Plasmonic Hyperbolic Dispersion in Epitaxial Rock Salt/Wurtzite Superlattices

Epitaxy of hybrid superlattices (HSLs) consisting of functional layers with different crystal geometries is crucial to expanding the family of artificially designed single-crystalline heterostructures for device applications. However, compared to the epitaxy of common heterostructures or superlattices consisting of materials with the same crystal structures, the epitaxy of heterostructures or superlattices with different crystal structures is more challenging. Here, epitaxial rock salt/wurtzite superlattices with ultrathin (<10 nm) TiN and AlOxNy layers have been successfully constructed on α-Al2O3(0001) substrates by magnetron sputtering epitaxy. Remarkably, the high-quality TiN/AlOxNy HSLs enable a type-II hyperbolic dispersion in visible-to-near-infrared ranges with high figures of merit. This study paves an easy and scalable route to the epitaxial rock salt-transition-metal-nitride/wurtzite-(oxy)nitrides HSLs, which can not only unlock the limits of isostructural epitaxial superlattices for high-performance plasmonic hyperbolic metamaterials but also enable their large-scale applications.