Highly anisotropic titanium nitride nanowire arrays for low-loss hyperbolic metamaterials fabricated via dynamic oblique deposition

Abstract

Hyperbolic metamaterials (HMMs) with highly anisotropic metal nanowires exhibit unique optical properties arising from their extraordinary optical anisotropy. Although metal nanowires are often fabricated by embedding noble metals such as silver in an anodic alumina membrane dielectric host, the low melting point of noble metals limits their utilization in high-temperature applications, and there are fabrication issues such as overfilling or discontinuous islands within the host pores. Thus, metal nanowires with a high melting point for HMMs and alternative fabrication techniques are desired. In this study, we fabricated a highly anisotropic nanowire array (NWA) using titanium nitride, which has a high melting point, via dynamic oblique deposition of titanium and subsequent thermal treatment in ammonia. Spectra of ellipsometric parameters were well-fitted by a Fresnel reflection theoretical model considering the optical anisotropy, in which the effective permittivity was described using effective medium theory and the Drude–Lorentz model. The out-of-plane component of the effective permittivity was negative at λ > 850 nm, whereas the in-plane component was positive, indicating that the fabricated NWA behaves as a HMM. The figure of merit in the near-infrared range was higher than that of conventional multilayer TiN HMMs. The NWA presented here is a promising candidate for HMMs in high-temperature applications due to the high melting point of titanium nitride.