Hopping and orbiting of Mie-resonant nanoparticles in optical tweezers

Abstract

Optical tweezers have become a powerful tool for measuring parameters of microscale and nanoscale local environments. Motion of particles within optical tweezer traps established itself as a probe for local viscosity, temperature as well as external fields. However, complex motions of trapped particles such as hopping or orbiting conventionally relies on intricate structure of the optical trap, which creates limitations to their applicability, in particular in challenging environments such as living matter. Here we demonstrate experimentally polarization-dependent hopping and orbiting of particles in an optical trap formed by a single Gaussian beam. This novel effect is observed for subwavelength particles with sizes above approximately $\lambda/3$, where $\lambda$ is the wavelength of the trapping beam. Our theoretical analysis reveals that the hopping and orbiting arises via the excitation of {\it higher-order Mie resonances}. While smaller particles experiencing dipolar optical forces remain trapped at the beam center, larger particles experience quadrupole optical forces exhibiting more complex behavior. Under linearly polarized illumination, larger particles show {\it bistable dynamics} hopping between two stable positions. Under circularly polarized illumination, trapped particles orbit around the beam center, revealing a light-induced spin-to-orbital angular momentum transfer. Experimental trajectory analysis reveals power-tunable hopping rates and angular velocities. We discuss the effect of VO$_2$ monoclinic-to-rutile phase change on the observed effect. Our results present a robust and reliable way to achieve complex behavior of a particle in simple single-beam tweezers. This should enhance sensitivity of parameter measurements for local environments via optical forces, such as viscosity and temperature inside cells.