Abstract
Considering the specific solar-thermal and optoelectronics applications of metamaterials, broadband absorbers with near-perfect absorption, tunability, and subwavelength dimensions are in high demand. Plasmonic meta-surface absorbers have been found to be band-limited due to resonant plasmonic excitations, thus leading to exploration of materials with high broadband absorption, along with salient features of robustness to environmental degradation and thermal stability. In order to attain broadband absorptance, materials having higher values of attenuation constant (imaginary part of refractive index) are employed so that loss mechanism can be introduced for the operational frequency. We have carried out full-wave numerical simulations to design a highly efficient, metasurface-based light absorber with MIM (Metal-Insulator-Metal) configuration. The insulator layer minimizes reflection by trapping light and acting as a Fabry-Perot cavity between two metal layers, and the thick ground plane blocks transmission. The sandwiched insulator layer is of silica, while the metal layers are of a refractory-metal-nitride (ZrN), having a melting point of 2980 degrees C. The thermal stability of the entire absorber can be extended by coating a thin layer of Hafnium oxide (HfO2). The proposed design has been optimized in terms of geometrical parameters, exhibiting an average absorptance of 95% for optical regime (400-800 nm), and 89.40% for ultra-broadband spectrum of 400-2000 nm. The study includes detailed analyses in terms of reflectance, transmittance, absorptance and free-space impedance-matching. The proposed ultrathin absorber has an overall height of 250 nm, is polarization-insensitive, angle-insensitive, and simple to fabricate with a high tolerance for fabrication errors. Moreover, a design for an emitter is proposed with a view to realize solar thermophotovoltaic (STPV) systems for solar energy harvesting. The emitter is designed in a way so as to have maximum efficiency for PV cells with a bandgap of 1.4 eV.