Xue-Bin Bian

High-order harmonic generation (HHG) in liquids exhibits striking characteristics that differ fundamentally from those in gases and crystalline solids, yet its microscopic origin remains elusive due to the internal complexity and disorder in liquid structures. The hydrogen bond (HB), responsible for many unique physical and chemical properties of liquids, is expected to play important roles in liquid HHG. However, how HB interactions are encoded in liquid HHG spectra remains largely unexplored. To solve these longstanding problems, here we propose an energy-space quantum multichannel transition model to account for the mechanism of liquid HHG, which is validated by the quantitative agreement with recent experimental measurements. We identify an electronic channel associated with HB interactions in liquid water, where HB-related modifications of the electronic states give rise to distinctive harmonic features that differ from those in other phases. Both the ab initio simulation and the simplified HB model capture this behavior and confirm the essential role of the HB network. Our results establish a universal microscopic framework for understanding liquid HHG and can be naturally generalized to interpret harmonic features in other liquids. It provides an HHG spectroscopic tool for probing HB-related chemical and biological processes in liquids.

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.

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.

Symmetry plays a central role in fundamental physics. Here we identify a phase-jump phenomenon in high-order harmonic generation in symmetry-breaking crystals. This phenomenon originates from the singularity in the effective mass of electron-hole pairs at nonzero-distance collisions, a factor usually ignored in the literature. However, it determines the basic properties of harmonic emissions. This phase-jump phenomenon results in a suppression of the harmonic spectrum by orders, which is strongly orientation dependent. It is universal and not limited to a specific crystal. Clues to this can be found in published ab initio simulations and experimental results, but not interpreted. Our findings pave the way for a better understanding of the ultrafast dynamics of electron-hole pairs in XUV photon emission.

Electron diffraction spectroscopy is a fundamental tool for investigating quasicrystal structures, which unveils the quasiperiodic long-range order. Nevertheless, it falls short in effectively distinguishing separate local isomorphism classes. This is a long outstanding problem. Here, we study the high-order harmonic generation in two-dimensional generalized Penrose quasicrystals to optically resolve different local isomorphism classes. The results reveal that: (i) harmonic spectra from different parts of a quasicrystal are identical, even though their atomic arrangements vary significantly. (ii) The harmonic yields of diverse local isomorphism classes exhibit variations, providing a way to distinguish local isomorphism classes. (iii) The rotational symmetry of harmonic yield can serve as a characteristic of quasicrystal harmonics and is consistent with the orientation order. Our results not only pave the way for confirming the experimental reproducibility of quasicrystal harmonics and identifying quasicrystal local isomorphism classes, but also shed light on comprehending electron dynamics influenced by the vertex environments.