Near-UV-to-Near-IR Hyperbolic Photonic Dispersion in Epitaxial (Hf,Zr)N/ScN Metal/Dielectric Superlattices

Abstract

Hyperbolic metamaterials (HMMs) with extreme dielectric anisotropy have shown great promise in nanophotonic applications such as superlensing, enhancement of spontaneous emission, negative refraction, and the diverging photonic density of states. Noble metal-based metal/dielectric multilayers (e.g., Au/SiO2 and Ag/TiO2) and metallic (Au and Ag) nanowires embedded inside a dielectric matrix have been traditionally used to demonstrate HMM properties and for implementations into devices. Noble metals are, however, unstable at high temperatures, complementary metal oxide semiconductor incompatible, and difficult to deposit in thin-film form due to their high surface energies that limit their potential applications. TiN has emerged as an alternative plasmonic material to Au in recent years, and epitaxial TiN/Al0.72Sc0.28N metal/semiconductor superlattices were developed that exhibit excellent HMM properties. As TiN exhibits ε-near-zero (ENZ) at ∼500 nm, TiN/Al0.72Sc0.28N HMM also operates from ∼500 nm to long-wavelength regions. However, for several energy-conversion-related applications as well as for fundamental studies, it is desirable to achieve HMM wavelengths from the near-UV to the near-IR region of the spectrum. In this article, we demonstrate hyperbolic photonic dispersion in (Hf,Zr)N/ScN, a class of metal/semiconducting superlattice metamaterial that covers the near-UV to the near-IR spectral range. Epitaxial HfN/ScN, ZrN/ScN, and Hf0.5Zr0.5N/ScN superlattices are deposited on (001) MgO substrates and characterized with synchrotron-radiation X-ray diffraction as well as high-resolution electron microscopy techniques. Superlattices grow with cube-on-cube epitaxy and with sharp interfaces. Optical characterization reveals both type-I and type-II hyperbolic photonic dispersions as well as low losses and high figures-of-merit. Along with its high-temperature thermal stability, demonstration of HMM properties in (Hf,Zr)N/ScN metal/dielectric superlattices makes them potential candidates for HMM devices.