Active hyperbolic metamaterials: progress, materials and design

Abstract

Hyperbolic metamaterials (HMMs) are a special class of metamaterials that exhibit a hyperbolic dispersion relation. Hyperbolic dispersion is rarely observed in natural materials as it typically requires nanostructured composites with constituent materials having both positive and negative electric permittivity or magnetic permeability tensors. In this article, we review the physics of HMMs, present their most common practical designs, explain how their design affects the frequency band where hyperbolic dispersion occurs, and introduce the common fabrication methods. We believe that active and reprogrammable HMMs, which have a dispersion relation that can be dynamically controlled, will be the next phase of the HMM development. Therefore, a substantial portion of this review is devoted to materials that can enable active tuning of the hyperbolic dispersion relation. We also review applications of HMMs, such as sub-wavelength imaging, spontaneous emission enhancement, focussing, biosensing and thermal emission engineering. Finally, we discuss how HMMs need to be further developed and the limitations that need to be overcome for HMMs make a real technological impact.