We demonstrate an integrated microparticle passive sorting system based on the near-field optical forces exerted by a 3-dB optical splitter that consists of a slot waveguide and a conventional channel waveguide. We show that 320 nm and 2 µm polystyrene particles brought into the splitter are sorted so that they exit along the slot waveguide and channel waveguide, respectively. Electromagnetic simulations and precise position tracking experiments are carried out to investigate the sorting mechanism. As the waveguides are separated by 200 nm, they provide two potential wells for the smaller particles, but only one broad potential well for the larger particles, since their diameters exceed the distance between the two field maxima. A structural perturbation consisting of a stuck bead transfers the smaller particles to the second well associated with the slot waveguide, while the larger particles are brought to the region between the two waveguides and eventually follow the channel waveguide, as it is associated with a deeper potential well. This label-free passive particle sorting system requires low guided power (20 mW in these experiments), and provides a new technique for sorting sub-micron particles.
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