Localized Dynamic Light Scattering: Probing Single Particle Dynamics at the Nanoscale

We developed an experimental technique which probes the dynamics of a single colloidal particle over many decades in time, with spatial resolution of a few nanometers. By scattering a focused laser beam from a particle observed in an optical microscope, we measure its fluctuations via the temporal autocorrelation function of the scattered intensity $g(t)$. This technique is demonstrated by applying it to a single Brownian particle in an optical trap of force constant k. The decay times of $g(t)$, which are related to the particle position autocorrelation function, scale as k^-1, as expected from theory.