Reversed orbital rotation of dielectric particles induced by coupled-dipoles in focused Gaussian beams

As an important application of optical manipulation, orbital rotation of nanoparticles in optical beams can find uses in constructing micro-motors/micro-machines and the measurement of physical quantity. Here, we report an intriguing orbital rotation of nanoparticles in focused circularly-polarized Gaussian beams at the wavelength scale. Through the transverse scattering forces, off-axis trapping of nanoparticles becomes possible in the off-focal plane, allowing us to create a rotation orbit with a small radius. During the focusing of the incident circularly-polarized Gaussian beam, the spin angular momentum is converted to the orbital angular momentum that drives the particle's orbital rotation. While gold nanoparticles always show rotation direction that is identical with that of the angular energy flux density under the dipole approximation, high-refractive index silicon nanoparticles can exhibit an opposite rotation direction. This directional inversion is due to the coupling between induced electric and magnetic dipoles in the dielectric particles. By adjusting the particle size and the position of the off-focus plane, the rotation radius and speed can be tuned. This microscale orbital rotation scheme here may advance the development of micro-motor/micro-machines and the investigation of micro-rheology. Our theoretical analysis also sheds what we believe to be new light to the particle dynamics in optical tweezers.