ENZ materials and Anisotropy: Enhancing Nonlinear Optical Interactions at the Nanoscale in Metal/Conducting-Oxide Multilayer Stacks

TL;DR

This paper demonstrates that metal/conducting-oxide multilayer stacks can access epsilon-near-zero regimes, leading to anisotropic nanostructures that significantly enhance nonlinear optical interactions at the nanoscale.

Contribution

It introduces a novel multilayer design that leverages anisotropy and nonlocal effects to boost nonlinear optical responses without relying on second-order bulk nonlinearity.

Findings

Field intensity enhanced by nearly two orders of magnitude.

Nonlocal effects blueshift the resonance.

Hot electron nonlinearity redshifts plasma frequency.

Abstract

Epsilon-near-zero materials are exceptional candidates for studying electrodynamics and nonlinear optical processes at the nanoscale. We demonstrate that by alternating a metal and a highly doped conducting-oxide, the epsilon-near-zero regime may be accessed resulting in an anisotropic, composite nanostructure that significantly enhances nonlinear interactions. Using two independent and different computation techniques we show that the structure can enhance the local field intensity by nearly two orders of magnitudes, in large part due to the onset of the effective anisotropy. The investigation of the multilayer nanostructure using a microscopic, hydrodynamic approach also sheds light on the roles of two competing contributions that are for the most part overlooked, but that can significantly modify linear and nonlinear responses of the structure: nonlocal effects, which blueshift the resulting resonance, and the hot electron nonlinearity, which redshifts the plasma frequency as the effective mass of free electrons increases as a function of incident power density. Finally, we show that, even in absence of second order bulk nonlinearity, second order nonlinear processes are also significantly enhanced by the layered structure.