Nonlinear ellipsometry and reflectometry of second harmonic generation: a new tool for photonics materials

Reflectivity of second-harmonic generation (R-SHG) has emerged as a powerful nonlinear optical technique for probing surfaces and thin films, where conventional bulk characterization methods often fail due to limited interaction volumes. In this work, we present a polarization-resolved nonlinear ellipsometry approach based on R-SHG, designed to provide rapid, non-destructive, and surface-sensitive access to the second-order nonlinear optical response of thin-film materials. We first establish the theoretical framework linking crystal symmetry, second-order susceptibility tensor components, and polarization-dependent R-SHG signals. An experimental polarization-resolved R-SHG setup is then introduced and validated using an isotropic glass reference sample, demonstrating excellent agreement with theoretical expectations. The sensitivity of the technique is further assessed through R-SHG mapping of molecular monolayers with controlled surface coverage, showing quantitative detection down to the monolayer regime. Finally, the capabilities of the technique are illustrated through large-area spatial mapping of thin-film lithium niobate (TFLN). By exploiting symmetry-specific polarization configurations, the method enables clear discrimination between samples fabricated by different processes, revealing variations in crystalline quality and spatial homogeneity of the nonlinear response. Overall, this work demonstrates that polarization-resolved R-SHG provides a versatile and robust metrology tool for characterizing nonlinear optical properties, crystallinity, and homogeneity in advanced photonic thin films, with strong potential for both fundamental studies and industrial process monitoring.