Near-perfect absorption throughout the visible using ultra-thin metal films on index-near-zero substrates [Invited]

Metals are highly reflective and therefore commonly overlooked as efficient absorbers. However, a subwavelength Fabry-Pérot-like resonance in ultra-thin metal films has been used to achieve absorption above 70%, approaching perfect absorption using traditional substrates. Here we take a different approach and show that near-perfect absorption is achievable provided that the ultra-thin metals are deposited on an index near zero (INZ) substrate. The optical contrast at the metal-INZ interface enhances the non-trivial reflections leading to destructive interference after multiple reflections. In this manuscript, we present design considerations for ultra-thin metal films on INZ substrates to obtain near-perfect absorption throughout the visible spectrum and into the near-infrared (NIR). We find that metals commonly used for plasmonics and hot carrier devices, such as Au and Ag, can obtain near-perfect absorption for near-ultraviolet and visible wavelengths, while metals such as Pd and Pt are efficient absorbers throughout the near-ultraviolet to near-infrared spectrum.