Mid-Infrared Perfect Absorption with Planar and Subwavelength-Perforated Ultrathin Metal Films

A straightforward analytical approach is proposed for the design of minimally thin metal absorbers. Unlike traditional resonant design principles, where shape, size, and periodicity of a nanostructured film determine the absorption properties, this study uses only the thickness and permittivity (i.e., sheet conductivity) of the material at hand to demonstrate maximal absorption in the minimal possible thickness at any given wavelength in planar layers - guided by only the derived material-agnostic equations. An alternative mechanism is further proposed and experimentally demonstrated to obtain precise control over the sheet conductivity of metal films necessary for such designs using metal dilution, enabling the tuning of both the amplitude and the phase of reflected waves. Finally, the concept of "phase doping" is proposed and experimentally demonstrated, wherein an ultrathin metal layer is placed within the spacer of the absorber cavity, which spectrally tunes the absorption feature without changing the spacer thickness or participating in the absorption. By judiciously combining the dilution of the absorbing and phase layers, a multifunctional ultrathin absorber architecture is demonstrated with customizable amplitude, spectral position, and selectivity, all leveraging the same vertical stack. These findings are promising for the design of ultrasensitive detectors, thermal emitters, and nonlinear optical components. Having bulk-like optical quality in ultrathin metal films is appealing for many applications including for efficient absorbers. Here, an analytical framework is built to address perfect absorption across the near- and mid-infrared regions, and it is shown that perforation of ultrathin metal films effectively mimics much thinner films with bulk quality, enabling a selective and spectrally flexible absorber architecture operating at the ultrathin limit. image