Tunable Sorting of Micro- and Nano-particles in a Microfluidic System Using Near-field Optics

In this paper, we propose a new structure that can trap and sort particles with size of micro and nanometers. To investigate its operating principles we calculate the optical forces exerted on a particle using Maxwell's stress tensor. The optical forces are originating from interaction between the particle and the propagating surface plasmons polaritons excited on gold stripes. We evaluate polystyrene sphere radius for which the potential depth reaches its threshold value of 10 k(B)T . The potential depth is computed from the lateral component of optical force (F-x) for three stripes with widths of 1,2 and 3 mu m. It is shown that this structure can sort polystyrene particles in four ranges with different radii. The boundaries of the ranges are 1010, 490 and 280nm. As advantage of this method, it can be tuned with changing illumination power or stripes widths that can be used for biological optical sorting.