Surpassing the Quarter-Wave Limit for Thin Film Perfect Absorbers with Epsilon-Near-zero Media

A conventional dielectric optical coating supports Fabry-Perot resonances if the thickness is at least a quarter-wavelength of the incident radiation. We show that for light incident obliquely, a low loss epsilon-near-zero (ENZ) thin film coating on a highly metallic substrate can support resonant interferences leading to perfect absorption, well below the quarter-wave thickness limit. The physical mechanism is based on evanescent fields in the ENZ medium, which is distinct from previous demonstrations of ultrathin-film interference achieved only on weakly metallic substrates and accompanied by significant loss. We also explain previously observed absorption features in metal-backed ENZ thin films based on this phenomenon and reveal the unique role of ENZ response in achieving perfect absorption using low loss materials. These readily configurable perfect absorbers are not only easy to engineer at surfaces and interfaces but the underlying design principles and the predictive model elucidated here will be of interest in fundamental optics research and amenable for functional applications.