Metamaterial absorber

Metamaterials

Metamaterials are artificial materials which exhibit unique properties which do not occur in nature. These are usually arrays of structures which are smaller than the wavelength they interact with. These structures have the capability to control electromagnetic radiation in unique ways that are not exhibited by conventional materials. It is the spacing and shape of a given metamaterial's components that define its use and the way it controls electromagnetic radiation. Unlike most conventional materials, researchers in this field can physically control electromagnetic radiation by altering the geometry of the material's components. Metamaterial structures are used in a wide range of applications and across a broad frequency range from radio frequencies, to microwave, terahertz, across the infrared spectrum and almost to visible wavelengths.

Absorbers

"An electromagnetic absorber neither reflects nor transmits the incident radiation. Therefore, the power of the impinging wave is mostly absorbed in the absorber materials. The performance of an absorber depends on its thickness and morphology, and also the materials used to fabricate it."

"A near unity absorber is a device in which all incident radiation is absorbed at the operating frequency–transmissivity, reflectivity, scattering and all other light propagation channels are disabled. Electromagnetic (EM) wave absorbers can be categorized into two types: resonant absorbers and broadband absorbers.

Principal conceptions

A metamaterial absorber utilizes the effective medium design of metamaterials and the loss components of permittivity and magnetic permeability to create a material that has a high ratio of electromagnetic radiation absorption. Loss is noted in applications of negative refractive index (photonic metamaterials, antenna systems metamaterials) or transformation optics (metamaterial cloaking, celestial mechanics), but is typically undesired in these applications.

Complex permittivity and permeability are derived from metamaterials using the effective medium approach. As effective media, metamaterials can be characterized with complex ε(w) = ε1 + iε2 for effective permittivity and μ(w) = μ1 + i μ2 for effective permeability. Complex values of permittivity and permeability typically correspond to attenuation in a medium. Most of the work in metamaterials is focused on the real parts of these parameters, which relate to wave propagation rather than attenuation. The loss (imaginary) components are small in comparison to the real parts and are often neglected in such cases.

However, the loss terms (ε2 and μ2) can also be engineered to create high attenuation and correspondingly large absorption. By independently manipulating resonances in ε and μ it is possible to absorb both the incident electric and magnetic field. Additionally, a metamaterial can be impedance-matched to free space by engineering its permittivity and permeability, minimizing reflectivity. Thus, it becomes a highly capable absorber.

This approach can be used to create thin absorbers. Typical conventional absorbers are thick compared to wavelengths of interest, which is a problem in many applications. Since metamaterials are characterized based on their subwavelength nature, they can be used to create effective yet thin absorbers. This is not limited to electromagnetic absorption either.

References

References

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