We present a detailed theoretical discussion and experimental analysis of an interferometric optical trapping device that allows efficient sorting of particles, including biological samples, either by size or refractive index. This technique involves no microfluidic flow, but it is based on the specific response of different microparticles to an interference pattern of fringes vibrating with a periodic but non-symmetric time modulation function. The performance of the system is analyzed in terms of the different control parameters, such as the period of the fringes, the vibration amplitude and frequency, and the power level in the sample. We discuss the possibility of using this system to characterize unknown samples.
We report on the observation of a normal streak effect on hollow micron sized spheres when illuminated by a focused Gaussian beam in a conventional optical tweezers setup. The hollow microspheres suspended in water can be optically trapped at the center of the illuminating beam. When the microsphere is illuminated off center, an emerging beam approximately perpendicular respect to the incoming beam is generated. This effect due to total internal reflections has been observed in microspheres with different external diameters, ranging from 5-20 microns. The generated normal beam is used to either pull or push other particles or objects around the microsphere or to remove particles stuck to the sphere due to radiation pressure.