Polarization characterization in surface second harmonic generation by nonlinear optical null ellipsometry Original Research Article

Nonlinear optical null ellipsometry (NONE) is developed as a novel approach to evaluate second-order nonlinearities on and off resonance in transparent and absorbing thin surface films. Despite the growing use of second harmonic and sum frequency spectroscopy in characterizing interfaces, common approaches designed to extract the χ(2) nonlinear susceptibility tensor elements from relative intensity measurements are generally only reliable for ultra-thin transparent films at the interface between transparent media. The simple expressions routinely used to treat intensity-based polarization measurements far from resonance become significantly more complicated on or near resonance, for which the χ(2) tensor elements and the thin film refractive indices are complex-valued parameters. In contrast, the mathematical formalism developed for NONE measurements is well suited for spectroscopic studies. The combined results of NONE and polarization-dependent intensity (PDI) measurements at fused silica/dye solution interfaces demonstrate that NONE retains complex phase information between the χ(2) nonlinear susceptibility tensor elements that is lost using comparable intensity-based approaches.