Rotation of optically bound particle assembly due to scattering induced spin-orbit coupling of light.

The optical binding of many particles has great potential to achieve the wide-area formation of a "crystal" of small materials. Unlike conventional optical binding, where the whole assembly of targeted particles is irradiated with light, if one can indirectly manipulate remote particles using a single trapped particle through optical binding, the degrees of freedom to create ordered structures will be greatly enhanced. In this Letter, we theoretically investigate the dynamics of the assembly of gold nanoparticles that is manipulated using a single particle trapped by a focused laser. As a result, we demonstrate that the spin--orbit coupling and angular momentum generation of light via scattering induce the assembly and rotational motion of particles through indirect optical force. This result opens the possibility of creating ordered structures with a wide area and manipulating them, controlling local properties using scanning laser beams.