Semi-OT Abstract- Waveguide Characterization of S-Band Microwave Mantle Cloaks for Dielectric and Conducting Objects

Antonino Vitiello

, Massimo Moccia

, Gian Paolo Papari

, Giuliana D’Alterio

, Roberto Vitiello

, Vincenzo Galdi

 & Antonello Andreone

http://www.nature.com/articles/srep19716

We present the experimental characterization of mantle cloaks designed so as to minimize the electromagnetic scattering of moderately-sized dielectric and conducting cylinders at S-band microwave frequencies. Our experimental setup is based on a parallel-plate waveguide system, which emulates a two-dimensional plane-wave scattering scenario, and allows the collection of near-field maps as well as more quantitative assessments in terms of global scattering observables (e.g., total scattering width). Our results, in fairly good agreement with full-wave numerical simulations, provide a further illustration of the mantle- cloak mechanism, including its frequency-sensitivity, and confirm its effectiveness both in restoring the near-field impinging wavefront around the scatterer, and in significantly reducing the overall scattering.

(My Note, From the paper:)
By comparison with the plasmonic implementation, mantle cloaks tend to be particularly suited for microwave and terahertz frequencies, providing low-profile, conformal, easy-to-fabricate configurations that are especially attractive for applications to reduction of antenna coupling

SpectroscopyNOW-Last Month's Most Accessed Feature: Invisibility cloak: Hiding the microscopic

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http://www.spectroscopynow.com/ir/details/highlight/14de329bf14/Last-Months-Most-Accessed-Feature-Invisibility-cloak-Hiding-the-microscopic.html

Cloaking device

A microscopic invisibility cloak based on brick-like blocks of gold nanoantennae could pave the way to a flexible device for making an object invisible in visible light.

Invisibility cloaks have been a staple of science fiction and fantasy for decade, who, after all, has not mused on what they might get up to if they could make themselves invisible? For several years camouflage covers that resemble the background environment to help hide a person were the best option, leafy greens allowing a soldier to crawl through undergrowth undetected perhaps, except when night-vision or thermal imaging is in place. Aircraft can be made "stealth" to hide them from the reflective waves of a radar tower but they can still be spotted as they fly by at altitude with a decent set of binoculars and a good eye.

In recent years, however, meta materials have emerged that might take invisibility to the next level. There have been demonstrations of infrared and other forms of invisibility but now, researchers at the US Department of Energy's Lawrence Berkeley National Laboratory and the University of California Berkeley have come up with a microscopic invisibility cloak that can hide a three dimensional object from the microscope's viewfinder. The team suggest that the principle should work on the macroscopic level to once it is scaled up.

Concealing to appeal

The team has worked with gold nanoantennae to fabricate a flexible skin a mere 80 nanometres in thickness that can be wrapped around a three-dimensional object of arbitrary shape the size of a clump of biological cells. Adopting the underlying lumps and bumps of the object. The meta-engineered surface of the skin cloak allows it to re-route reflected light waves so impinge on it so that the object's lumps and bumps are rendered invisible to optical detection when the cloak is activated.

"This is the first time a 3D object of arbitrary shape has been cloaked from visible light," explains meta materials expert Xiang Zhang, director of Berkeley Lab's Materials Sciences Division. "Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects," he adds.

Zhang, working with Xingjie Ni, Zi Jing Wong, Michael Mrejen and Yuan Wang, point out that that it is the scattering of electromagnetic radiation, whether that is visible light, infrared, X-ray, or another band in the spectrum, and its interaction with matter that enables us to detect and observe objects. The researchers explains that the rules that govern these interactions in natural materials can be circumvented using meta materials whose optical properties arise from their physical structure rather than their chemical composition. The surface of a butterfly's wing has no coloured pigment, it's surface texture interacts with visible light to cause iridescence that gives rise to its beautiful colours and patterns and so might be thought of as a natural meta material.

For the past ten years, Zhang and his research group have been pushing the boundaries of how light interacts with fabricated meta materials. They have managed to curve the path of light or bend it backwards, creating negative refractive index meta materials, a phenomenon not seen before in nature, and to render objects optically undetectable. In the past, their meta material-based optical carpet cloaks were bulky and hard to scale-up, and entailed a phase difference between the cloaked region and the surrounding background that made the cloak itself detectable, although what it was concealing could not be detected, which defeats the object of being invisible one has to say.

