Optical tweezers setup with optical height detection and active height regulation under white light illumination

An optical tweezers setup with optical detection in three dimensions and active height regulation has been developed. The presented novel method to determine the relative height of a microparticle from its microscopic image is based on the analysis of the integrated light intensity of the main maximum of the diffraction pattern. After the determination of a master curve as reference, the height can be detected with an accuracy of up to 2 nm. The method is applicable under microscopic white light illumination and is simple to implement. As an example of measurements where active height regulation is indispensable, force–distance curves are discussed. Furthermore, the colloid height is calculated geometrically. In the range where the geometrical estimation provides reliable results, the values are found to be in quantitative agreement with the suggested algorithm