Evaluating the most efficient 2D ZrN nanostructures for broadband metasurface absorbers

Clean energy harvesting applications of metasurfaces employ perfect broadband absorbers with a high thermal stability. An evaluation of a set of several numerically investigated, refractory metal nitride (Zirconium Nitride - ZrN)-based, sub-wavelength broadband absorbers is carried out using wide-angle (0° ≤ θ ≤ 70°) and variously polarized (0° ≤ φ ≤ 90°) incident light. Following a metal-dielectric-metal configuration with the same order of dimensions, each absorber features a dielectric layer of SiO2, separating the top (nanostructure) and bottom (ground) layers made of ZrN. The choice of material is supported by a high melting point (2980 °C), interesting optical and plasmonic properties along with chemical and mechanical stability. Each of the proposed designs was first analyzed for geometrical parameters to achieve optimum absorption. The simulated output for the wavelength range 280-4000 nm, in terms of normally incident light for both TE and TM polarizations is presented alongside solar irradiance. The normalized impedance for each structure was computed for comparison with that of the free space. The analyses led towards an important consequence that while preserving the order of optimized dimensions for each geometry, only a few designs are independent of incident angle, insensitive to polarization variation and easy to fabricate. Out of the twenty absorber profiles investigated, the "cross nanoresonator"showed the highest absorbance of 99.61% at 613 nm, with an average of 94.60% for visible spectrum and 66.72% for 280- 4000 nm. The study adds confidence to the understanding that absorption is a function of both structure geometry and its dimensions.