An optical tweezer is able to trap small dielectric particles like micrometer-sized glass spheres by a tightly focused laser beam. The transmitted light is used to measure very small displacements of the trapped microspheres with position sensitive detectors. We established an optical tweezers setup that is capable of detecting the movements of molecular motors on DNA. The setup has two traps where one can be moved with respect to the other using a Piezo-driven rotatable mirror. In a typical measurement, an enzyme is tied to the microspheres and its position can be measured under different external forces. Especially, the ability to generate external forces by the optical traps provides valuable information about the enzymatic cycle which is often not accessible by other single-molecule techniques.
Michaelis J., Muschielok, A., Andrecka, J., Kügel, W., and Moffitt, J.R. Physics of Life Reviews (2009), 6,250-266