A novel optical manipulation system based on photonic nanojets (PNJs) is numerically investigated based on the finite element method. It is found that nanoscale particles can be trapped stably in a standing-wave PNJ generated by the constructive interference between two coherent PNJs. In particular, we show that the elongated standing-wave PNJs generated by using two-layer microcylinders or microspheres can provide larger manipulation platforms and stronger optical forces. To assess the trapping stability of the particle under the Brownian motion in the elongated PNJ, the relationship between the stability number and the particle size is studied. The simulation results show that the proposed elongated standing-wave PNJs can provide the stable and tunable manipulation for dielectric nanoparticles that are smaller than 100 nm. (C) 2016 Optical Society of America
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