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

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.

A remote laboratory for optical levitation of charged droplets

“…For instance, the set-up has been used to study the collision of micrometersized droplets using high-speed cameras [18]. Further, the same experimental platform has been employed to construct a sensitive way to track the position of particles using a Sagnac interferometer [19].…”

Optical manipulation for studies of collisional dynamics of micron-sized droplets under gravity

“…We generate droplets in the size range from 4 to 60 µm in diameter using a commercial printer cartridge (Hewlett-Packard C6614). The cartridge works as a drop-on-demand system, where a droplet is ejected each time a TTL pulse is sent to the cartridge from a pulse generator (see [17] and [18] and references within for the droplet generation technique). The cartridge, washed and filled with a solution mixed with 90% (by volume) of distilled water and 10% of glycerol (produced by Fisher Scientific with a purity of 99.6%), produces droplets with a uniform diameter of approximately 22 µm.…”