Diffusion in Laser Gradient Field Studied by Fluorescence Correlation Spectroscopy

We discuss the effects of a laser gradient field on particle diffusion in solvent. A tightly focused laser beam may change both the local concentration and the diffusion time obtained in fluorescence correlation spectroscopy (FCS). A quantitative relation between the gradient field strength and the local particle concentration has been derived. Using the data generated by Monte Carlo simulations, we demonstrate that the quantitative relation can be used to obtain number density and polarizability of the particles by FCS experiments in the laser gradient field. A nonlinear growth of diffusion time with laser power is also shown by both the theoretical analysis and the simulations.