Broadband LWIR and MWIR metamaterial absorbers with a simple design topology: almost perfect absorption and super-octave band operation in MWIR band

Infrared absorbers are essential structures in the design of thermal emitters and thermal infrared imagers. In this study, we propose simple topologies of wideband metamaterial absorbers operating in the long-wave infrared or in the mid-wave infrared (MWIR) wavelengths of the electromagnetic spectrum where the atmosphere shows transparent behavior. Suggested metamaterial absorbers are mostly thin structures that consist of three functional layers from top to bottom: a periodically patterned metal layer, a planar dielectric layer, and a ground metal layer. The pattern of the top metal layer is four-fold symmetric to guarantee the polarization insensitivity of the absorber under normal incidence of light. In addition, a geometrically simple metamaterial pattern is preferred to facilitate the process of lithography. As titanium is known to be a high-loss metal, it is deliberately used at the top layer of the absorber to increase the overall absorption bandwidth. Highly satisfactory absorber results, such as almost perfect absorption and super-octave band operation are demonstrated, especially in the MWIR region. As oxidation of the top titanium layer may cause performance degradation in long-term use, a design modification is also suggested where a very thin protective coating layer is applied over the titanium metasurface.