Ultratransparent Media: Towards the Ultimate Transparency

From left to right: (top row) Jie Luo, Yuting Yang, Zhongqi Yao, Weixin Lu; (bottom row) Bo Hou, Zhi Hong Hang, C. T. Chan, and Yun Lai.

Authors: Jie Luo¹, Yuting Yang¹, Zhongqi Yao¹, Weixin Lu¹, Bo Hou¹, Zhi Hong Hang¹, Che Ting Chan², Yun Lai¹

Affiliation:
¹College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China
²Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong.

Transparent media are the foundation of almost all optical instruments, such as optical lens, etc. However, perfect transparency has never been realized in natural transparent solid materials such as glass because of the impedance mismatch with free space or air. As a consequence, there generally exist unwanted reflected waves at the surface of a glass slab, as illustrated in Fig. 1(a). It is well known that non-reflection only occurs at a particular incident angle for a specific polarization, which is known as the Brewster angle effect [1]. Our question is: is it possible to extend the Brewster angle from a particular angle to a wide range of or all angles, so that there is no reflection for any incident angle.

In addition, the virtual image formed by a glass slab placed in air is usually blurred to a certain extent [Fig. 1(a)]. Such a blur indicates the aberration of virtual images, and is caused by the mismatch of equal frequency contours (EFCs) between air (grey lines) and the glass (blue lines).

Figure 1: (a) Virtual image formation through a glass slab, with general reflection and aberration. (b) Aberration-free virtual image formation through an ultratransparent photonic crystal without any reflection due to omnidirectional impedance matching. The black arrows and blue dashed lines in represent the light rays from a point source, and the back tracing lines, respectively. The yellow dashed curves in (b) indicate equal phase surfaces of transmitted rays. The inset graphs show the corresponding EFCs. The figure is adapted from Reference [2].

The purpose of our work [2] is to explore the possibility of realizing the ultimate transparency by artificial optical structures such as photonic crystals (PhCs) [3] and metamaterials [4]. In other words, we pursue the realization of transparent media with the extreme property of omnidirectional impedance matching and the ability of forming aberration-free virtual images, which are hereby denoted as ultratransparent media.

In this work, we propose that omnidirectional impedance matching can be realized by utilizing effective medium with nonlocal parameters, i.e. permittivity and permeability that are dependent on the incident angle. Interestingly, such an effective medium can be realized by using pure dielectric PhCs. Moreover, the EFC of the ultratransparent PhC can be tuned to be a shifted ellipse (red lines) with the same height of the EFC of air (grey lines). By using ray optics, we prove that such an EFC endows the valuable ability of forming aberration-free virtual images, as presented in Fig. 1(b).

Because of the shift of EFC, the PhC is beyond the local medium framework, and effective parameters are nonlocal (i.e. spatially dispersive). Interestingly, such nonlocality leads to additional phase modulation p d, where p is the shift magnitude and d is the slab thickness [Fig. 1(b)].

Figure 2: (a) Illustration of the unit cell of the ultratransparent PhC. (b) The EFC of the PhC. (c) Transmittance through the PhC slab with N (=4, 5, 6, 15) layers of unit cells as functions of the incident angle. The figure is adapted from Reference [2].

An extreme example with almost complete transparency (T>99%) for nearly all incident angles (-89^o, +89^o) is shown in Fig. 2. The PhC is two-dimensional and its unit cell is shown in Fig. 2(a). For transverse electric polarization, at the working frequency, the EFC is a shifted ellipse (red dashed curve) with the same height as that in free space (grey dashed curve), as shown in Fig. 2(c). The transmittance through such a PhC slab is always near unity (>99%) for nearly all incident angles (<89^o), and is almost irrespective of the layer number, N.

Figure 3: (a) Photo of the simplified PhC composed of alumina bars (white) placed inside the microwave field mapper. (b) The EFC of the PhC. (c) Transmittance through the PhC slab in simulations (solid lines) and experiments (triangular dots) and an alumina slab having the same thickness (dashed lines) as the function of incident angles. The figure is adapted from Reference [2].

