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 counter-propagating 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.
Highlights
The techniques of optical levitation and optical tweezers were introduced by Ashkin et al in 1971 and 1986, respectively [1, 3]
We have extended the theory presented by Taylor et al [23] to a case of arbitrarily-sized spherical particles trapped in orthogonally-polarized counter-propagating optical tweezers
We experimentally demonstrated a significant improvement in the signal-to-background ratio for position detection of 11 μm-sized particles by embedding counterpropagating optical tweezers into a Sagnac interferometer
Summary
The techniques of optical levitation and optical tweezers were introduced by Ashkin et al in 1971 and 1986, respectively [1, 3]. A weakly focused, vertically aligned laser beam creates an optical pressure force on a dielectric particle which can become sufficiently strong to compensate gravity. A weak gradient force provides stability in the horizontal plane, creating a stable trap [2]. Optical levitation does not require high numerical apertures (NA) for trapping. Is the large working distance of the focusing optics. This property has been used, for example, to determine the electric charge of levitated oil droplets [12]
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