Surface Phonon Polaritons from a Sapphire Nanocone Array

Ionic crystals have attracted much attention due to their ability to achieve efficient light confinement with minimal loss in the infrared (IR) range. This is primarily based on interactions between the incident light and the ionic lattice vibrations. Sapphire (Al₂O₃) is a material widely used as a substrate in multi-layered structures and optics such as windows. Sapphire is also a hyperbolic material with multiple Reststrahlen bands (RBs) in the IR. While the optical properties of sapphire have been measured and studied from the visible to the IR, there is still a need for further investigations into the material’s unexplored functionalities based on both positive and negative permittivities. In this study, Raman scattering and infrared (10 to 30 µm) reflectivity of a sapphire nanocones array sample have been investigated with special attention to its in-plane and out-of-plane permittivity components.  Three confined modes have been observed in the sapphire nanocones array sample. The high positive in-plane refractive index of the material close to the hyperbolic transition point (at 17.5 µm) results in a dielectric resonance. Additionally, two surface phonon polariton excitations have been observed at wavelengths of 12.6 µm and 13.5 µm where the material exhibits metallic behavior and the permittivity is negative in all directions. Finite element modeling of the electromagnetic fields has been performed and is in excellent agreement with reflection spectroscopy results. This study demonstrates that the sapphire nanostructures are a promising platform for nanophotonic applications within the IR spectral range.