Optical measurement of torque exerted on an elongated object by a noncircular laser beam

We have developed a scheme to measure the optical torque exerted by a laser beam on a phase object by measuring the orbital angular momentum of the transmitted beam. The experiment is a macroscopic simulation of a situation in optical tweezers, as orbital angular momentum has been widely used to apply torque to microscopic objects. A hologram designed to generate LG₀₂ modes and a CCD camera are used to detect the orbital component of the beam. Experimental results agree with theoretical numerical calculations, and the strength of the orbital component suggest its usefulness in optical tweezers for micromanipulation.