Abstract
We design, fabricate and test optically driven microrotors a few microns in size. The rotors are trapped and rotated in optical tweezers using an LG02 Laguerre–Gaussian laser beam. We verify that we can accurately measure the total optical torque by measuring the spin angular momentum transfer for three different polarizations, by comparing the optical torque with the optical torque calculated using computational electrodynamics and the viscous drag torque determined from the rotation rate and computational fluid dynamics. The torque agrees with that expected from the design principles and electromagnetic modelling of the torque within the optical trap.
Highlights
One end to a fixed surface and with the other to an object that can be optically trapped and rotated in the optical trap
Even if we restrict ourselves to two-photon photopolymerization, there is a wide range of biocompatible materials suitable for the attachment of biomolecules available, and their size can be tailored to the studied systems
A convenient way to represent such a non-paraxial beam is as a superposition of vector spherical wavefunctions (VSWFs) (Nieminen et al 2003), which have an azimuthal phase variation of exp(imφ), where m is the azimuthal mode index, with the z-component of the angular momentum being equal to mhper photon
Summary
The shape of a particle plays a central role in its interaction with light. Perhaps the most important aspect is the symmetry of the particle—consideration of the effects of particle symmetry allows some quite general aspects of optical torque to be understood (Nieminen et al 2009, Nieminen et al 2004). For an elongated or flattened particle, with p = 2, there will be interference between the left- and right-hand scattered modes, and the torque due to a linearly polarized beam will be angle dependent. Interference between the left- and right-hand scattered modes can result in angle-dependent torques acting to align the structure, even for p > 2—higher-order rotational symmetry does not rule out the use of orbital angular momentum for alignment. Alignment of this type has seen extensive use for the generation of optical torque (Bingelyte et al 2003, Paterson et al 2001, Sato et al 1991). This is a thickness of approximately double the free-space wavelength of the trapping beam
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