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.

众所周知，与电磁场相互作用的金属结构表现出类似于原子和分子中的特性，例如多光子和隧道电离。开发这种超越电子发射电流的相似性，我们概括了著名的 Fowler-Nordheim 模型，并预测了迄今为止未被认识的来自暴露于强光脉冲照射的纳米结构的非线性光学响应来源。我们发现这些结构产生的谐波是非扰动的，因为高次谐波几乎与低次谐波一样强。同时，预测频谱与通常的高次谐波产生不同，因为它没有表现出明显的截止。