Hamburg 2009 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 36: Laseranwendungen

Q 36.6: Talk

Wednesday, March 4, 2009, 17:45–18:00, Audi-A

Full spatial range characterization of transverse forces in optical tweezers — •Karl Probst, Márton Gelléri, and Carsten Fallnich — Institut für Angewandte Physik, WWU Münster, Corrensstraße 2-4, 48149 Münster

While the trapping forces applied by optical tweezers systems can in principle be calculated rigorously using Mie theory, in practice this possibility is limited by knowledge about the incident light distribution. A drawback of commonly applied methods employing Boltzmann statistics to characterize optical tweezers is that the displacement of particles relative to the trap center must never cross the position of maximum force or the particles are dropped from the trap.

We present a new scheme for the characterization of a single beam gradient trap that allows characterization of the radial trapping force up to several trap diameters. For this we set a trap in circular motion with a speed relative to the surrounding medium chosen so a particle is dropped from the trap with a low but finite probability and retrapped on the next cycle of the trap's motion.

A mathematical formulation describing the position, distribution width and amplitude of the particle count density in this experiment is supplied for the case of periodic sampling. First experimental results show that this method can be successfully applied for characterization of the transverse trapping force of a single beam gradient trap with a good reproduction of predictions made by rigorous theory.