Speckle correlation measurement in a disordered medium observed through second-harmonics generation

We examined speckle correlation in a disordered medium consisting of ${\mathrm{LiNbO}}_3$ corpuscles, by observing the total intensity of the second-harmonics (SH) light generated by angular correlated two excitation beams. The total SH intensity showed a correlation peak at θ∼0, where θ was the angle between two excitation beams, and decreased as θ was increased. The observation could be understood as the effect of the constructive interference between two volume speckle patterns produced by the excitation beams inside the medium, and the peak could reflect the mutual correlation between two patterns. The experimental results showed good agreements with a theoretical analysis based on a diffusion approximation.