Nonlinear light scattering from clusters and single particles.

We present sum-frequency-scattering experiments on colloidal dispersions with various concentrations and in different scattering geometries. At small scattering angles, large fluctuations are observed in the intensity of the scattered sum-frequency photons. By considering the angular dependence of the signal, the particle concentration dependence, and the surface vibrational spectra of the particle, we have determined that the fluctuations are caused by scattering from clusters of particles. We further demonstrate that dynamic nonlinear light scattering may be used to measure the size of the correlated particle clusters.