Simultaneous optical resonances at visible and mid-infrared frequencies with epitaxial TiN/Al0.72Sc0.28N/TiN metal/polar-dielectric/metal multilayers

Abstract

Traditionally, light-matter interactions in the visible spectral range utilize tailored plasmonic nanostructure in metals that often require advanced nanofabrication techniques. Concomitantly, polar lattice vibrations in dielectrics are used to achieve mid-to-long wavelength infrared (IR) light-matter coupling. Achieving optical resonances simultaneously in the visible and mid-to-long wavelength IR spectral range in one host medium has been challenging due to mutually conflicting material and optical property requirements. In this article, we show simultaneous light-matter coupling and development of super absorbers in the visible and mid-IR spectral ranges with a lithography-free planar wide-angle metal-(polar)-dielectric-metal (MDM) Fabry-Pérot cavity composed of ultrathin refractory metallic TiN as the top layer and polar dielectric Al0.72Sc0.28N as the spacer layer. Lattice-matched TiN/Al0.72Sc0.28N/TiN MDM cavities exhibit super absorption resonance with maximum absorptivity of ∼99%. The absorption maxima spectral position is tuned with changes in spacer layer thickness and by utilizing multiple cavity interactions. Al0.72Sc0.28N inside the MDM is also used for light coupling to the transverse optical phonon mode and to the Berremann mode near the longitudinal phonon frequency with strong selective absorption. Demonstration of refractory epitaxial TiN/Al0.72Sc0.28N/TiN MDM metamaterials mark significant progress towards developing compact optical meta-structures with on-demand optical resonances at different spectral ranges.