Present-day methods for determining the performance of third-order nonlinear optical materials include Z-scan, degenerate four-wave mixing and third-harmonic generation (THG). All these techniques possess severe drawbacks; for example, in THG, since all media (air and glass walls of the cell) present a third-order effect, eliminating these contributions requires careful, complex analysis or use of vacuum chambers. We have developed nonlinear scattering as a sensitive, straightforward technique for determining the second hyperpolarizability of samples in solution. Herein, we will for the first time show the applicability of the technique to measure organometallic Ru-complexes, optimized for high nonlinear responses. The investigated compounds showed a significant second hyperpolarizability |γ|, ranging from 1.1 for the least efficient to 2.8 ∙ 10-33 esu for the most efficient molecule, and comparable to fullerene C60 in thin films. It was deemed infeasible to extract hyperpolarizabilities using a high-frequency femtosecond laser source by a modified z-scan setup, which, in contrast to nonlinear scattering, could not account for the high degree of thermal lensing present in the investigated compounds.
We studied the effects of two types of ultrasonic waves, shear waves and longitudinal waves, using two nonlinear optical techniques, second-harmonic generation and hyper-Rayleigh scattering. Since shear waves hardly propagate in liquids, their influence on molecules at the interface between a surface and a liquid was studied using second-harmonic generation. Longitudinal waves propagate easily in solution, thus we used hyper-Rayleigh scattering to probe the ultrasonic effects on chromophores in solution. While we did not find shear waves to alter the second-harmonic generation from chromophores at the liquid/surface interface, the longitudinal waves caused effects comparable to our earlier observations. Longitudinal ultrasound caused a strong intensity modulation of the nonlinear optical signal according to a wave-pattern.