  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.
