Phase-matching mechanism for high-harmonic generation in the overdriven regime driven by few-cycle laser pulses

TL;DR

This paper investigates the phase-matching mechanism enabling high-harmonic generation in the overdriven regime with few-cycle laser pulses, demonstrating how plasma effects facilitate isolated attosecond pulse production at high photon energies.

Contribution

It reveals the role of plasma-induced phase-matching gating in generating IAPs in the overdriven regime, supported by experiments and simulations across different gases and wavelengths.

Findings

IAPs in argon are generated via ionization-induced transient phase-matching gating.

Plasma defocussing and blue-shift are crucial for phase-matching at high plasma densities.

The mechanism is applicable for scaling attosecond pulses to x-ray energies using high-pressure gas targets.

Abstract

Isolated attosecond pulses (IAPs) produced through laser-driven high-harmonic generation (HHG) hold promise for unprecedented insight into biological processes via attosecond x-ray diffraction with tabletop sources. However, efficient scaling of HHG towards x-ray energies has been hampered by ionization-induced plasma generation impeding the coherent buildup of high-harmonic radiation. Recently, it has been shown that these limitations can be overcome in the so-called 'overdriven regime' where ionization loss and plasma dispersion strongly modify the driving laser pulse over small distances, albeit without demonstrating IAPs. Here, we report on experiments comparing the generation of IAPs in argon and neon at 80 eV via attosecond streaking measurements. Contrasting our experimental results with numerical simulations, we conclude that IAPs in argon are generated through ionization-induced transient phase-matching gating effective over distances on the order of 100 $\mu$m. We show that the decay of the intensity and blue-shift due to plasma defocussing are crucial for allowing phase-matching close to the XUV cutoff at high plasma densities. We perform simulations for different gases and wavelengths and show that the mechanism is important for the phase-matching of long-wavelength, tightly-focused laser beams in high-pressure gas targets, which are currently being employed for scaling isolated attosecond pulse generation to x-ray photon energies.