High-harmonic generation in liquids with few-cycle pulses: effect of laser-pulse duration on the cut-off energy

TL;DR

This study investigates high-harmonic generation in liquids using few-cycle pulses, revealing that the cut-off energy remains unaffected by pulse duration and confirming the fundamental nature of this property in liquids.

Contribution

It demonstrates that the HHG cut-off energy in liquids is independent of pulse duration, unlike in gases, and confirms this through ab-initio simulations, advancing understanding of HHG in liquids.

Findings

Cut-off energy is independent of pulse duration in liquids.

Transition from discrete harmonics to continuous emission with shorter pulses.

Few-cycle pulses can generate isolated attosecond pulses from liquids.

Abstract

High-harmonic generation (HHG) in liquids is opening new opportunities for attosecond light sources and attosecond time-resolved studies of dynamics in the liquid phase. In gas-phase HHG, few-cycle pulses are routinely used to create isolated attosecond pulses and to extend the cut-off energy. Here, we study the properties of HHG in liquids, including water and several alcohols, by continuously tuning the pulse duration of a mid-infrared driver from the multi- to the sub-two-cycle regime. Similar to the gas phase, we observe the transition from discrete odd-order harmonics to continuous extreme-ultraviolet emission. However, the cut-off energy is shown to be entirely independent of the pulse duration. This observation is confirmed by ab-initio simulations of HHG in large clusters. Our results support the notion that the cut-off energy is a fundamental property of the liquid, independent of the driving-pulse properties. Combined with the recently reported wavelength-independence of the cutoff, these results confirm the direct sensitivity of HHG to the mean-free paths of slow electrons in liquids. Our results additionally imply that few-cycle mid-infrared laser pulses are suitable drivers for generating isolated attosecond pulses from liquids.