Nonperturbative effects in second harmonic generation

Keisuke Kitayama; Masao Ogata

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arXiv · 2604.05710

Nonperturbative effects in second harmonic generation

Keisuke Kitayama·Masao Ogata

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Abstract

Second-harmonic generation (SHG) is a quintessential probe of inversion symmetry breaking in condensed matter. While perturbative $\chi^{(2)}$ processes are well-documented, the nonperturbative regime under intense driving remains largely unexplored. In this Letter, we develop a nonperturbative Floquet-Keldysh theory to describe SHG in two-band systems. Our analysis reveals the emergence of two distinct types of nonperturbative saturation: a transition from the conventional $E^2$ scaling to a linear $E$ dependence, and a stronger saturation regime where the SHG response becomes independent of the field amplitude. These behaviors are analytically shown to be governed by one-photon and two-photon resonance processes, respectively. By applying our formalism to a tight-binding model of monolayer GeS, we demonstrate that these specific scaling behaviors are observable in realistic materials and are fully consistent with large-scale numerical Floquet-matrix calculations.

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Keisuke Kitayama, Masao Ogata. 2026-04-07. Nonperturbative effects in second harmonic generation. https://arxiv.org/abs/2604.05710

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