Optical deflection and sorting of microparticles in a near-field optical geometry

Near-field optical micromanipulation permits new possibilities for controlled motion of trapped objects. In this work, we report an original geometry for optically deflecting and sorting micro-objects employing a total internal reflection microscope system. A small beam of laser light is delivered off-axis through a total internal reflection objective which creates an elongated evanescent illumination of light at a glass/water interface. Asymmetrical gradient and scattering forces from this light field are seen to deflect and sort polystyrene microparticles within a fluid flow. The speed of the deflected objects is dependent upon their intrinsic properties. We present a finite element method to calculate the optical forces for the evanescent waves. The numerical simulations are in good qualitative agreement with the experimental observations and elucidate features of the particle trajectory. In the size range of 1 μm to 5 μm in diameter, polystyrene spheres were found to be guided on average 2.9 ± 0.7 faster than silica spheres. The velocity increased by 3.0 ± 0.5 μms-1 per μm increase in diameter for polystyrene spheres and 0.7 ± 0.2 μms-1 per μm for silica. We employ this size dependence for performing passive optical sorting within a microfluidic chip and is demonstrated in the accompanying video.