Skin to carpet

"Creating a carpet cloak that works in air was so difficult we had to embed it in a dielectric prism that introduced an additional phase in the reflected light, which made the cloak visible by phase-sensitive detection," explains team member and co-lead author Ni, who has recently moved to Pennsylvania State University. "Recent developments in meta surfaces, however, allow us to manipulate the phase of a propagating wave directly through the use of sub-wavelength sized elements that locally tailor the electromagnetic response at the nanoscale, a response that is accompanied by dramatic light confinement."

In their experiments, the team shone red light struck on a sample object with an area of about 1300 square micrometres. When it was sheathed in the gold nanoantennae skin cloak, light reflected from the object's surface produced the same effect as if the light were simply reflecting from a plane mirror. The 3D object cloaked in this way is thus invisible even by phase-sensitive detection. The team points out that their cloaking device can be turned on or off simply by switching the polarization of the nanoantennae.

"A phase shift provided by each individual nanoantenna fully restores both the wavefront and the phase of the scattered light so that the object remains perfectly hidden," explains Wong. Ironically, this ability to manipulate the interactions of light and a meta material for invisibility hints at a future of high resolution optical microscopes and superfast optical computers and as a component of a future 3D display technology. Conversely, such a device could be used for security through obscurity applications allowing microscopic components to be hidden for privacy or security applications. purposes.

Related Links

Science 2015, 349, 1310-1314: "An ultrathin invisibility skin cloak for visible light"

Article by David Bradley

• Read more about David and our other columnists >>>

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

OBJECTS INVISIBLE WITHOUT METAMATERIAL CLOAKING

                                            

This is the radio-frequency anechoic chamber used for the experiment. Credit ITMO University
http://www.eurasiareview.com/14042015-objects-invisible-without-metamaterial-cloaking/

Physicists from ITMO University, Ioffe Institute and Australian National University managed to make homogenous cylindrical objects completely invisible in the microwave range.

Contrary to the now prevailing notion of invisibility that relies on metamaterial coatings, the scientists achieved the result using a homogenous object without any additional coating layers. The method is based on a new understanding of electromagnetic wave scattering. The results of the study were published in Scientific Reports.

The scientists studied light scattering from a glass cylinder filled with water. In essence, such an experiment represents a two-dimensional analog of a classical problem of scattering from a homogeneous sphere (Mie scattering), the solution to which is known for almost a century. However, this classical problem contains unusual physics that manifests itself when materials with high values of refractive index are involved. In the study, the scientists used ordinary water whose refractive index can be regulated by changing temperature.

As it turned out, high refractive index is associated with two scattering mechanisms: resonant scattering, which is related to the localization of light inside the cylinder, and non-resonant, which is characterized by smooth dependence on the wave frequency. The interaction between these mechanisms is referred to as Fano resonances. The researchers discovered that at certain frequencies waves scattered via resonant and non-resonant mechanisms have opposite phases and are mutually destroyed, thus making the object invisible.

The work led to the first experimental observation of an invisible homogeneous object by means of scattering cancellation. Importantly, the developed technique made it possible to switch from visibility to invisibility regimes at the same frequency of 1.9 GHz by simply changing the temperature of the water in the cylinder from 90 °C to 50 °C.

“Our theoretical calculations were successfully tested in microwave experiments. What matters is that the invisibility idea we implemented in our work can be applied to other electromagnetic wave ranges, including to the visible range. Materials with corresponding refractive index are either long known or can be developed at will,” said Mikhail Rybin, first author of the paper and senior researcher at the Metamaterials Laboratory in ITMO University.

The discovery of invisibility phenomenon in a homogenous object and not an object covered with additional coating layers is also important from the engineering point of view. Because it is much easier to produce a homogeneous cylinder, the discovery could prompt further development of nanoantennas, wherein invisible structural elements could help reduce disturbances. For instance, invisible rods could be used as supports for a miniature antenna complex connecting two optical chips.

