Fingerprint of the Interbond Electron Hopping in Second-Order Harmonic Generation

TL;DR

This study investigates how interbond electron hopping influences second-order harmonic generation in materials, revealing new nonlinear optical behaviors and proposing a unified susceptibility model applicable across different field strengths.

Contribution

It introduces the role of interbond electron hopping as a key factor in SHG and proposes a generalized susceptibility model for both weak and strong fields.

Findings

Interbond electron hopping induces a novel nonlinear polarization source.

Anomalous anisotropic SHG structures are observed in ZnO crystals.

A unified susceptibility model describes nonlinear responses across regimes.

Abstract

We experimentally explore the fingerprint of the microscopic electron dynamics in second-order harmonic generation (SHG). It is shown that the interbond electron hopping induces a novel source of nonlinear polarization and plays an important role even when the driving laser intensity is 2 orders of magnitude lower than the characteristic atomic field. Our model predicts anomalous anisotropic structures of the SHG yield contributed by the interbond electron hopping, which is identified in our experiments with ZnO crystals. Moreover, a generalized second-order susceptibility with an explicit form is proposed, which provides a unified description in both the weak and strong field regimes. Our work reveals the nonlinear responses of materials at the electron scale and extends the nonlinear optics to a previously unexplored regime, where the nonlinearity related to the interbond electron hopping becomes dominant. It paves the way for realizing controllable nonlinearity on an ultrafast time scale.