Measurement of particle motion in optical tweezers embedded in a Sagnac interferometer

TL;DR

This study demonstrates that embedding optical tweezers in a Sagnac interferometer significantly enhances particle position detection sensitivity, enabling more precise measurements of particle motion in optical trapping experiments.

Contribution

The paper extends existing theory to larger particles and experimentally confirms a over 30-fold improvement in signal-to-noise ratio using Sagnac-enhanced interferometry.

Findings

Over 5 times improvement in signal-to-background ratio

More than 30-fold enhancement in signal-to-noise ratio

Successful experimental validation of theoretical predictions

Abstract

We have constructed a counterpropagating optical tweezers setup embedded in a Sagnac interferometer in order to increase the sensitivity of position tracking for particles in the geometrical optics regime. Enhanced position determination using a Sagnac interferometer has previously been described theoretically by Taylor et al. [Journal of Optics 13, 044014 (2011)] for Rayleigh-regime particles trapped in an antinode of a standing wave. We have extended their theory to a case of arbitrarily-sized particles trapped with orthogonally-polarized counterpropagating beams. The working distance of the setup was sufficiently long to optically induce particle oscillations orthogonally to the axis of the tweezers with an auxiliary laser beam. Using these oscillations as a reference, we have experimentally shown that Sagnac-enhanced back focal plane interferometry is capable of providing an improvement of more than 5 times in the signal-to-background ratio, corresponding to a more than 30-fold improvement of the signal-to-noise ratio. The experimental results obtained are consistent with our theoretical predictions. In the experimental setup, we used a method of optical levitator-assisted liquid droplet delivery in air based on commercial inkjet technology, with a novel method to precisely control the size of droplets.