Unveiling unique ultrafast nonlinearities in liquid-phase high-order harmonic generation

TL;DR

This study uncovers unique ultrafast nonlinear optical behaviors in liquid-phase high-order harmonic generation, revealing distinct spectral features and underlying mechanisms that differ from gases and solids, with implications for probing liquid internal structures.

Contribution

It provides the first combined experimental and theoretical analysis of liquid-phase HHG, identifying unique spectral signatures and their origins related to electronic states and molecular interactions.

Findings

Liquid HHG exhibits spectral redshift and broadening distinct from gases and solids.

Redshift and broadening depend linearly on laser intensity.

Water shows stronger nonlinear response than ethanol.

Abstract

High-order harmonic generation (HHG) provides a powerful optical tool for probing ultrafast dynamics on the attosecond timescale. While its mechanisms in gases and solids are well-established, understanding nonlinear optical responses in liquids remains challenging. The absence of long-range order in liquids questions the applicability of the existing HHG models developed in other media. Through combined experimental and theoretical investigations, we identify unique characters of liquid-phase HHG -- spectral redshift and broadening, which are fundamentally distinct from both the gaseous and solid-state counterparts. Quantitative measurements and simulations of HHG in liquids illustrate a near linear dependence of harmonic redshift and broadening on the laser intensity, with the nonlinear response of water exceeding that of ethanol. The simulations reveal that these features arise from delocalized electronic states with energy loss in multiple scatterings and transient Stark shift during their transitions in laser fields. Meanwhile, we find that liquid polarity or hydrogen bond exerts decisive control over the transition dipole momentum distributions of delocalized states. Our findings establish a nonlinear spectral method for probing the internal network in liquids, paving the way for studying its role in chemical and biological processes.