title: Measuring the complete force field of an optical trap.
abstract: The use of optical traps to measure or apply forces on the molecular level requires a precise knowledge of the trapping force field. Close to the trap center, this field is typically approximated as linear in the displacement of the trapped microsphere. However, applications demanding high forces at low laser intensities can probe the light-microsphere interaction beyond the linear regime. Here, we measured the full nonlinear force and displacement response of an optical trap in two dimensions using a dual-beam optical trap setup with back-focal-plane photodetection. We observed a substantial stiffening of the trap beyond the linear regime that depends on microsphere size, in agreement with Mie theory calculations. Surprisingly, we found that the linear detection range for forces exceeds the one for displacement by far. Our approach allows for a complete calibration of an optical trap.
authors.affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, 01307 Dresden, Germany.
authors.forename: Marcus
authors.initials: M
authors.lastname: Jahnel
authors.forename: Martin
authors.initials: M
authors.lastname: Behrndt
authors.forename: Anita
authors.initials: A
authors.lastname: Jannasch
authors.forename: Erik
authors.initials: E
authors.lastname: Schäffer
authors.forename: Stephan W
authors.initials: SW
authors.lastname: Grill
mesh_terms: D007834:Lasers; D008863:Microspheres; D052898:Optical Tweezers