  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.
