Directional perfect absorption using deep subwavelength low permittivity films

TL;DR

This paper demonstrates near-infrared directional perfect absorption using ultrathin ITO films on silver, revealing new conditions for perfect absorption in deep subwavelength regimes and analyzing the underlying eigenmodes.

Contribution

It introduces analytical conditions for perfect absorption in subwavelength films and provides a modal analysis framework to understand the phenomenon.

Findings

Perfect absorption occurs slightly above the ENZ frequency of ITO.

Ultrathin films (~1/50th of wavelength) achieve near-perfect absorption.

Eigenmode analysis explains the crossover between bound and leaky modes.

Abstract

We experimentally demonstrate single beam directional perfect absorption (to within experimental accuracy) of p-polarized light in the near-infrared using unpatterned, deep subwavelength films of indium tin oxide (ITO) on Ag. The experimental perfect absorption occurs slightly above the epsilon-near-zero (ENZ) frequency of ITO where the permittivity is less than one. Remarkably, we obtain perfect absorption for films whose thickness is as low as ~1/50th of the operating free-space wavelength and whose single pass attenuation is only ~ 5%. We further derive simple analytical conditions for perfect absorption in the subwavelength-film regime that reveal the constraints that the ITO permittivity must satisfy if perfect absorption is to be achieved. Then, to get a physical insight on the perfect absorption properties, we analyze the eigenmodes of the layered structure by computing both the real-frequency/complex-wavenumber and the complex-frequency/real-wavenumber modal dispersion diagrams. These analyses allow us to attribute the experimental perfect absorption behavior to the crossover between bound and leaky behavior of one eigenmode of the layered structure. Both modal methods show that perfect absorption occurs at a frequency slightly larger than the ENZ frequency, in agreement with experimental results, and both methods predict a second perfect absorption condition at higher frequencies attributed to another crossover between bound and leaky behavior of the same eigenmode. Our results greatly expand the list of materials that can be considered for use as ultrathin perfect absorbers and also provide a methodology for the design of absorber systems at any desired frequency.