Multi-plateau high-harmonic generation in liquids driven by off-site recombination.

Area of Science:

• Attosecond science
• Nonlinear optics
• Quantum dynamics

Background:

• Non-perturbative high-harmonic generation (HHG) in liquids differs from gases/solids.
• Current models involve recollision with scattering, explaining cut-off energy but not intensity independence.
• Increased laser intensity typically extends cut-off energy, contrary to liquid HHG observations.

Purpose of the Study:

• Investigate the origin of a second plateau in liquid-phase HHG.
• Explore the underlying physical mechanism responsible for this phenomenon.
• Determine the role of electron recombination sites and solvation shells.

Main Methods:

• Experimental observation of HHG in various liquids (water, D2O, propanol, ethanol).
• Computational modeling to simulate electron trajectories and recombination dynamics.
• Theoretical analysis correlating HHG yields with laser ellipticity and recombination sites.

Main Results:

• Observed a distinct second plateau in HHG spectra from multiple liquids.
• Identified electron recombination at neighboring molecular sites as the cause, not the ionization site.
• Confirmed this mechanism via the second-plateau yield's dependence on driving field ellipticity.
• Found the second plateau is influenced by recombination within the first/second solvation shells, indicating hole delocalization.

Conclusions:

• Established a new physical phenomenon in the nonlinear optical response of liquids.
• Demonstrated that recombination at neighboring sites, facilitated by hole delocalization, generates the second plateau.
• Theoretical predictions suggest the possibility of even higher plateaus, indicating a general trend in liquid HHG.