Cover Story：Liu J, Zheng M, Xiong ZJ, Li ZY. 3D dynamic motion of a dielectric micro-sphere within optical tweezers. Opto-Electron Adv 4, 200015 (2021).

With nanometer accuracy of positioning and pico-newton precision of force measurement, optical tweezers offer an ideal platform to exert and measure the forces and torques on single bio-molecule or micro-particle. Although nowadays, various visualization techniques have been developed including quadrant photodiode detection, stereomicroscopy and V-shaped micro-mirrors, tracking the particle trajectory are still very hard. These experimental techniques are limited by sampling rate and in three-dimensional (3D) imaging. Solution and analysis of the 3D micro-scale particle dynamic process within optical trapping is essential for understanding various mechanisms for engineering the mechanical motion behavior of molecules or micro-particles in liquid. In order to solve the dynamic process with the thousands or even millions of trajectory points, Dr. Jing Liu from South-Central University for Nationalities and Prof. Zhiyuan Li from South China University of Technology have proposed a method based on semi-analytical ray optics model to simulate the 3D trajectory of a microscopic sphere in the optical tweezers. Based on a large amount of simulations, they find the dielectric micro-sphere exhibits abundant phases of mechanical motions, including acceleration, deceleration, and even turning, in a simple set of optical tweezers. This compound mechanical motion model will give insight into the roles of optical force and optical torque in studying life science, biology and physics. Moreover, investigation of these dynamical processes will open up a new way to promote optical external control in various applications including single bio-molecular kinematics, light-driven micro-machines and in vivo manipulations.