We employ two-band density-matrix equations and time-dependent density-functional theory to calculate high-order harmonic generation (HHG) in graphene under femtosecond laser irradiation. Our investigation uncovers a striking harmonic enhancement structure (HES) in the HHG spectrum. We find that the HES is associated with the bunching of multiple interband electron-hole recombination trajectories, in analogy to the focusing behavior of the light rays known as caustics. In contrast to the atom situation, where the caustic effects are confined to a narrow energy regime around the HHG cutoff energy, for graphene, the caustic effects can dominate the entire interband harmonic generation regime. The magnitude of enhancement is significant and can be estimated to be on the order of $\sim N^{2/3}$, with $N$ representing the harmonic order, according to catastrophe theory.

Close

PrevNext

View figure in article

• Atomic & molecular collisions
• High-order harmonic generation
• Graphene
• Density functional theory
• Dipole approximation
• Tight-binding model