Intensity-dependent speckle contrast of second harmonic light in a nonlinear disordered medium

Coherent light propagation and scattering in diffuse media realize an interference effect that manifests as a random spatial distribution of bright and dark spots called speckle. Laser speckle contrast is a crucial parameter that quantifies light behavior in diffuse media, often of help in biomedical imaging applications. With several ultra-intense lasers coming onto the scene in recent years, the probability of nonlinearly generating harmonic light has considerably increased. To our knowledge, there are no studies on the simultaneous measurement of local speckle contrast (LSC) at fundamental and harmonic wavelengths. In this work, we experimentally characterize the intensity-dependent global and local speckle contrast at λ=1064nm (IR) and λ=532nm [second-harmonic generation (SHG)] in a diffuse sample made of potassium dihydrogen phosphate. Under pulsed excitation with IR light, second-harmonic light is generated and undergoes scattering, leading to speckle at both wavelengths. The radiation pressure on the sample induces displacement of the particles of the medium, which leads to the temporal averaging and hence reduction in speckle contrast of IR. The decrease in speckle contrast with input power is consistent with the theoretically predicted behavior. Next, the probability distribution of LSC for IR and SHG are observed to follow log-normal distributions with distinctive parameters, measured for co-polarized and counter-polarized channels. Parameter evolution with box-size is fit to the established global model for speckle contrast and also another alternative model. The fit parameters, illustrated as a function of input power, indicate possibilities of widening the global model to second-harmonic light.