
• Angular Momenta and Spin-Orbit Interaction of Nonparaxial Light in Free Space
• Conservation of the spin and orbital angular momenta in electromagnetism
• Measurement of the total optical angular momentum transfer in optical tweezers
• Optical measurement of torque exerted on an elongated object by a non-circular laser beam