High-order harmonic generation in argon driven by short laser pulses: effects of post-pulse propagation and windowing

TL;DR

This paper uses ab initio RMT calculations to study high-order harmonic generation in argon, revealing how post-pulse effects and analysis choices influence the observed spectra, especially near the ionization threshold.

Contribution

It demonstrates the dependence of HHG spectral features on analysis parameters and post-pulse propagation, clarifying the interpretation of experimental and theoretical spectra.

Findings

HHG spectra show strong CEP sensitivity.

Spectral features near ionization threshold depend on windowing and propagation time.

HHG spectrum below ionization threshold is analysis-dependent rather than a unique observable.

Abstract

We present ab initio calculations using the $R$-matrix with time dependence (RMT) method for high-order harmonic generation (HHG) in argon in a short, intense pulse regime. The calculations employ a $6$-cycle $\sin^2$ pulse at $850$ nm with peak intensity $2.3\times 10^{14}$ W/cm$^2$ and, for comparison with the experiment by Guo et al. [J. Phys. B: At. Mol. Opt. Phys. 51, 034006 (2018)], a Gaussian pulse with the same frequency and peak intensity. Both pulse shapes yield the expected harmonic structure in the region above the ionization threshold (approximately $15.82$ eV in $LS$-coupling). The spectra exhibit strong carrier-envelope-phase (CEP) sensitivity. The energy region leading up to the ionization threshold contains spectral features arising from residual coherent dipole oscillations (free-induction decay) that strongly depend on spectral windowing and the post-pulse propagation time. We show that the HHG spectrum, particularly below the ionization threshold, is a defined quantity that depends on analysis choices rather than being a uniquely determined observable. Comparison between theoretical predictions and experimental observations in this energy regime, therefore, requires explicit specification of these parameters.