Abstract
Tightly-focused laser beams that carry angular momentum have been used to trap and rotate microrotors. In particular, a Laguerre-Gauss mode laser beam can be used to transfer its orbital angular momentum to drive microrotors. We increase the torque efficiency by a factor of about 2 by designing the rotor such that its geometry is compatible with the driving beam, when driving the rotation with the optimum beam, rather than beams of higher or lower orbital angular momentum. Based on Floquet's theorem, the order of discrete rotational symmetry of the rotor can be made to couple with the azimuthal mode of the Laguerre-Gauss beam. We design corrugated donut rotors, that have a flat disc-like profile, with the help of the discrete dipole approximation and the T-matrix methods in parallel with experimental demonstrations of stable trapping and torque measurement. We produce and test such a rotor using two-photon photopolymerization. With a rotor that has 8-fold discrete rotational symmetry, an outer radius of 1.85 μm and a hollow core radius of 0.5 μm, we were able to transfer approximately 0.3 h̄ per photon of the orbital angular momentum from an LG04 beam.
Highlights
An optically-driven microrotor can be used as a component of micromachines where it can act as an impeller for a pump, an agitator for a mixing chamber, a probe for viscosity measurements as well as being used in many other applications in the microfluidic regime [1,2,3]
We focus on orbital angular momentum transfer due to the coupling between the specific geometry of a structure to specific modes of the beam carrying OAM
Our attempts in making the corrugated donut rotor (CDR) with the intended shape shown in Fig. 8 by two-photon photopolymerization resulted in distortions that caused the structure to resemble a “ring cake” structure (Fig. 1)
Summary
An optically-driven microrotor can be used as a component of micromachines where it can act as an impeller for a pump, an agitator for a mixing chamber, a probe for viscosity measurements as well as being used in many other applications in the microfluidic regime [1,2,3]. The microrotor can acquire torque from a trapping and driving beam via spin angular momentum transfer due to its birefringence (material [4, 5] or structural form [6]) or orbital angular momentum (OAM) transfer due to azimuthal mode coupling with its structure. In the latter case, torque may be imparted on the rotor either from an incident beam carrying OAM [7] or, if the beam does not carry OAM, via asymmetric reflection from rotor’s chiral structure [1, 8]. OAM transfer may occur via absorption [9])
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