Optical Characterization of Epsilon-Near-Zero, Epsilon-Near-Pole and Hyperbolic Response in Nanowire Metamaterials

TL;DR

This paper investigates the optical properties of gold nanowire metamaterials, revealing their epsilon-near-zero and epsilon-near-pole resonances, which differ from multilayer structures and are highly tunable for various optical applications.

Contribution

It provides a detailed fabrication process, physical and optical characterization, and theoretical analysis of nanowire metamaterials exhibiting unique epsilon-near-zero and epsilon-near-pole responses.

Findings

Nanowire metamaterials show uniaxial anisotropic dielectric response.

The ENZ resonance is tunable from 583 nm to 805 nm.

ENP and ENZ resonances are highly absorptive and can be excited from free space.

Abstract

We report on the optical and physical characterization of metallic nanowire (NW) metamaterials fabricated by electrodeposition of ~30 nm diameter gold nanowires in nano-porous anodic aluminum oxide. We observe a uniaxial anisotropic dielectric response for the NW metamaterials that displays both epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) resonances. We show that a fundamental difference in the behavior of NW-metamaterials from metal-dielectric multilayer (ML) metamaterials is the differing directions of the epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) dielectric responses relative to the optical axis of the effective dielectric tensor. In contrast to multilayer metamaterials, nanowire metamaterials exhibit an omnidirectional ENP and an angularly dependent ENZ. Also in contrast to ML metamaterials, the NW metamaterials exhibit ENP and ENZ resonances that are highly absorptive and can be effectively excited from free space. Our fabrication allows a large tunability of the epsilon-near-zero resonance in the visible and near IR spectrum from 583 nm to 805 nm as the gold nanorod fill fraction changes from 26% to 10.5%. We support our fabrication process flow at each step with rigorous physical and optical characterization. Energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses are used to ascertain the quality of electrochemically deposited Au nanowires prior to and after annealing. Our experimental results are in agreement with simulations of the periodic plasmonic crystal and also analytical calculations in the effective medium metamaterial limit. We also experimentally characterize the role of spatial dispersion at the ENZ resonance and show that the effect does not occur for the ENP resonance. The application of these materials to the fields of biosensing, quantum optics and thermal devices shows considerable promise.