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 $g\left(t\right)$. This technique is demonstrated by applying it to a single Brownian particle in an optical trap of force constant $k$. The decay times of $g\left(t\right)$, which are related to the particle position autocorrelation function, scale as $k^{-1\mathrm{}}$, as expected from theory.