Optical tracking of single Brownian particles.

A random access scanning microscope system was built to track the Brownian motion of submicrometer colloidal particles in all three dimensions. Transverse tracking resolutions of 20 nm were achieved for 1-ms clock rates. Based on mean-square displacements, the diffusion coefficient and hence the size of the individual particles was determined. Even better size resolution was demonstrated through the measurement of an axial drift motion, which was induced by radiation pressure. This experiment was simplified, if the particles could be trapped transversally by ponderomotive optical forces. The degree of trapping was directly observable from histograms of particle positions.