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2Physics Quote:
"Lasers are light sources with well-defined and well-manageable properties, making them an ideal tool for scientific research. Nevertheless, at some points the inherent (quasi-) monochromaticity of lasers is a drawback. Using a convenient converting phosphor can produce a broad spectrum but also results in a loss of the desired laser properties, in particular the high degree of directionality. To generate true white light while retaining this directionality, one can resort to nonlinear effects like soliton formation."
-- Nils W. Rosemann, Jens P. Eußner, Andreas Beyer, Stephan W. Koch, Kerstin Volz, Stefanie Dehnen, Sangam Chatterjee
(Read Full Article: "Nonlinear Medium for Efficient Steady-State Directional White-Light Generation"
)

Sunday, October 25, 2015

Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption

Ihar Faniayeu (left) and Viktar Asadchy

Authors: Ihar Faniayeu1,2, Viktar Asadchy2,3, Younes Ra'di3, Sergey Khakhomov2, Igor Semchenko2, Sergey Tretyakov3

Affiliation:
1Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan,
2Department of General Physics, Francisk Skorina Gomel State University, Gomel, Belarus,
3Department of Radio Science and Engineering, Aalto University, Aalto, Finland.

People these days exhibit strong desire to control their surroundings, which -- in addition to tangible objects -- involves electromagnetic radiation omnipresent as radio waves, heat, and light. In this regard, the latest trend is to use electromagnetic metamaterials for transforming flow and absorption of electromagnetic waves. This trend has opened up new possibilities in imaging, telecommunications, signal processing, environmental sensing, medicine and other areas of science and technology. While metamaterial-based absorbers are typically tailored to exhibit efficient absorption at the desired resonance frequency, they are usually not transparent at other non-operative frequencies, and may exhibit strong unwanted back-reflections [1], which limit their functionality. Here we demonstrate for the first time a metamaterial-based absorber which uses 3D architecture without an opaque ground plane, which leads to complete off-resonance transparency, as is illustrated schematically in Fig. 1 (a) and (b).
Figure 1(a): Schematic design and working principle of metamaterial absorber.
Figure 1(b): Transmission, reflection and absorption spectra of the structure illustrate its perfect absorbance (R=1) at the resonance, and complete transparency (T=1) away from it.

Our work published in Physical Review X [2], presents both theoretical concept and experimental realization of such invisible metamaterial absorber for the microwave range. Conceptually, the structure consists of a periodic array of right- and left-handed single- and double-turn helices made of lossy metal, embedded in a dielectric. Balanced periodic arrangement of these bi-anisotropic elements leads to extremely broadband resonant response, which can be utilized in transmission arrays and absorbers [3]. In the case of absorber, strong resistive losses occurring in the metal transforms the absorbed electromagnetic field energy into heat. This concept is quite general and is therefore applicable in the entire electromagnetic spectrum. Practical demonstration of such absorber uses thin chromium-nickel wire helices embedded in a plastic foam sheet as shown in Fig. 2. As expected, strong absorption resonance is seen around 3 GHz frequency (see Fig. 3), whereas reflection remains low in the entire measured range.
Figure 2(a): Fabricated absorbers of single-turn helices comprising 480 elements embedded in plastic foam.
Figure 2(b): Fabricated absorbers of double-turn helices comprising 324 elements embedded in plastic foam.
Figure 3: Measured and numerically simulated reflection, transmission, and absorption coefficients for the fabricated metasurfaces with (a) single- and (b) double-turn helical inclusions. Experimental data is shown by points, the solid lines are guides to the eye, and the numerically simulated data are shown by dashed lines.

This concept and its experimental verification suggest that it is possible to realize metamaterial-based absorbers having significant advantages over other existing designs. We stress here that off-resonance transparency of the single absorber layer allows realization of multilayer structures where layers operate at different frequencies simultaneously without cross-talk, thus drastically expanding functionality of the device. The structure is tunable by changing its unit cell size. Terahertz and even visible wavelength range can be reached, provided that electromagnetic dispersion of the metal is taken into account and fabrication technique allowing realization of downscaled lattice is available. It is expected that currently available nanofabrication techniques, such as 3D printing and direct laser writing lithography will allow practical realization of such metamaterial structures, thus making a further step toward more versatile tailoring of electromagnetic radiation.

We thank Prof. Vygantas Mizeikis for helpful discussions and support in this article.

References:
[1] Claire M. Watts, Xianliang Liu, Willie J. Padilla, "Metamaterial Electromagnetic Wave Absorbers", Advanced Materials, 24, OP98 (2012). Abstract.
[2] V.S. Asadchy, I.A. Faniayeu, Y. Ra’di, S.A. Khakhomov, I.V. Semchenko, S.A. Tretyakov, "Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption", Physical Review X, 5, 031005 (2015). Abstract.
[3] V.S. Asadchy, I.A. Faniayeu, Y. Ra'di, I.V. Semchenko, S.A. Khakhomov, "Optimal arrangement of smooth helices in uniaxial 2D-arrays", Advanced Electromagnetic Materials in Microwaves and Optics (Metamaterials), 7th International Congress, pp. 244–246 (16-21 September, 2013). Abstract.

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