A key element in the generation of optical torque in optical traps, whichoccurs when electromagnetic angular momentum is transferred from the trappingbeam to the trapped particle by scattering, is the symmetries of the scatteringparticle and the trapping beam. We discuss the effect of such symmetries on thegeneration and measurement of optical torque in optical tweezers, and someconsequent general principles for the design of optically-driven micromachines.
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