High-temperature-resilient hyperbolicity in a mixed-dimensional superlattice

Hyperbolic superlattices are used for subwavelength focusing, cloaking, and optical thermal management. Typically, these superlattices are constructed of layers of noble metals and insulators. Despite these systems displaying excellent optical performance, the poor thermal stability of noble metals prevents their application in high-temperature environments. Instead, CMOS-compatible transition-metal nitrides are often substituted for noble metals in plasmonic systems since they have high thermal stability at the expense of optical properties. Here, we fabricate hyperbolic titanium nitride (TiN)/hexagonal boron nitride (hBN) superlattices with 3D-2D interfaces. The mixed-dimensional nature of the interfaces prevents atoms from diffusing across the interface at high temperatures. The hyperbolicity of the superlattice is found to be unaffected by annealing at high temperature (800°C for 5 days), and TiN/hBN is found to have a larger hyperbolic figure of merit than similar superlattices.