Generation of vectorial structured light carrying bright and dark focus for multifunctional optical trapping via spin-decoupled metasurfaces

Optical trapping and manipulation of small particles are important in many fields, like biophysics and nanoscience. The use of structured light can greatly improve the degree of freedom of optical manipulation. Here, we propose what we believe to be a novel vectorial structured light field whose orthogonal circularly polarized components have the characteristics of bright and dark focus, respectively. A scheme based on a spin-decoupled metasurface is then proposed to generate this structured light for multifunctional optical trapping. Owing to the ultra-thin profile and spin-decoupled property of the designed metasurface, structured light fields of dark focus and bright focus can be generated by tuning the polarization of the illuminating beam, and chip-scale optical trapping of dielectric nanoparticles with different refractive indices is realized. Longitudinally varying optical chirality density can also be realized by the same metasurface illuminated by linearly polarized light, which can be used for chiral nanoparticle sorting. Our proposed device can provide a platform for multi-functional on-chip optical manipulation of different kinds of nanoparticles.