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

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.

Gearing up for optical microrobotics: micromanipulation and actuation of synthetic microstructures by optical forces

“…One of the proposed applications of optical rotation, that directly mimic macroscale applications, is for optically driven motors , which can be used as micropumps for microfludics . The noncontact trapping and rotation of such micropumps allows versatile deployment, e.g.…”

Precise switching control of liquid crystalline microgears driven by circularly polarized light

“…The coincidence of rotational frequencies upon a direction reversal validates the mirror symmetry of the exerted torque due to left-and righthanded circularly polarized light. The present reversal requires only the π/2 rotation of the phase plate, which is an advantage over optically driven micromachines consisting of nonbirefringent resins [13,14].…”

“…Yang et al reported that continuous rotation of chiral nematic LC droplets is driven by a linearly polarized light [10]. Another alternative strategy is to use micro-objects that can scatter light in a helical manner [11][12][13]. The photopolymerization of light-curing resins can create microscopic propellers, which have been demonstrated to rotate efficiently due to helical scattering.…”

“…While several types of polymeric microrotors [11][12][13][14] have potential as driving components of microfluidic devices, fine optical adjustments, including the change of trapping positions, are required for achieving the rotational reversal [13]. In this letter, we impart birefringence to photopolymerizable resins for more precise switching control of microrotors.…”

“…We used photopolymerizable LC materials instead of genuine LC droplets and fabricated microgears of various shapes using two-photon microstereolithography. Our aim was to demonstrate the precise switching control of LC microgears compared with previously reported microrotors [1][2][3][4][5][6][7][8][9][10][11][12][13][14].…”