Extended high-harmonic spectra through cascade resonance in confined quantum systems

TL;DR

This paper reveals how cascade resonance in confined quantum systems like graphene nanoribbons significantly alters high-harmonic generation, introducing new cutoff laws and mechanisms distinct from atomic or bulk solid systems.

Contribution

It introduces a multilevel model demonstrating cascade resonance effects in high-harmonic generation within confined quantum systems, especially in nanoribbons, and redefines the harmonic cutoff laws.

Findings

Cascade resonance causes a new high-harmonic cutoff law.

Resonant fields induce charge pumping to higher subbands.

Distinct mechanisms from atomic and bulk solid harmonic generation.

Abstract

The study of high-harmonic generation in confined quantum systems is vital to establishing a complete physical picture of harmonic generation from atoms and molecules to bulk solids. Based on a multilevel approach, we demonstrate how intraband resonances significantly influence the harmonic spectra via charge pumping to the higher subbands and, thus, redefine the cutoff laws. As a proof of principle, we consider the interaction of graphene nanoribbons, with zigzag as well as armchair terminations, and resonant fields polarized along the cross-ribbon direction. Here, this effect is particularly prominent due to many nearly equi-separated energy levels. In such a scenario, a cascade resonance effect can take place in high-harmonic generation when the field strength is above a critical threshold, which is completely different from the harmonic generation mechanism of atoms, molecules and bulk solids. We further discuss the implications not only for other systems in a nanoribbon geometry, but also systems where only a few subbands (energy levels) meet this frequency-matching condition by considering a generalized multilevel Hamiltonian. Our study highlights that cascade resonance bears fundamentally distinct influence on the laws of harmonic generation, specifically the cutoff laws based on laser duration, field strength, and wavelength, thus unraveling new insights in solid-state high-harmonic generation.