Trapping of low-refractive-index nanoparticles in a hollow dark spherical spot

Optical trapping techniques have been of great interest and have advantages that enable the direct handling of nanoparticles. However, stable trapping of low-refractive-index nanoparticle remains challenging because the conventional two-dimensional hollow beams are only capable of trapping nanoparticle in the transverse plane. In this work, we propose a novel strategy to optically trap low-refractive-index nanoparticle in three-dimensional space with a hollow dark spherical focal spot, which is generated by 4Pi focusing of radially polarized first-order Laguerre-Gaussian beam. With the assumption that the laser power is 100 mW, the nanoparticles can be stably trapped with the maximal optical force of 0.3 pN, potential depth of 10 KBT and stiffness of 80 pN/mu m. Moreover, both the number and the position of the focal spot can be controlled by modulating the focusing condition and the gradient phase of the illumination respectively, enabling the simultaneously trapping of multiple nanoparticles with complex motion trajectory. The technique demonstrated in this work may open up new avenues for optical manipulation and their applications in various scientific fields.