Mitigating thermal and nonlinear lensing for optimized performance in nonlinear optical scattering

Nonlinear optical light scattering is used in a wide range of applications, from screening organic electro-optic materials to analyzing the structure and chemistry of buried interfaces in colloidal and biological systems. It is also applied in real-time studies, such as monitoring nucleation and crystal growth. Given that NLO effects are intrinsically weak, experiments require high laser power to achieve measurable signals. However, the onset of nonlinear effects, such as thermal and Kerr lensing, limits the maximum laser power that can be applied, which in turn limits the signal strength. In this study, we systematically investigated the impact of thermal and Kerr lensing on hyper-Rayleigh scattering experiments in pure water and a methanolic solution of para-nitro aniline, using an amplified femtosecond laser system with medium-range repetition rates (100–500 kHz). Through systematic evaluation of focusing optics, cuvette path length, and laser repetition rate and carefully managing these effects we achieved significant gains in signal strength, allowing sub-millisecond measurements for pure water and para-nitroaniline, while avoiding photodegradation of the organic compound. These findings present a practical route to harness the available power more effectively, by balancing intensity and undesired lensing effects. This optimization is crucial for advancing the capabilities of nonlinear optical scattering techniques, enabling time-resolved measurements across a wide range of chemical and biological systems, from aggregation and nucleation to the dynamic measurement of adsorption, surface potentials, hydrogen-bonding networks. Our results also extend to biomedical applications such as real-time volumetric imaging of tissue morphology, label-free monitoring of cellular metabolism and dynamic processes in living tissues and organisms.