  Traditionally, efforts to achieve perfect absorption have required the use of
complicated metamaterial-based structures as well as relying on destructive
interference to eliminate back reflections. Here, we have demonstrated both
theoretically and experimentally that such perfect absorption can be achieved
using a naturally occurring material, hexagonal boron nitride (hBN) due to its
high optical anisotropy without the requirement of interference effects to
absorb the incident field. This effect was observed for p-polarized light
within the mid-infrared spectral range, and we provide the full theory
describing the origin of the perfect absorption as well as the methodology for
achieving this effect with other materials. Furthermore, while this is reported
for the uniaxial crystal hBN, this is equally applicable to biaxial crystals
and more complicated crystal structures. Interference-less absorption is of
fundamental interest to the field of optics; moreover, such materials may
provide additional layers of flexibility in the design of frequency selective
surfaces, absorbing coatings and sensing devices operating in the infrared.
