Multiple Epsilon-Near-Zero Resonances in Multilayered Cadmium Oxide: Designing Metamaterial-Like Optical Properties in Monolithic Materials

In this Letter, we demonstrate a new class of infrared nanophotonic materials based on monolithic, multilayered doped cadmium oxide (CdO) thin films, where each CdO layer is individually tuned to support a separate epsilon-near-zero (ENZ) resonance. Infrared reflectivity measurements reveal that the optical response of the multilayered stack combines multiple discrete absorption events, each associated with an individual ENZ plasmonic polaritonic mode. Structural and chemical characterization confirm that the multilayers are homoepitaxial and monolithic, with internal interfaces defined by discrete steps in dopant density and carrier concentration. Structurally, the layers are indistinguishable as they differ from their neighbors by only â1 in 10000 constituent atoms. The optoelectronic property contrast, however, is pronounced, as each layer maintains an independent electron concentration, as corroborated by secondary ion mass spectroscopy and numerical solutions to Poisson's equation. It is this electron confinement that imbues each individual layer with the ability to independently resonate at separate mid-infrared frequencies. We additionally demonstrate simultaneous thermal emission of infrared light from each individual layer at its respective ENZ frequency, pursuant to Kirchhoff's law of radiation. The highly localized property contrast intrinsic to these monoliths offers great potential in nanophotonics, plasmonics, and physics thanks to the ability to engineer infrared response and achieve metamaterial-like optical properties without the need for lithography or micro/nanofabrication. New possibilities arising from this work include strongly tunable and multimodal perfect absorbers as well as spectrally engineered and narrow-band light emitters.