Radiative volume plasmon and phonon-polariton resonances in TiN-based plasmonic/polar-dielectric hyperbolic optical metamaterials

Ferrell and Berreman modes are absorption resonances in thin metal films and polar-dielectric media that arise from radiative bulk plasmon-polariton and phonon-polariton excitations. Compared to surface polaritons, Ferrell and Berreman modes occur due to volume charge oscillations across the medium and provide a unique pathway for light-matter interactions. Though the resonances are studied individually, stringent polarization and material requirements have prevented their observation in one host medium. Here, we show simultaneous excitation of Ferrell and Berreman absorption resonances in refractory epitaxial TiN/Al0.72Sc0.28N plasmonic metal/polar-dielectric hyperbolic metamaterials in the visible and far-infrared spectral ranges. The nanoscale periodicity of the superlattices enables the coupling of bulk plasmons (and longitudinal optical phonons) across different TiN (and Al0.72Sc0.28N) layers and allows polarization matching with free-space light that results in Ferrell (and Berreman) mode excitations. Ferrell and Berreman absorption resonances can be used for strong light confinement in radiative cooling, thermophotovoltaics, and other dual-band applications.