Quantum-orbit analysis of high-order harmonic generation by resonant plasmon field enhancement

TL;DR

This paper analyzes high-order harmonic generation in atoms under inhomogeneous laser fields, revealing how quantum orbits and field inhomogeneity influence the harmonic spectra and cutoff extensions beyond classical limits.

Contribution

It introduces a detailed quantum orbit analysis for HHG in inhomogeneous fields, showing new behaviors and cutoff extensions not captured by previous models.

Findings

Inhomogeneous fields produce both odd and even harmonics.

Electron tunneling involves two different canonical momenta.

HHG cutoff extends beyond semiclassical predictions with increased inhomogeneity.

Abstract

We perform a detailed analysis of high-order harmonic generation (HHG) in atoms within the strong field approximation (SFA) by considering spatially inhomogeneous monochromatic laser fields. We investigate how the individual pairs of quantum orbits contribute to the harmonic spectra. We show that in the case of inhomogeneous fields, the electron tunnels with two different canonical momenta. One of them leads to a higher cutoff and the other one develops a lower cutoff. Furthermore, we demonstrate that the quantum orbits have a very different behavior in comparison to the homogeneous field. We also conclude that in the case of the inhomogeneous fields, both odd and even harmonics are present in the HHG spectra. Within our model, we show that the HHG cutoff extends far beyond the semiclassical cutoff as a function of inhomogeneity strength. Our findings are in good agreement both with quantum mechanical and classical models.