WO2009065519A8 - Method for measurement of the force which acts on an object which is held in optical tweezers/an optical catch, and optical tweezers/an optical catch - Google Patents

Abstract

The invention relates to a method for measurement of the force which acts on an object which is held in optical tweezers/an optical catch, in which the light which is scattered back from the held object through the objective of the optical tweezers/catch is detected by a detector and the detector signal is used to determine the force in the case of which a laser beam (1) which is used to produce the optical tweezers/catch and is polarized linearly in a first direction passes through a polarizing beam splitter (2) then passes through a polarization-rotating element (8) and an objective (3), wherein the laser beam (1) is circular-polarized by the polarization-rotating element (8) and is focussed by the objective (3) and holds an object in a focus area, and wherein, furthermore, light which is scattered back from the held object through the objective (3) from the same laser beam (1) passes through the polarization-rotating element (8) in the opposite direction, is linear-polarized by this polarization-rotating element (8) in a second direction at right angles to the first direction, and passes through the polarizing beam splitter (2), wherein the polarizing beam splitter (2) separates the illuminating laser beam (1) and the back-scattered laser light, and the back-scattered laser light is detected by means of the detector (7). The invention also relates to optical tweezers/an optical catch having an objective (3) for focussing of laser radiation (1) for holding an object in the focus area of this laser radiation (1) and to a detector (7) for detection of a detector signal produced by the laser light which is scattered back onto the detector (7) from an object through the objective (3), wherein a force which acts on an object can be determined on the basis of the detector signal, which force comprises, in a common area of the beam path of the incident laser beam and the back-scattered laser beam, a polarizing beam splitter (2) and a polarization-rotating element (8) which follows the beam splitter (2) in the direction of the illuminating laser beam (1), in order to form an optical switch between incident laser radiation (1) and the laser light which is scattered back on the object through the objective (3), wherein the detector (7) is arranged in the beam path of the back-scattered light, which beam path is separate from the incident laser light (1).