Measurement of Orbital Angular Momentum in Optical Tweezers

Several techniques have been proposed and used for the rotation or alignment of microparticles in optical tweezers. In every case the optical torque results from the exchange of angular momentum between the beam and the particle, and, in principle, can be measured by purely optical means. Measurement of this torque could be useful for quantitative measurements in biological systems and is required to measure properties such as viscosity of liquids in microlitre (or less) volumes. Although elongated particles will align with the plane of polarisation, the torque efficiency is low, typically about 0.05h̄ per photon. The use of a beam with an elongated focal spot can increase this torque by a factor of 10-20 times, due to the transfer of orbital angular momentum. We report measurements of the orbital component using an analysing (Laguerre–Gauss) hologram. As a proof of principle experiment, an elliptical beam scattered off a glass rod was simulated on a macroscopic scale. The torque was found to be as much as 0.8h̄ per photon. Microscopic elongated objects have been aligned and rotated in optical tweezers and we plan to make measurements of the torques involved.