To prove the theory, we performed proof-of-principle microwave experiments by utilizing a simplified PhC consisting of rectangular alumina bars in a square lattice, as shown in Fig. 3(a). Such a PhC exhibits a shifted elliptical EFC [Fig. 3(b)] and a wide-angle impedance matching effect. The measured transmission data (triangular dots) and simulation results (solid lines) both show high transmittance, which is great enhancement compared to the transmittance through an alumina slab with the same thickness (dashed lines).

Finally, we also note that such ultratransparent media can extend transformation optics (TO) [5, 6] to the general realm of nonlocal media. The traditional TO was founded in the local medium framework and require local media. Here, we demonstrate that ultratransparent media with controllable refractive indexes are also good candidates for TO applications such as invisibility cloaks. Interestingly, due to the nonlocality, the ultratransparent media also enable additional freedom in phase modulation, which is absent in the traditional TO. At optical frequencies, ultratransparent PhCs exhibit the significant advantages of omnidirectional impedance matching, low loss and micro fabrication requirement.

The concept and theory of ultratransparency gives a guideline for realizing the ultimate transparency which is broadband, omnidirectional and polarization-insensitive. Recently, we designed broadband, wide-angle and polarization-insensitive transparent media by using one-dimensional dielectric PhCs [7]. In the future, ultratransparent solid materials may be optimized to exhibit an unprecedented level of transparency and produce no reflection at all in certain ranges of frequencies.

References:
[1] John D. Jackson, "Classical Electrodynamics" (3rd edition, Wiley, New York, 1975).
[2] J. Luo, Y. Yang, Z. Yao, W. Lu, B. Hou, Z. H. Hang, C. T. Chan, and Y. Lai, "Ultratransparent media and transformation optics with shifted spatial dispersions", Physical Review Letters, 117, 223901 (2016). Abstract.
[3] John D. Joannopoulos, Steven G. Johnson, Joshua N. Winn, Robert D. Meade, "Photonic Crystals: Molding the Flow of Light" (2nd edition, Princeton University Press, Princeton, USA, 2008).
[4] Yongmin Liu, Xiang Zhang, "Metamaterials: a new frontier of science and technology", Chemical Society Reviews, 40, 2494-2507 (2011). Abstract.
[5] J. B. Pendry, D. Schurig, and D. R. Smith, "Controlling electromagnetic fields", Science, 312, 1780-1782 (2006). Abstract.
[6] Ulf Leonhardt, "Optical conformal mapping", Science, 312, 1777-1780 (2006). Abstract.
[7] Zhongqi Yao, Jie Luo, Yun Lai, "Photonic crystals with broadband, wide-angle, and polarization-insensitive transparency", Optics Letters, 41, 5106 (2016). Abstract.

A Magnetic Wormhole

(From left to right) Carles Navau, Alvaro Sanchez, Jordi Prat-Camps

Authors: Jordi Prat-Camps, Carles Navau, Alvaro Sanchez 

Affiliation: Departament de Física, Universitat Autònoma de Barcelona, Spain.

Link to Superconductivity Group UAB >>

Is it possible to build a wormhole in a lab? Taking into account that large amounts of gravitional energy would be required [1], this seems an impossible task. However, redefining a wormhole into a path between two points in space that is completely undetectable, Greenleaf and colleagues [2] suggested in 2007 a (theoretical) way of realizing an electromagnetic wormhole capable of guiding light through an invisible path. They demonstrated that this is topologically equivalent as if the light had been sent through another spatial dimension. However, such a wormhole required metamaterials with extreme properties, which prevented its construction.

In our work, we have constructed an actual 3D wormhole working for magnetostatic fields. It allows the passage of magnetic field between distant regions while the region of propagation remains magnetically invisible. Our wormhole takes advantage of the possibilities that magnetic metamaterials offer for shaping static magnetic fields [3]. These metamaterials can be constructed using existing magnetic materials that can provide extreme magnetic permeability values ranging from zero - superconductors - to effectively infinity - ferromagnets.

