7] Optical-trap force transducer that operates by direct measurement of light momentum

Abstract

This chapter describes the optical-trap force transducer that operates by the direct measurement of light momentum. By focusing a laser beam through a microscope objective, it has been found that particles with high indexes of refraction—such as glass, plastic, or oil droplets—are attracted to intense regions in the beam and could be held permanently at a focal point. Optical tweezers are useful in molecular and cell biology because several important forces are in an accessible piconewton (pN) range—for example, ligand-receptor binding, DNA stretching, protein unfolding, and molecular motor stall forces. Methods to measure such forces in the optical trap are under continuous development. The position of a spherical bead inside a trap can be measured with subnanometer precision by one of several optical methods, and so this technique has proven very effective. Methods to measure such forces in the optical trap have been under continuous development. Most force-measurement methods treat the optical trap as a harmonic potential well or virtual spring that pulls the bead toward the trap center.