Liquid Lasers Reveal Multiple Paths for Ultrafast Attosecond Spectroscopy Advances

Researchers investigating high-harmonic generation (HHG) from liquids have long sought to extend attosecond spectroscopy to the study of complex chemical systems. Wanchen Tao, Ruisi Zhang, Qihe Guo, et al. from the Wuhan National Laboratory for Optoelectronics and the China University of Geosciences now demonstrate clear separation of quantum trajectories within liquid HHG, resolving a key question regarding attosecond synchronisation and multi-trajectory contributions. Their experiments, achieved through optimised laser focusing and phase-controlled two-colour driving fields, provide direct evidence for distinct short- and long-trajectory pathways and establish a trajectory-resolved energy-time mapping. This work confirms that liquid HHG operates under the same principles as its gaseous and solid-state counterparts, paving the way for robust attosecond-resolved studies of ultrafast processes in liquid environments.

This work demonstrates, for the first time, direct evidence of multiple quantum trajectories contributing to harmonic emission within a liquid, despite inherent disorder and scattering.

By carefully optimising laser focusing geometry, the team distinguished between contributions from electrons following short and long paths, revealing that both persist and play a role in the harmonic signal. Utilising a phase-controlled two-colour driving field, they independently measured the attosecond chirp, the energy-dependent time delay, associated with each trajectory.
Observations revealed opposite energy-time correlations for short and long trajectories, establishing a trajectory-resolved energy-time mapping in liquid high-harmonic generation. This precise mapping allows researchers to link the energy of emitted photons to the timing of electron recollision with the parent ion, a crucial step for quantitative attosecond spectroscopy.

Semiclassical recollision simulations accurately reproduced all experimental findings, validating the interpretation of the observed phenomena. These results place liquid high-harmonic generation on a comparable footing with its gas and solid-state counterparts, solidifying a robust foundation for probing ultrafast electronic and chemical dynamics in liquid environments.

The ability to resolve and control quantum trajectories within liquids opens new avenues for studying complex systems, including chemical reactions in solution and biological processes. This advancement promises to unlock a deeper understanding of fundamental dynamics occurring on the attosecond timescale within disordered and chemically rich media.

Spatially resolved harmonic generation characterises trajectory dynamics in liquids with high precision

High-harmonic generation (HHG) experiments were conducted to investigate trajectory-dependent temporal structures within liquid environments. The research focused on spatially discriminating between short- and long-trajectory contributions by carefully optimising the laser focusing geometry. This precise control enabled direct observation of multiple trajectories occurring simultaneously within the liquid sample.

A phase-controlled two-color driving field was implemented to independently retrieve the attochirp associated with each trajectory, allowing for detailed analysis of their temporal characteristics. Measurements revealed opposite energy-time correlations for the short and long trajectories, establishing a trajectory-resolved energy-time mapping in liquid HHG.

This mapping demonstrates how different electron trajectories contribute to the harmonic signal with distinct temporal properties. The experimental setup relied on generating HHG signals from liquids, a challenging environment for attosecond spectroscopy due to inherent disorder and scattering. By resolving the temporal structure of these signals, the study aimed to determine if liquids could support well-defined attosecond synchronization, a crucial requirement for time-resolved measurements.

Semiclassical recollision simulations were used to corroborate the experimental observations, validating the interpretation of trajectory-dependent harmonic emission. These simulations accurately reproduced the measured attochirps and energy-time correlations, confirming the theoretical understanding of the process. The work establishes a conceptual link between liquid HHG and its counterparts in gas and solid phases, paving the way for robust attosecond-resolved spectroscopy of ultrafast electronic and chemical dynamics in liquid environments.

Trajectory separation and characterisation in liquid high-harmonic generation are crucial for understanding the process

Researchers demonstrated spatial discrimination of short- and long-trajectory contributions in liquid high-harmonic generation, providing direct evidence for the existence of multiple quantum trajectories within liquids. Optimisation of the laser focusing geometry facilitated clear separation of these trajectories at the generation plane.

The study independently retrieved the attochirp associated with each trajectory using a phase-controlled two-color driving field, revealing opposite energy, time correlations for short and long trajectories. This established a trajectory-resolved energy, time mapping in liquid HHG, a crucial step towards attosecond spectroscopy.

Spatial measurements of harmonic emission revealed distinct signatures corresponding to different quantum pathways. Far-field spectra simulated for a large beam size differed significantly from those obtained with a small beam size, indicating sensitivity to trajectory characteristics. Time, frequency spectrograms of on-axis harmonic emission further highlighted these differences, showing variations in temporal structure dependent on beam parameters.

Off-axis harmonic emission at 5 milliradians exhibited a distinct temporal profile, confirming the spatial separation of short and long trajectories. All observations were successfully reproduced by semiclassical recollision simulations, validating the experimental findings. These simulations corroborated the existence of multiple trajectories and their associated temporal characteristics in the liquid environment.

The work places liquid HHG on a comparable conceptual footing with gas- and solid-phase HHG, establishing a robust foundation for attosecond-resolved spectroscopy of ultrafast electronic and chemical dynamics in liquid environments. This advancement opens possibilities for studying complex systems with unprecedented temporal resolution.

Trajectory mapping elucidates attosecond dynamics in liquid harmonic generation by revealing the pathways of electron motion

Researchers have demonstrated trajectory-resolved high-harmonic generation in liquids, establishing a firm conceptual foundation comparable to that established in gases and solids. By optimising laser focusing geometry, clear spatial discrimination between short and long quantum trajectories was achieved, providing direct evidence for the existence of multiple trajectories within liquid environments.

Independent retrieval of the attosecond chirp associated with each trajectory revealed opposite energy-time correlations for short and long trajectories, establishing a trajectory-resolved energy-time mapping in liquid high-harmonic generation. These findings resolve a long-standing question regarding liquid high-harmonic generation and validate the preservation of well-defined attosecond synchronization within liquids.

The observed ability to separate and characterise short and long trajectories suggests that liquid-phase high-harmonic generation is sensitive to electron-environment interactions, offering potential for probing characteristic length scales such as the electron mean free path. While acknowledging that long trajectories can be suppressed by mid-infrared fields due to wave-packet spreading, the use of shorter wavelengths enables their survival in disordered liquids. Future research will likely focus on combining trajectory separation with wavelength-tunable driving fields to systematically control electron excursion time and return energy, ultimately establishing attosecond high-harmonic spectroscopy as a tool for tracking ultrafast electronic and chemical dynamics in complex liquid environments.