Understanding second-harmonic scattering from water
Machine-learning the second-harmonic response of water molecules © COSMO / 2017 EPFL
Second-harmonic Scattering is an exquisitely sensitive technique to probe molecular correlations in solutions, at interfaces and close to nanoparticles. Recent work, in collaboration between the Laboratory of Computational Science and Modelling and the Laboratory for Fundamental Biophotonics have made progress towards quantitative modelling, by combining large-scale molecular dynamics simulations, mean-field models, and machine-learning predictions of the non-linear optical response of water molecules.
Second-Harmonic Scatteringh (SHS) experiments provide a unique approach to probe noncentrosymmetric environments in aqueous media, from bulk solutions to interfaces, living cells and tissue. Modelling of experiments has been based on several assuptions: for instance, the signal generated by water and other bulk molecular liquids have long been assumed to be insensitive to interactions between the molecules. The measured intensity is generally thought to arise from incoherent scattering due to individual molecules. Furthermore, it is generally assumed that each molecule scatters light according to a constant molecular hyperpolarizability tensor.
Recent work performed in collaboration between the Laboratory of Computational Science and Modelling and the Laboratory for Fundamental Biophotonics have started to put these assumptions to the test, at the same time developing modelling techniques that can account fully for the correlations that exist in dipolar fluids, and for the strong dependence of the molecular non-linear response on the environment, and the quantum mechanical fluctuations of the atoms. Quantitative modelling of the SHS measurements might soon be possible, making it possible to use this sensitve technique to investigate solutions, interfaces, and biological systems.