Abstract
Second-order nonlinear optical processes do not manifest in the bulk of centrosymmetric materials, but may occur in the angstroms-thick layer at surfaces. At such length scales, quantum mechanical effects come into play which could be crucial for an accurate description of plasmonic systems. In this article, we develop a theoretical model based on the quantum hydrodynamic description to study free-electron nonlinear dynamics in plasmonic systems. Our model predicts strong resonances induced by the spill-out of electron density at the metal surface. We show that these resonances can boost second-harmonic generation efficiency up to four orders of magnitude and can be arbitrarily tuned by controlling the electron spill-out at the metal surface with the aid of thin dielectric layers. These results offer a possibility to artificially increase nonlinear susceptibilities by engineering optical properties at the quantum level.
Highlights
Second-order nonlinear optical processes do not manifest in the bulk of centrosymmetric materials, but may occur in the angstroms-thick layer at surfaces
The parameter λ, weighting the von Weizsäcker functional, is of extreme importance as it defines the decay of the electron density and it is usually taken as 1/9 ≤ λ ≤ 1
Our approach is based on the quantum hydrodynamic theory (QHT), which can efficiently handle the realistic profiles of the ground-state electron density
Summary
Second-order nonlinear optical processes do not manifest in the bulk of centrosymmetric materials, but may occur in the angstroms-thick layer at surfaces. We show that these resonances can boost second-harmonic generation efficiency up to four orders of magnitude and can be arbitrarily tuned by controlling the electron spill-out at the metal surface with the aid of thin dielectric layers These results offer a possibility to artificially increase nonlinear susceptibilities by engineering optical properties at the quantum level. The SHG surface process occurs within a very thin layer of the order of few angstroms (a few Thomas–Fermi screening lengths) at the surface where the induced charges are confined At such near-atomic length scales, classical electrodynamics fails to address the microscopic details and consideration of nonlocal and quantum mechanical effects may become crucial for accurately characterizing optical behavior of a metallic system. The QHT can incorporate classical nonlinear effects, such as Lorentz and convective contributions, in multiscale plasmonic systems with deep sub-wavelength features, where the quantum effects cannot be neglected
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