Extended optical binding with single and two focal laser beams

A non-contacting method to manipulate and array many nanoparticles is an important subject for physics, chemistry, and biology. The optical manipulation and binding provide rich functionalities and degrees of freedom to form a ordered assembly of particles by combining with heat generation, hydrodynamic effect, chemical reaction, etc. In this study, we extend a concept of the optical binding to achieve a wide-area particle formation by an optical method. In contrast to a conventional optical binding with a wide-area laser irradiation, we consider a tightly focal laser irradiation, where one or a few gold particles are directly trapped at the focal center and the others are bound indirectly at the out of irradiation area. For the indirect binding, the interference in multiple scattering is significant. The bound particles revolve by the spin angular momentum of laser. The revolution is stable dynamics since the revolution axis is well defined. For the focal laser, we have several control parameters, e.g., NA, focal depth, and number of beams. Then, we consider the optical binding by two focal laser beams. This extension brings an increase of control parameters. We focus on the circular polarized two lasers with seven particles binding in the respective irradiation area. The scattered light from one laser spot disturbs the dynamics of particles at other laser spot and revolution is frozen in each other. It suggests a nonlocal optical manipulation. When the lasers become close, 14 particles form a single stable array. It enables an on-demand formation of particle array.