Harmonic Generation in Metal-Insulator and Metal-Insulator-Metal Nanostructures

TL;DR

This study investigates harmonic generation in metal-insulator and metal-insulator-metal nanostructures, revealing how electron spill-out and nonlocal effects influence harmonic efficiencies, with experimental measurements supported by detailed simulations.

Contribution

It introduces a method to measure electron density in spill-out regions via harmonic signal reduction and demonstrates enhancement saturation and quenching in MIM structures compared to MI samples.

Findings

Harmonic signals decrease with insulator coverage, indicating electron spill-out effects.

MIM structures show higher harmonic enhancement saturation and quenching.

Simulations suggest nonlocal electron phenomena primarily cause observed effects.

Abstract

We report that the second and third harmonic signal reductions with insulator film surface coverage over a gold substrate gives a measure of the electron density in the spill out volume of the insulator, which is dubbed metal insulator gap states. For metal-insulator-metal (MIM) structures we observe enhancement saturation and quenching of the third harmonic efficiencies well above the efficiencies for metal-insulator (MI) samples. The measured optical harmonics of scattered light from MI and MIM systems are compared with detailed simulations of the nonlinear interactions including free electron spill out into the insulator, nonlocal and electron quantum tunneling effects. Gold coated substrates are covered with variable thin insulator film thicknesses using atomic layer deposition. Optical harmonics of light scattered from two insulator materials (ZnO and Al2O3) are measured in our experiments. Based on our simulations we conclude that the observed MIM signal enhancement effects are primarily due to nonlocal phenomena in an electron gas.