Enhanced light trapping in the mid-infrared using gold-black phosphorus nanostructured absorbers

We propose a gold -based nanostructured design for achieving enhanced absorption in black phosphorus (BP) thin films in the 3 – 5 microns wavelength range. We tune the design parameters to excite strongly localized radiation modes in layers of thicknesses ranging from 5 to 50 nm at 4 microns. In addition, we compute the absorption enhancement factor of these absorbers and compare it against the conventional 4n2 limit. For a BP layer thickness of 5 nm, we are able to achieve an enhancement of around 561 at a wavelength of 4 microns which is significantly greater than the conventional value of 34 for an isolated textured layer.