Optical tweezers use highly focussed beams to trap microscopic particles in three dimensions. It is possible to carry out quantitative force measurements, on the order of piconewtons, if calibration of the system is done first. This requires finding the optical force for a given trapping power and position in the trap. Two tools commonly used for calibration are the camera and position-sensitive detector (PSD). Both are commonly used to track trapped particles, but they give complementary information. The camera gives the position of the particle. The PSD measures the defection of the beam, which is the force exerted on the particle. Since these data are obtained on different instruments, usually at vastly different rates, there is difficulty in synchronising the force and position data. Here we look at a force calibration method, without synchronising the data, by mapping force and position measurements. If the force-position relation is monotonic, then the median of the force distribution corresponds to the median of the position distribution; in general, the nth percentile of one corresponds to the (100-n)th percentile of the other. This intuitively works for traps whose force-position relations are monotonic, which includes Hookean traps like a single round symmetric trap. We discuss the limits at which this method can be applied to non-Hookean trapping arrangements, such as independent or coherent double-well traps.
Ann A. Bui, Alexander B. Stilgoe, Timo A. Nieminen, and Halina Rubinsztein-Dunlop, "Mapping of independent force and position measurements for calibration of non-Hookean optical traps (Presentation Recording)," Proc. SPIE 9548, Optical Trapping and Optical Micromanipulation XII, 95480X (Presented at SPIE Nanoscience + Engineering: August 11, 2015; Published: 5 October 2015); https://doi.org/10.1117/12.2186721.4519370463001.
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