Beyond Fowler–Nordheim model: harmonic generation from metallic nano-structures

Metallic structures interacting with electromagnetic fields are known to exhibit properties similar to those found in atoms and molecules, such as multi-photon and tunnel ionization. Developing this similarity beyond the electron emission current, we generalize the well-known Fowler–Nordheim model, and predict a heretofore unrecognized source of nonlinear optical response from nano-structures exposed to illumination with intense optical pulses. We find that the harmonic generation by these structures is non-perturbative in the sense that higher harmonic orders can be almost as strong as the lower orders. At the same time, the predicted spectrum is distinct from the usual high-harmonic generation in that it does not exhibit a sharp cutoff.