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MOSCOW, December 17. /TASS/. A team of scientists from Moscow Institute of Physics and Technology (MIPT), Kansas State University and the US Naval Research Laboratory have demonstrated that it is possible to fully absorb electromagnetic radiation using an anisotropic crystal. The observations are of fundamental importance for electrodynamics and will provide researchers with an entirely new method of absorbing the energy of electromagnetic waves. The paper has been published in Physical Review B.
Effective absorption of electromagnetic waves is also important for use in sensing, nanochemistry, and photodynamic therapy. A classic example of an electromagnetic absorber that is familiar to many people is ordinary black paint. It looks black because a significant amount of the light that falls on it is absorbed in the layer of paint and does not reach the observer. However, black paint is a relatively poor absorber — a certain amount of energy from the incident light (typically a few percent) is still reflected back into the surrounding space.
In order to absorb incident radiation completely, we need to use interference. When light falls on an layered absorbing system, if the coating parameters have been chosen properly, the reflected waves cancel each other out — reflected radiation vanishes completely and the absorption becomes perfect. This type of interference is called destructive interference. Absorption in such systems is very sensitive to the geometry of the structure. With the slightest variation in thickness or refractive indices of the layers the absorption is no longer perfect and reflected radiation reappears.
In their paper, the Russian and American researchers showed that destructive interference is not a necessary requirement for perfect absorption. The scientists used an anisotropic crystal 3 hexagonal boron nitride — as their specific absorbing system.
The approach proposed by the researchers is currently only able to achieve perfect absorption for a fixed wavelength and angle of incidence, both of which are determined by the electronic properties of the material. However, for practical applications the possibility of energy absorption in a wide range of wavelengths and angles of incidence is of more interest. The use of alternative strongly anisotropic materials such as biaxial absorbing media will likely help to bypass these limitations in the future, making this approach more flexible.
Nevertheless, this experiment is of interest from a fundamental point of view. It demonstrates that it is possible to completely absorb radiation without the incorporation of destructive interference. This effect offers a new tool for controlling electromagnetic absorption. In the future, these materials could give a greater level of flexibility when designing absorbing devices and sensors that operate in the infrared range.