Nonlinear optical characterization of opaque and scattering media: from rough surfaces to powder

We introduce the reflection intensity correlation scan (RICO-scan), a nonlinear (NL) optical technique designed to characterize opaque and scattering media, where traditional transmittance methods fail. By analyzing variations in the intensity correlation functions of speckle patterns generated from backscattered light, the RICO-scan was applied to an unpolished silicon surface and silicon powders, providing information on the intensity dependence of the complex refractive index. Numerical simulations based on Fresnel equations and speckle propagation corroborated the experimental results, demonstrating RICO-scan's robustness and versatility. The RICO-scan is the first, to the best of our knowledge, NL optical technique capable of characterizing the third-order nonlinearity of powdered materials, offering a reliable tool for studying disordered complex systems.