Rotation of a Trapped Microsphere in a Misaligned Dual-Beam Optical Tweezer

In this paper, we experimentally realize the rotation of a microsphere in an optical tweezer and conduct comprehensive simulation on trapping, rotation and escaping of a microsphere in dual-beam optical tweezer. According to our simulation, in a dual-beam optical tweezer, the radial misalignment and angle between two beams determines the movement properties of the trapped microsphere in the optical tweezer. When two beams are well aligned, the microsphere can be stably trapped in the optical tweezer. With a radial misalignment of the dual-beam, the microsphere starts to rotate, and its rotation speed and trajectory are influenced by the offset and separation of two beams. When the misalignment is increased to the escaping offset, the microsphere will escape the optical tweezer. We simulate the motion profiles of a trapped microsphere under various conditions such as beam waist, misalignments and tilting angles of two beams. Our dynamic analysis is based on ray optics approximation. The gravity, environmental resistance force and forces from air molecule collision are taken into consideration in our dynamic analysis. We also conduct experiments to verify our simulation results. We achieve rotation of a microsphere by controlling the offset of two beams in an optical tweezer and analyze the rotation phenomenon of the microsphere. In experiments, power spectrum analysis method can be used to measure rotation rates of the microsphere. Simulation and experiment show consistent results regarding rotation rates and motion trajectory, which verifies the validity and accuracy of dynamic analysis. Our work is an important step toward explaining the motion properties of an optical tweezer and can be applied in microsphere manipulation.