Signatures of electronic structure in bi-circular high-harmonic spectroscopy

TL;DR

This paper advances bi-circular high-harmonic spectroscopy by extending it to the mid-infrared regime, providing detailed experimental spectra and a quantum-orbit analysis that elucidates the influence of atomic electronic structure on harmonic emission.

Contribution

It introduces a quantitative interpretation framework for bi-circular high-harmonic spectra, incorporating scattering-wave matrix elements and propensity rules related to atomic orbitals.

Findings

Detailed high-harmonic spectra of rare-gas atoms in the mid-infrared regime.

Identification of propensity rules linked to angular momentum of atomic orbitals.

Explanation of harmonic intensity ratios through interference of electron emissions.

Abstract

High-harmonic spectroscopy driven by circularly-polarized laser pulses and their counter-rotating second harmonic is a new branch of attosecond science which currently lacks quantitative interpretations. We extend this technique to the mid-infrared regime and record detailed high-harmonic spectra of several rare-gas atoms. These results are compared with the solution of the Schrodinger equation in three dimensions and calculations based on the strong-field approximation that incorporate accurate scattering-wave recombination matrix elements. A quantum-orbit analysis of these results provides a transparent interpretation of the measured intensity ratios of symmetry-allowed neighboring harmonics in terms of (i) a set of propensity rules related to the angular momentum of the atomic orbitals, (ii) atom-specific matrix elements related to their electronic structure and (iii) the interference of the emissions associated with electrons in orbitals co- or counter-rotating with the laser fields. These results provide the foundation for a quantitative understanding of bi-circular high-harmonic spectroscopy.