Waveguide-grating-based optical tweezers for particle trapping

On-chip photonic devices are promising for large-scale optical trapping and parallel particle manipulation. Based on a waveguide grating, which can convert light from a localized optical mode to a freely propagating radiation beam, we propose two waveguide-grating-based optical tweezers for trapping micro- and sub-micro-particles. By optimizing the structural parameters of the etched trenches on the waveguide, the intensity distribution of the upward diffraction beam out of the grating is altered to approximate a target beam that can produce large gradient forces. The optimization method generates a focused quasi-Gaussian beam with a waist height of 7 μm above the grating and a beam with quasi-spherical wave wavefronts at ± 20°. The optimized upward radiation beams generate gradient forces up to hundreds of pN/W, which is strong enough for trapping microparticles with a trapping range as high as 20 μm. Besides, the proposed tweezer shows a possibility of trapping sub-micron particles and the potential for large-scale parallel particle manipulation.