Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms

TL;DR

This paper reports the first experimental observation of viscoelastic nonlocalities in the infrared optical response of doped cadmium-oxide nanofilms, revealing new insights into electron dynamics in epsilon-near-zero materials.

Contribution

It introduces a hydrodynamic viscoelastic model to describe conduction electron motion and demonstrates its agreement with experimental infrared optical data.

Findings

Viscoelastic nonlocalities are observable in doped cadmium-oxide nanofilms.

Electron elasticity causes blue-shifting of infrared plasmonic resonances.

Evidence of nonlocal damping (viscosity) in conduction electron motion is provided.

Abstract

Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of doped cadmium-oxide, epsilon-near-zero nanofilms. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity, in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.