A design of thin metal-nitrides broadband meta-absorber for application of solar energy

A wideband metasurface that is capable of absorbing more than 90% of electromagnetic energy is presented. The performance of the design is validated through numerical simulations for the materials including Niobium-nitride (NbN), Hafnium-nitride (HfN) and Tantalum nitride (TaN) for their broadband absorption behavior. We have employed a symmetric, square-structured unit-cell assuming subwavelength dimensions, where the spacer layers are of SiO₂ or Al₂O₃. In general, the excitation of localized electromagnetic resonances (in the top layer and the cavity), controls the devices’ absorption mechanism and is a strong function of the geometry of the top metal layer and the thickness of the dielectric layer. The meta-absorber is a periodic array of patches comprising of refractory-metal nitrides top on a thin dielectric layer over a highly reflecting thick layer. The highest values of absorption with silica as spacer for TaN-, HfN- and NbN-based subwavelength absorbers are 99.90%, 98.79% and 99.66%, at peak wavelengths of 549 nm, 584 nm and 664 nm, respectively. The average values of absorption obtained over the range 300 to 2500 nm are 89.10%, 91.71% and 79.84%, respectively. On the other hand, the peak absorption values with Al₂O₃ spacer for TaN-, HfN- and NbN-based designs are 96.12%, 96.93% and 99.99% at peak wavelengths of 338 nm, 349 nm and 330 nm, respectively. The spectral location of resonance does not shift significantly when light is incident at different angles and the average of the absorption remains higher than 84% for an incident angle of 70°. Furthermore, the design is absolutely insensitive to the change of polarization. The presented design has unit impedance at peak absorption wavelengths. Thus, the presented design offers all the favorable characteristics of an ideal absorber.