Ultra-wideband terahertz (THz) and infrared (IR) absorbers offer enormous technical and economic promise for applications such as THz/IR-imaging, medical diagnoses, communication, spectroscopy, explosive detection, and weather monitoring. However, their usefulness is limited since they suffer from fabrication difficulty due to multi layered complex structure, narrowband and weak absorption. It is highly challenging to develop absorbers having compact, wide bandwidth, high gain and good radiation efficiency. We have here proposed a planar absorber that gives the most promising solutions to the rapid development of the aforementioned technologies. In this article a new design concept of gradient planar absorber based on the combination of circular split rings and log-spiral structures has been proposed for ultra-wideband EM wave absorption. A large absorption bandwidth of 46 THz (4 THz to 50 THz) covering the terahertz (THz) and mid-infrared (MIR) frequency bands with a mean absorption h
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