Abstract
A major problem with holographic optical tweezers (HOTs) is their incompatibility with laser-based position detection methods, such as back-focal-plane interferometry (BFPI). The alternatives generally used with HOTs, like high-speed video tracking, do not offer the same spatial and temporal bandwidths. This has limited the use of this technique in precise quantitative experiments. In this paper, we present an optical trap design that combines digital holography and back-focal-plane displacement detection. We show that, with a particularly simple setup, it is possible to generate a set of multiple holographic traps and an additional static non-holographic trap with orthogonal polarizations and that they can be, therefore, easily separated for measuring positions and forces with the high positional and temporal resolutions of laser-based detection. We prove that measurements from both polarizations contain less than 1% crosstalk and that traps in our setup are harmonic within the typical range. We further tested the instrument in a DNA stretching experiment and we discuss an interesting property of this configuration: the small drift of the differential signal between traps.
Highlights
Multiple laser beams are increasingly required in many optical trapping experiments [1]
As pointed out by Moffitt et al [22], the resistance found to holographic optical tweezers (HOTs) turning into a standard technique outside their own field of application is based on the lack of a method for measuring the displacements and forces exerted on the trapped samples “that rivals the resolution and bandwidth offered by back-focal plane interferometry” [22, 23]
We limited the numerical aperture of the condenser lens used to collect the light scattered by the sample to an effective value of about 0.75, which is sufficient for accurate back-focal-plane interferometry (BFPI) position measurements
Summary
Multiple laser beams are increasingly required in many optical trapping experiments [1]. Acousto-optic deflectors (AODs) [6] or galvano-mirrors [7] are the preferred methods for steering the laser beam in two dimensions They show limitations for creating multiple traps by scanning one single laser through different positions (time-sharing), since the refreshing takes place at a limited frequency, which can produce significant position fluctuations [8]. As pointed out by Moffitt et al [22], the resistance found to HOTs turning into a standard technique outside their own field of application is based on the lack of a method for measuring the displacements and forces exerted on the trapped samples “that rivals the resolution and bandwidth offered by back-focal plane interferometry” [22, 23]. We present an optical trap design that combines the wide range of possibilities of HOTs for manipulation and the measurement of positions and forces through BFPI. Molecule experiment to assess its performance and discuss the long-term stability of the proposed configuration
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