Metamaterial-based perfect absorbers

Metamaterials are artificially-engineered materials, possessing properties which are not readily observable in materials existing in nature. Since they show very novel properties such as left-handed behavior, negative refractive index, classical analog of electromagnetically-induced transparency, extraordinary transmission, negative Doppler effect, and so on, they can be used for perfect lens, invisibility cloaking, perfect absorption and transmission, etc. Metamaterial-based perfect absorbers (MMPAs) are promising candidates for the practical application of perfect absorbers. MMPA is usually composed of three layers. The first layer is periodically-arranged metallic patterns, whose structure and geometrical parameters should be carefully adjusted to fulfill the impedance-matching condition with the ambient, allowing no reflection of incident electromagnetic (EM) waves. The second layer is a dielectric layer, which allows a space for the EM waves to be dissipated, and sometimes plays a role of resonance cavity to prolong the time taken by the EM waves inside the second layer. Finally, the third layer is a continuous metallic plate, blocking remnant transmission. For practical usage, several aspects of MMPAs are to be considered seriously. Some of them are broadband operation, polarization-independent response, omni-directional response, and tunability. These aspects are basically determined by the structures of MMPA. Another important aspect is flexibility, which is determined by the material used in the fabrication. In this review, the basic operating principles of MMPAs and brief introduction of recent progresses in the field of MMPAs operating in different frequency ranges (GHz, THz and infrared/visible) are presented. Perspectives and future works for the investigation and the real application of MMPAs are also presented.