An international scientific team have demonstrated that it is possible to fully absorb electromagnetic radiation using an anisotropic crystal, which primarily important for reducing of the radar
A team of authors from MIPT, Kansas State University, and the U.S. 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.
On the left is an absorbing medium lying on a reflective substrate. On the right is an absorbing medium with an anti-reflective coating applied on top. In both cases the interference of light results in the complete absorption of energy within the artificial structure.
Image courtesy of the authors of the study
Effective absorption of the energy of electromagnetic radiation is the cornerstone of a wide range of practical applications. Electromagnetic energy harvesting in the visible spectrum is very important for photovoltaics - the conversion of solar energy into direct current electricity. Absorbing materials in the microwave range of frequencies have an application that is equally as important - they are able to reduce the radar visibility of an aircraft. 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. A layer of absorbing material is placed on a reflective substrate or is combined with a specially designed anti-reflective coating. According to the laws of classical electrodynamics, there emerges a sequence of waves having different amplitudes and phases that are reflected from the structure. Such series of reflections also occurs in a soap film. When white light falls on the film, we see reflected light of a certain colour depending on the thickness of the film. When light falls on an 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 researchers from Russia and the US showed that destructive interference is not a necessary requirement for perfect absorption. The scientists used an anisotropic crystal - hexagonal boron nitride - as their specific absorbing system.
This medium belongs to the class of unique van der Waals crystals which consist of atomic layers bound together by van der Waals forces from adjacent layers. Van der Waals forces occur between atoms and molecules that are electrically neutral but possess a dipole moment - the charges in them are not uniformly distributed. Due to this arrangement of the lattice, the dielectric permittivity of the crystal in the mid-infrared range (wavelength of about 10 microns) differs considerably for the in- and out-of-plane directions - it becomes anisotropic and is not described by a single number, but by a tensor - a matrix of numbers (each number is responsible for its own direction). It is the dielectric permittivity tensor that determines how light is reflected from the surface of any substance.
Due to the unusual properties of its crystal lattice, hexagonal boron nitride has already found a number of applications in optics and nanoelectronics. In this particular case, the strong anisotropy of dielectric permittivity works in our favour and helps to absorb electromagnetic waves. Incident infrared radiation at a certain wavelength enters the crystal without reflections and is completely absorbed within the medium. There is no need for any anti-reflective layers or a substrate to provide destructive interference - reflected radiation simply does not occur, unlike in an isotropic (i.e. identical in all directions) absorbing medium.
"The ability to fully absorb electromagnetic radiation is one of the key areas of focus in electrodynamics. It is believed that destructive interference is needed to do this, which therefore requires the use of anti-reflective coatings, substrates and other structures. Our observations indicate that interference is not a compulsory requirement and perfect absorption can be achieved using simpler systems," says Denis Baranov, the corresponding author of the paper.
For the experimental observation of the predicted phenomenon, the researchers grew an optically thick sample of hexagonal boron nitride and measured the reflectance spectrum in the mid-infrared range. At the wavelengths and angles of incidence predicted analytically, the authors observed a strong drop in the reflected signal - less than 10-4 of the incident energy was reflected from the system. In other words, more than 99.99% of the incident wave energy was absorbed in the anisotropic crystal.
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.
Valerii Roizen | EurekAlert!
Heating quantum matter: A novel view on topology
22.08.2017 | Université libre de Bruxelles
Engineering team images tiny quasicrystals as they form
18.08.2017 | Cornell University
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
22.08.2017 | Health and Medicine
22.08.2017 | Materials Sciences
22.08.2017 | Life Sciences