  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.
