Role of Excited States In High-order Harmonic Generation

TL;DR

This paper explores how excited states influence high-order harmonic generation in argon, revealing their role in emission mechanisms and temporal delays that affect the spectral characteristics of the generated radiation.

Contribution

It demonstrates the impact of excited states on harmonic generation, including direct emission and delayed spectral shifts, using advanced attosecond measurement techniques.

Findings

Excited states enable direct XUV emission via Free Induction Decay.

Harmonics from excited states are temporally delayed by a few femtoseconds.

The emission exhibits a nonadiabatic spectral redshift.

Abstract

We investigate the role of excited states in High-order Harmonic Generation by studying the spectral, spatial and temporal characteristics of the radiation produced near the ionization threshold of argon by few-cycle laser pulses. We show that the population of excited states can lead either to direct XUV emission through Free Induction Decay or to the generation of high-order harmonics through ionization from these states and recombination to the ground state. By using the attosecond lighthouse technique, we demonstrate that the high-harmonic emission from excited states is temporally delayed by a few femtoseconds compared to the usual harmonics, leading to a strong nonadiabatic spectral redshift.