Dynamic correlation in high-harmonic generation for helium atoms in intense visible lasers

TL;DR

This paper explores how dynamic electron correlation influences high-harmonic generation in helium atoms exposed to intense visible lasers, using two approximate theoretical methods that agree well on key spectral features.

Contribution

It demonstrates that simplified models can effectively capture electron correlation effects in high-harmonic generation under specific conditions, highlighting the roles of collective and single-electron responses.

Findings

Good agreement between theories for polarization and spectra at high intensities

Identification of a secondary maximum below the classical cut-off in emission spectra

Carrier-envelope-phase imprint observed at high frequencies

Abstract

We investigate the effect of dynamic electron correlation on high-harmonic generation in helium atoms using intense visible light (\lambda=390nm). Two complementary approaches are used which account for correlation in an approximate manner: time-dependent density-functional theory and a single-active-electron model. For intensities I~10^{14} W/cm^2, the theories are in remarkably good agreement for the dynamic polarization and harmonic spectrum. This is attributed to a low-frequency collective mode together with a high-frequency single-electron response due to the nuclear singularity, both of which dominate electron correlation effects. A time-frequency analysis is used to study the timing and emission spectrum of attosecond bursts of light. For short pulses, we find a secondary maximum below the classical cut-off. The imprint of the carrier-envelope-phase, for the time-integrated spectral density appears at frequencies above the high-frequency drop-off, consistent with previous studies in the infrared \lambda~800nm.