Abrupt Vortex‐Induced Stop Yields Ratcheted Rotation of Microparticles

ABSTRACT In low‐Reynolds‐number environments, where viscous forces dominate, microparticles driven by optical vortex beams exhibit a distinct abrupt stop phenomenon when the light is switched off, exhibiting overdamped motion, unlike classical mechanics, where an object in circular motion would continue tangentially and decelerate. Viscous damping dissipates rotational kinetic energy within a few microseconds after torque removal. Even for particle speeds of 1000 µm/s, stopping time and distance are only ∼8.2 µs and ∼0.82 nm, far below the 40 ms temporal resolution of standard imaging. Exploiting this property, a programmable vortex‐grating optical switch is constructed to periodically alternate holograms that apply and revoke optical torque, thereby enabling precise timing control of rotation stop. A repeating light switch can drive the particles rotate by a user‐defined angular step, with the step size tunable via the switch interval, laser power, and topological charge, effectively converting a continuous micro‐rotor into a programmable digital optical ratchet. This contact‐free brake‐and‐step strategy operates without the need for high‐speed imaging and provides an optical route for digitally timed valves, micropumps, and reconfigurable biomechanical actuators, as well as for probing nanoscale dissipation dynamics and tracking the motion of micro‐ and nanoscale objects with high temporal control.