Doppler velocimetry of levitated particles in dual-beam optical tweezers

Accurate velocity measurement of optically levitated particles is critical to achieving optimal control and high-precision metrology in optical tweezers. Conventional particle tracking techniques-such as those utilizing quadrant photodetectors, D-shaped mirrors, or direct imaging-primarily measure particle position. To derive the instantaneous particle velocity, these methods require precise trajectory calibration and the calculation of time derivatives. However, these operations introduce significant errors into the retrieved velocity data. Here we introduce a Doppler-based technique for tracking the absolute instantaneous velocities of levitated micro- and nanospheres within a dual-beam trap. The method enables high-precision velocimetry of levitated particles in modulated motion, with substantially improved spatiotemporal resolution compared to conventional methods. Its capability for simultaneously tracking of multiple particles is also verified. The Doppler-based approach addresses key limitations of position-derived velocity measurements, providing a robust basis for precise studies of particle dynamics in optical tweezers.