Computer-controlled orientation of multiple optically-trapped microscopic particles

An all-optical manipulation method has been used for transferring both linear and angular momentum resulting in translation, rotation and alignment of multiple dielectric polystyrene particles and cell structures. The trapping-beams are generated using the generalised phase contrast method where a direct correspondence exists between the input phase modulation and the output intensity distribution. A computer-controlled spatial light modulator is used to produce a dynamically programmable input phase pattern that is transformed into parallel optical traps without the need of mechanical movement of the optical components. This is advantageous for future miniaturised systems, e.g. lab-on-a-chip and micro-fluidic devices. Our method provides for computer-controlled laser trapping-beams with real-time orientation of the particles in a micro-fluidic system.