Modulated optical sieve for sorting of polydisperse microparticles

We present an all-optical technique that permits sorting within a polydisperse sample of microparticles in the absence of any microfluidic flow. We can sort colloidal samples based on their size and their refractive index. We show experimental and theoretical data for this method. It is based on the specific response of different microparticles to an interference pattern of fringes vibrating with an asymmetric time modulation. The size selectivity arises from the spatial fringe periodicity whereas selection based on refractive index is controlled by the beam power.