Array of optical tweezers with individual beam-steering and polarization control

We report a new method for generating multiple optical tweezers that enables beam-steering functionality and polarization state control for each trapping beam. The method employs a spatial light modulator (SLM) that imparts a two-dimensional (2-D) phase distribution onto a linearly polarized input beam from a CW-laser (lambda = 830nm). A microlens array is used to divide the beam resulting in a matrix of point sources that are subsequently imaged onto the sample plane of a microscope as trapping beams. For beam steering, the plane of polarization of the input field is aligned parallel to the slow axis of the SLM, on which an array of phase grating patterns is encoded. The blazed grating parameters dictate the positional deflections of the optical traps. When the input polarization plane makes a 45degrees-angle with the SLM slow axis, an elliptic polarization state is produced for each optical trap and the degree of ellipticity depends on the phase difference introduced between the extraordinary and ordinary output field components at each SLM sub-aperture. It is known that elliptically polarized light changes its angular momentum when focused to a birefringent particle thereby inducing an axial rotation of the trapped object. We experimentally demonstrate the two modes of operation: (1) controlled lateral deflection of simultaneous trapped polystyrene microspheres and (2) polarization-induced rotation of simultaneously trapped calcite crystals.