Measurement of the total optical angular momentum transfer in optical tweezers

Abstract

We describe a way to determine the total angular momentum, both spin and orbital, transferred to a particle trapped in optical tweezers. As an example an LG 02 mode of a laser beam with varying degrees of circular polarisation is used to trap and rotate an elongated particle with a well defined geometry. The method successfully estimates the total optical torque applied to the particle. For this technique, there is no need to measure the viscous drag on the particle, as it is an optical measurement. Therefore, knowledge of the particle's size and shape, as well as the fluid's viscosity, is not required.

Optical angular momentum transfer to microrotors fabricated by two-photon photopolymerization

“…An important consequence of this is that Parkin's method (Parkin et al 2006) for measuring the total optical torque from measurements of the rotation rate and spin torque for left-and right-circular and plane polarizations can be expected to be accurate if the spin torque due to plane polarization is very small (which indicates very similar scattering of the left-and right-circular modes).…”

“…In order to measure the orbital torque τ o , we used a method previously described in the literature (Parkin et al 2006). It is based on the steady rotation of the microstructure, which means that the total optical torque is equal to the drag torque due to rotation in the liquid.…”

“…Parkin's method (Parkin et al 2006) was used to measure the total optical torque, including the contribution due to OAM. The relationship between rotation rate and the spin component of the torque is shown in figure 8.…”

Torque transfer in optical tweezers due to orbital angular momentum

“…The slope and intercept of this plot yield the orbital angular momentum transferred to the trapped particle. Please refer to Parkin et al 10 for a more detailed description of this result. It should be mentioned here that the orbital component transferred was five times greater than the spin component, which demonstrates the advantage of using orbital angular momentum for rotating asymmetric objects.…”

Optimization of optically-driven micromachines

“…We fabricate microrotors using two-photon photopolymerization 9 and they are usually driven using LG 02 or LG 04 beams. The method we use to determine torque efficiency experimentally 10,11 involves measuring the rate of rotation and the change in circular polarization.…”