Figure 1. High-harmonic generation (HHG) assisted by graphene plasmons.

(a) Schematic illustration of a doped graphene nanoribbon illuminated by an intense optical pulse that is resonant with the ribbon transverse dipole plasmon. The latter produces strong in-plane electric-field intensity enhancement (see colour scale) that boosts the generation of high harmonics. (b) The low-energy band structures of graphene (upper left) and a conventional 2D semiconducting crystal (upper right) respond differently to a monochromatic light electric field E(t)=E₀ cos(ωt): in graphene, the induced current J(t) (lower left) acquires a square-wave temporal profile in the high-intensity limit, which contains all odd-order harmonics in its Fourier decomposition, while the semiconductor responds harmonically at the driving frequency ω. A 2D free-electron gas also shows a harmonic response⁵².