Ab-initio cluster approach for high-harmonic generation in liquids

TL;DR

This paper introduces an ab-initio cluster approach based on density functional theory to model high-harmonic generation in liquids, providing new insights into their nonlinear optical responses and spectral features.

Contribution

It develops a reliable, ab-initio method for HHG in liquids, bridging the gap between gases, solids, and liquids, and demonstrates its effectiveness with multiple liquids.

Findings

Identification of a structural minima at 15-17 eV in liquid methane

Discovery of a shape resonance-like feature in the HHG spectra

Comparison of HHG responses between polar and non-polar liquids

Abstract

High harmonic generation (HHG) takes place in all phases of matter. In gaseous atomic and molecular media, it has been extensively studied and is very well-understood. In solids research is ongoing, but a consensus is forming for the dominant microscopic HHG mechanisms. In liquids on the other hand, no established theory yet exit and approaches developed for gases and solids are generally inapplicable, hindering our current understanding. We develop here a powerful and reliable ab-initio cluster-based approach for describing the nonlinear interactions between isotropic bulk liquids and intense laser pulses. The scheme is based on time-dependent density functional theory and utilizes several approximations that make it feasible yet accurate in realistic systems. We demonstrate our approach with HHG calculations in water, ammonia, and methane liquids, and compare the characteristic response of polar and non-polar liquids. We identify unique features in the HHG spectra of liquid methane that could be utilized for ultrafast spectroscopy of its chemical and physical properties: (i) a structural minima at 15-17eV, and (ii) a well-like shape in the perturbative region that is reminiscent of a shape resonance. Our results pave the way to accessible calculations of HHG in liquids and illustrate the unique nonlinear nature of liquid systems.