Measurement of the total optical angular momentum transfer in optical   tweezers
  We describe a way to determine the total angular momentum, both spin and orbital, transferred to a particle trapped in optical tweezers. As an example an LG02 mode of a laser beam with varying degrees of circular polarisation is used to trap and rotate an elongated particle with a well defined geometry. The method successfully estimates the total optical torque applied to the particle. For this technique, there is no need to measure the viscous drag on the particle, as it is an optical measurement. Therefore, knowledge of the particle's size and shape, as well as the fluid's viscosity, is not required. 
Simon Parkin, Gregor Knoener, Timo A. Nieminen, Norman R. Heckenberg   and Halina Rubinsztein-Dunlop