Plasmon signatures in high harmonic generation

High harmonic generation in polarizable multi-electron systems is investigated in the framework of multi-configuration time-dependent Hartree-Fock. The harmonic spectra exhibit two cut-offs. The first cut-off is in agreement with the well-established, single active electron cut-off law. The second cut-off presents a signature of multi-electron dynamics. The strong laser field excites nonlinear plasmon oscillations. Electrons that are ionized from one of the multi-plasmon states and recombine to the ground state gain additional energy, thereby creating the second plateau.