Metamaterials are already described in another post. As described earlier, these artificially structured composites could potentially fill the gap in the electromagnetic spectrum where the material response is limited and thereby facilitate building of new devices.
Despite early consideration of loss (in particular ohmic loss in metals) as a major limiting factor against the performance of plasmonic metamaterials, the absorptivity is regarded as a standalone function of them following the first experimental demonstration of metamaterial perfect absorber in 2008. Although the initially proposed absorbers were simply designed for a specific frequency (narrow-band), the requirement of broadband absorption of light for energy collection (e.g. solar cells) initiated an ever growing research field which is known today as metamaterial perfect absorbers (MPA).
A MPA is a device wherein all incident waves are absorbed at the operating frequency whereas transmissivity, reflectivity and scattering are hindered. They are generally composed of two main layers: a back metal plate and a composite top layer where the latter can be composed of nanoparticles (appended image), micro-structures, grating and resonators. MPAs can be classified into two main categories: resonant (narrow-band) absorbers and broadband (multi-band) absorbers.
In the resonant absorbers, as the name suggests, resonance at the designed frequency is the origin of high absorption. However, the broadband absorbers usually rely on composite materials (metamaterial) with multiple frequency resonances which are coupled with each other to realize a wide-band light absorption. The current trends in the field of metamaterial absorbers is directed toward finding a cheap but up-scalable fabrication method for development of MPAs for operation in desired frequency. This subject will be discussed in more details in upcoming posts.