Optical rotor capable of controlling clockwise and counterclockwise rotation in optical tweezers by displacing the trapping position

Appl Opt. 2010 Apr 1;49(10):1991-6. doi: 10.1364/AO.49.001991.

Abstract

A clockwise rotor and a counterclockwise rotor (a clockwise rotor placed upside down) are linked on the optical axis to control the rotation direction in optical tweezers by displacing the trapping (focus) position. The dependence of optical torque on the trapping position of this linked rotor is analyzed using an upward-directed focused beam as illumination, via an objective lens with a numerical aperture of 1.4, using a ray optics model under the condition that laser light is incident to not only the lower surfaces, but also to the side surfaces of both rotors. The rotation rate in water is also simulated for an SU-8 linked rotor with 20 microm diameter at a laser power of 200 mW, with rotor thickness as a parameter, by balancing the optical torque with the drag force evaluated using computational fluid dynamics. It is confirmed that the rotation direction changes from clockwise to counterclockwise with the displacement of the trapping position, that almost the same rotation speed is possible in both directions, and that both speeds increase, reach a maximum at a rotor thickness of 9 microm, and then decrease as the thickness increases.