We show how measurements of forces through the analysis of light momentum changes can be combined with holographic optical tweezers (HOTs) to leverage the potential of this force detection method. As the magnitude is not derived from the sample displacement, no in situ calibration is required, and measurements are not restricted to specific conditions. In particular, we show that forces on irregular particles and beams can also be measured with optical traps by simultaneously applying a force in the same direction to multiple holographically-trapped particles through a constant flow. Finally, we measure forces exerted on micro-cylinders in order to assess their transversal and longitudinal drag coefficients.
In this work, we present and discuss several developments implemented in an instrument that uses the detection of the light momentum change for measuring forces in an optical trap. A system based on this principle provides a direct determination of this magnitude regardless of the positional response of the sample under the effect of an external force, and it is therefore to be preferred when in situ calibrations of the trap stiffness are not attainable or are difficult to achieve. The possibility to obtain this information without relying upon a harmonic model of the force is more general and can be used in a wider range of situations. Forces can be measured on non-spherical samples or non-Gaussian beams, on complex and changing environments, such as the interior of cells, or on samples with unknown properties (size, viscosity, etc.). However, the practical implementation of the method entails some difficulties due to the strict conditions in the design and operation of an instrument based on this method. We have focused on some particularly conflicting points. We developed a process and a mechanism to determine and systematically set the correct axial position of the device. We further analyzed and corrected the non-uniform transmittance of the optical system and we finally compensated for the variations in the sensor responsivity with temperature. With all these improvements, we obtained an accuracy of ~5% in force measurements for samples of different kinds.
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