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.