Nanoscale rotational optical manipulation.

Light has momentum, and hence, it can move small particles. The optical tweezer, invented by Ashkin et al. [Opt. Lett. 11, 288 (1986)] is a representative application. It traps and manipulates microparticles and has led to great successes in the biosciences. Currently, optical manipulation of "nano-objects" is attracting growing attention, and new techniques have been proposed and realized. For flexible manipulation, push-pull switching [Phys. Rev. Lett. 109, 087402 (2012)] and super-resolution trapping by using the electronic resonance of nano-objects have been proposed [ACS Photonics 5, 318 (2017)]. However, regarding the "rotational operation" of nano-objects, the full potential of optical manipulation remains unknown. This study proposes mechanisms to realize rotation and direction switching of nano-objects in macroscopic and nanoscopic areas. By controlling the balance between the dissipative force and the gradient force by using optical nonlinearity, the direction of the macroscopic rotational motion of nano-objects is switched. Further, conversion between the spin angular momentum and orbital angular momentum by light scattering through localized surface plasmon resonance in metallic nano-complexes induces optical force for rotational motion in the nanoscale area. This study pieces out fundamental operations of the nanoscale optical manipulation of nanoparticles.