A hybrid method to calculate optical torque: Application to a nano-dumbbell trapped by a metalens

The hyper-fast rotation frequency realized in an optical levitation system provides an essential platform for various applications. Benefiting from the development of integrated photonics, optically trapping and manipulating a micro-particle via a metalens has been a significant development trend. The metalens’ powerful and flexible controlling ability of the optical field opens the door to tailoring optical trapping potential. However, the existing methods are difficult to compute optical forces and torques on a non-spherical particle trapped by a metalens-based trapping system, especially when the trapping potential is tailored by a delicately designed metalens. Therefore, a hybrid method by combining the finite difference in time-domain and discrete dipole approximation method is proposed in this paper to realize this goal. The relative error of this method is verified to be below 10%. Based on this hybrid method, the fractional vortex field is found in a metalens-based trapping system for the first time. Then, the optical torque’s dependency on a nano-dumbbell’s geometrical parameters and spatial orientation angles are studied. It is found that there is a torque driving the nano-dumbbell to rotate about the optical axis, and the long axis of the nano-dumbbell tends to be aligned to the polarization plane because of the transverse optical torques if the long axis of the nano-dumbbell is not aligned to the optical axis.