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.

Flexible but Refractory Single-Crystalline Hyperbolic Metamaterials

“…As shown in Figure S7, the SE data for SL1 and SL2 can be fitted well with a mean-square error (MSE) of ∼6. Consistent with other metal nitride SLs on rigid substrates (e.g., MgO), − the TiN/ScN SLs on F-mica here also show two different hyperbolic regions with the real parts of dielectric constants (ε ⊥ ′ and ε ∥ ′) showing the opposite signs, i.e., type I HMM (ε ⊥ ′ < 0, and ε ∥ ′ > 0) in visible ranges and type II HMM (ε ⊥ ′ > 0, and ε ∥ ′ < 0) in vis–NIR ranges (Figure a–c). As one can see, SL1 and SL2 are type I HMMs in the wavelength ranges of 496–534 and 496–583 nm, respectively, with very small dielectric losses (ε ∥ ″ < 2, and ε ⊥ ″ < 11), which are very promising for hyperlens applications .…”

“…Above ∼750 nm, the TiN/ (Al,Sc)N* SL on a MgO substrate with very small lattice misfits (<1%) among TiN, (Al,Sc)N, and MgO has very few misfit-induced dislocations and the highest FOM. 30 Despite being lower than the FOMs (e.g., ∼0.5 at 1500 nm) of TiN/ (Al,Sc)N* SL and ZrN/ScN* SL above ∼750 nm, 30,32 the FOMs (e.g., ∼0.18 at 1500 nm) of SL1 and SL2 are very close to the FOM of HfN/ScN* SL in a previous work. 32 It is also important to note that the multilayers based on noble metals (e.g., Au and Ag) and dielectric oxides (e.g., TiO 2 and Al 2 O 3 ) are the most widely applied HMMs for the applications of hyperlens, spontaneous emission engineering, negative refraction, and nonlinear optics.…”

“…As an important paradigm of plasmonic/photonic components, hyperbolic metamaterials (HMMs) with extreme optical anisotropy that rarely exist in nature have been applied in the fields of negative refraction, subwavelength-resolution imaging, highly sensitive sensors, and spontaneous emission engineering . Single-crystalline HMMs can be directly grown on rigid substrates. − However, their flexible counterparts have not yet been successfully fabricated. It is also important to note that typical organic substrates are extremely vulnerable to harsh environments, e.g., high temperature or intensive light illumination, − which strongly limit the applications of flexible plasmonic/photonic components.…”

“…Summary of the figures of merit for flexible HMMs (TiN/ScN SLs on F-mica) in this work and FOMs of multilayer HMMs on rigid substrates in previous reports (denoted with asterisks) . Reprinted in part with permission from ref . Copyright 2022 American Chemical Society.…”

Refractory Plasmonic Hafnium Nitride and Zirconium Nitride Thin Films as Alternatives to Silver for Solar Mirror Applications

“…HfN and ZrN films exhibit ENZ at ∼340–380 nm, similar to Ag. Utilizing HfN and ZrN as metallic components, HfN/ScN and ZrN/ScN metal/semiconductor superlattices are also developed that exhibit both type-I and type-II hyperbolic photonic dispersion starting from the near-UV to near-IR spectral ranges . However, despite their ENZ at ∼340 to ∼380 nm, which is close to the edge of the solar spectrum at short wavelengths, the measured reflectivity values of HfN and ZrN are relatively small (68 and 75% in the 320–900 nm range, respectively). , Such a lower reflection coefficient results from a somewhat tapered rise in the reflection edge from its plasma frequency or ENZ point due to higher optical losses.…”