Using a focussed laser beam, it is possible to trap and manipulate microscopic particles. Scattering by the particle in the trap results in a force acting on the particle, due to the transfer of momentum from the trapping beam. Similarly, the transfer of angular momentum through scattering generates an optical torque, which can be used to rotate or align trapped microscopic particles. We examine the basic processes involved in the transfer of angular momentum in scattering, and show that geometrically simple non-spherical particles can be aligned and rotated merely by using a plane-polarised Gaussian beam
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