Figure 1: (Left) 3D sketch of the magnetic wormhole, showing how the magnetic field lines (in red) of a small magnet at the right are transferred through it. (Right) From a magnetic point of view the wormhole is magnetically undetectable so that the field of the magnet seems to disappear at the right and reappear at the left in the form of a magnetic monopole. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

The magnetic wormhole requires three properties: (i) to magnetically decouple a given volume from the surrounding 3D space, (ii) to have the whole object magnetically undetectable, and (iii) to have magnetic fields propagating through its interior. The first two properties are achieved by constructing a 3D magnetic cloak. Based on previous ideas [4] such a cloak could be made by surrounding a superconducting sphere with a specially created ferromagnetic (meta)surface, such that the magnetic signature of the superconductor was cancelled by the ferromagnet. For the third property, we use magnetic hoses, also made by magnetic metamaterials, as developed in [5].

The parts composing the magnetic wormhole are shown in fig. 2: a central magnetic hose to guide the magnetic field from one end of the hose to the opposite one, and a magnetic cloak composed of a superconducting-ferromagnetic bilayer to make the hose magnetically invisible.

Figure 2: (a) 3D image of the magnetic wormhole, formed by concentric shells: from outside inwards, an external metasurface made of ferromagnetic pieces (b), an internal superconducting shell made of coated conductor pieces (c), and a magnetic hose made of ferromagnetic foil (d). (e) Cross-section view of the wormhole, including the plastic formers (in green and red) used to hold the different parts. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

Experimental results clearly demonstrate [6] the two desired properties for the wormhole: (i) magnetic field from a source at one end of the wormhole appear at the opposite end (actually as a kind of isolated magnetic monopole), (ii) the overall device is magnetically undetectable (it does not noticeably distort an applied magnetic field, even a non-uniform one).

Besides the scientific interest per se in the realization of an object with properties of a wormhole, our device may have applications in practical situations where magnetic fields have to be transferred without distorting a given field distribution, as in magnetic resonance imaging.

Acknowledgements: We thank Spanish project MAT2012-35370 and Catalan 2014-SGR-150 for financial support. A.S. acknowledges a grant from ICREA Academia, funded by the Generalitat de Catalunya. J. P.-C. acknowledges a FPU grant form Spanish Government (AP2010-2556).

References:
[1] Michael S. Morris, Kip S. Thorne, Ulvi Yurtsever, "Wormholes, Time Machines, and the Weak Energy Condition", Physical Review Letters, 61, 1446 (1998). Abstract.
[2] Allan Greenleaf, Yaroslav Kurylev, Matti Lassas, Gunther Uhlmann, "Electromagnetic Wormholes and Virtual Magnetic Monopoles from Metamaterials", Physical Review Letters, 99, 183901 (2007). Abstract.
[3] Steven M. Anlage, "Magnetic Hose Keeps Fields from Spreading", Physics, 7, 67 (2014). Full Article.
[4] Fedor Gömöry, Mykola Solovyov, Ján Šouc, Carles Navau, Jordi Prat-Camps, Alvaro Sanchez, "Experimental realization of a magnetic cloak", Science, 335, 1466 (2012). Abstract.
[5] C. Navau, J. Prat-Camps, O. Romero-Isart, J. I. Cirac, A. Sanchez. "Long-Distance Transfer and Routing of Static Magnetic Fields", Physical Review Letters, 112, 253901 (2014). Abstract.
[6] Jordi Prat-Camps, Carles Navau, Alvaro Sanchez. "A Magnetic Wormhole", Scientific Reports 5, 12488 (2015). Full Article.

Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption

Ihar Faniayeu (left) and Viktar Asadchy

Authors: Ihar Faniayeu^1,2, Viktar Asadchy^2,3, 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.