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Potential mapping of optical tweezers.

Optical tweezers are very often used for measurement of piconewton range forces. Depending on the displacement of the trapped bead, the trap may become stiffer which causes considerable underestimation of the measured force. We have shown, both by theory and experiment, that such a stiffening occurs for beads larger than 0.5 μm in radius. For the first time, we have shown that the displacement at which the stiffening starts is size dependent and that the stiffening starts at higher forces for larger beads. We have shown that for the applications, which simultaneous force measurement and position sensing are on demand (such as biopolymer stretching), mid-range sized (∼1.5 μm in radius) beads could be the best choice.