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

Domenico De Ceglia,Michael Scalora,Salvatore Campione,Evan L Runnerstrom,Jon-Paul Maria,Gordon A Keeler,Ting S Luk,Kyle Kelley,Maria A Vincenti

doi:10.1038/s41598-018-27655-z

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 epsilon-near-zero nanofilms made of low-loss doped cadmium-oxide. 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.

Highlights

The interaction of light with electrons in motion produces plasmon polaritons, i.e., light-driven collective oscillations of conduction electrons[1]
Longitudinal plasmons attributed to the hydrodynamic behavior of conduction electrons were observed in thin films of potassium[12] and magnesium[13,14] using electron-spectroscopy techniques, and in silver films[15] via spectrophotometry
We report, for the first time to our knowledge, infrared optical nonlocalities due to conduction electrons in indium-doped cadmium oxide (CdO) thin films

Summary

Introduction

The interaction of light with electrons in motion produces plasmon polaritons, i.e., light-driven collective oscillations of conduction electrons[1]. Semiconductors such as conducting-metal oxides and transition-metal nitrides are emerging in the context of plasmonic materials as alternatives to noble metals because of their low absorption losses[25], CMOS compatibility, and tunability, offering new opportunities for the realization of photonic and electro-optic devices[26] in the infrared range Among these materials, indium-doped CdO lends itself well to the study of quantum and nonlocal hydrodynamic effects for at least two reasons: (i) the optical properties of doped CdO are dominated by conduction electrons near the plasma frequency, where interband transitions are practically absent, to other doped conducting-metal oxides (e.g., indium tin oxide and aluminum zinc oxide); (ii) absorption losses can be controlled with doping – films with carrier mobility around 500 cm2/(V·s) and imaginary part of the (local) bulk dielectric constant as low as 0.15 have been measured near the zero-crossing frequency of the real part of the bulk dielectric constant[27]. The model that we adopt generalizes the Bloch hydrodynamic theory[1] and shows that the nonlocal hydrodynamic interaction of the electron-fluid with p-polarized light induces: (i) a significant blue-shift of the dominant ENZ mode; (ii) additional, thickness-dependent resonances above the bulk plasma frequency of the film associated with the elasticity of electrons and higher-order pressure modes; and (iii) nonlocal damping due to the electrons’ viscosity that augments absorption losses

Objectives

Results

Conclusion