Articles techniques et présentations

Optical tweezers enable the non-contact manipulation of micro- and nanoparticles through momentum exchange between focused optical fields and the particles, while the local energy-flow distribution plays an important role in the rotational behavior of nonspherical metallic nanostructures. To investigate the mechanism by which energy-flow reversal in a focused optical field regulates the optical rotation of metallic nanostructures, a three-dimensional full-wave electromagnetic model was developed in COMSOL Multiphysics. A helicity-preserving focused optical field with a wavelength of 532 nm was used as the excitation, and a silver nanorod with a diameter of 300 nm and a length of 1.5 μm was numerically simulated in water with a refractive index of 1.33.By calculating the electromagnetic field and time-averaged energy-flow distribution around the nanorod, the optical torque exerted on the nanorod was obtained by integrating the Maxwell stress tensor, and its rotational characteristics at different axial positions before and after the focal plane were analyzed. Meanwhile, the orientation angle of the nanorod and its lateral displacement relative to the optical axis were parametrically varied to investigate the effects of particle position and orientation on the optical torque. The simulation results show that the azimuthal energy-flow direction reverses across the focal plane, causing the direction of the optical torque exerted on the silver nanorod to reverse as its axial position changes. Although the orientation angle and lateral displacement of the nanorod significantly affect the magnitude of the optical torque, the torque direction remains stable within the same axial region. This model reveals the relationship between the local energy-flow distribution in a focused optical field and the bidirectional rotation of metallic nanostructures, providing a reference for the numerical analysis and parameter optimization of switchable rotational manipulation of micro- and nanoparticles in structured optical fields.