The subject of invisibility came into prominence with the development of metamaterials – artificially designed structures with optical properties that are not encountered elsewhere in nature. Metamaterials are capable of changing the direction of light in exotic ways, including making light curve around the cloaked object. Nevertheless, coating layers based on metamaterials are extremely hard to fabricate and are not compatible with many other invisibility ideas. The method developed by the group is based on a new understanding of scattering processes and leaves behind the existing ones in simplicity and cost-effectiveness.

Quick History of Cloaking Devices

AMANDA B. WOMAC

http://www.memphisdailynews.com/news/2015/feb/28/quick-history-of-cloaking-devices

Dr. Ramki Kalyanaraman, associate professor of materials science engineering, and his colleague Dr. Gerd Duscher at UT are not the only people working to develop cloaking technologies.

From its 1966 debut in a Star Trek episode to its present form, the science of invisibility has captured the imaginations of everyone from screenwriters to physicists.

Invisibility as a concept has been used in fiction since the days of Plato.

Most commonly associated with magic, people become invisible by putting on a cloak or a cap or by gaining magical powers through a pact with the devil as seen in Christopher Marlow’s Doctor Faustus. It first appeared on the big screen in the 1933 science fiction film, The Invisible Man and continues to be a theme in comic books, video games and literature to this day.

However, it wasn’t until 1968 that the term “cloaking device” was introduced to the general public. If you are a Trekkie, you probably remember when a Romulan Bird of Prey armed with a cloaking device attacked the Starship Enterprise in the 1966 episode titled “Balance of Terror.”

It was in the 1968 episode “The Enterprise Incident” that Star Trek screenwriter Paul Schneider gives invisibility technology a name: Cloaking device.

In 1997, J.K. Rowling published Harry Potter and the Philosopher’s Stone, in which Harry Potter receives a cloak of invisibility as a gift. However, up until this point, invisibility and cloaking were still a concept of science fiction.

The British military experimented with what is known as “active cloaking” technologies in 2009. This technology uses video cameras to record the landscape and then projects the image on the front of an object, such as the hull of a tank.

Delays in the video feed and its one-dimensional view turned out to be the main problems with this cloaking technology.

The real break-through with cloaking devices came in 2006 when physicists from Duke University announced they had built the world’s first invisibility cloak using what was novel in 2006: Metamaterials.

The physicists developed a matrix of metal wires and loops on the nanoscale to control electromagnetic radiation. However, the technology only worked in two dimensions and on microwaves.

Picking up on the theme of metamaterials as the potential answer to true invisibility, scientists at the University of California, Berkeley, developed metamaterials in 2008 with extraordinary capabilities to bend electromagnetic waves. For the first time, scientists used 3-D materials to reverse the natural direction of visible and near-infrared light.

A year later, they develop a nanostructured silicon, or carpet cloak, that hid objects underneath it from optical detection. The main limitation in this case is the cloak is still visible.

In 2010, scientists at Tufts and Boston universities developed an invisibility cloak fit for a queen and capable of manipulating terahertz waves.

The cloak is made from a one centimeter square piece of silkworm silk and is stenciled with 10,000 gold resonators. Scientists blasted the metamaterial with terahertz waves and detected a resonance when usually, they would blast right through.

Since that time, advances in the use of metamaterials have increased significantly and scientists take one step closer to making a cloak of invisibility a possibility for aspiring Romulans and Harry Patter fans everywhere.

Three-dimensional invisibility cloaks functioning at terahertz frequencies

Wei Cao, Fan Zhou, Cheng Sun, Jianqiang Gu, Dachuan Liang, Jiaguang Han, Shuang Zhang and 

Weili Zhang

http://spie.org/x108531.xml

Both the geometric and spectroscopic signatures of an object were completely concealed under 3D terahertz invisibility cloaks made of either homogeneous or inhomogeneous media.

3 June 2014, SPIE Newsroom. DOI: 10.1117/2.1201405.005440

Achieving invisibility cloaking in the terahertz regime has recently garnered a great deal of attention due to unique and promising emerging applications of terahertz technology. Particularly with recent advances in terahertz communications and radar, there has been an increasing demand for cloaking devices functioning at terahertz frequencies. The ultimate goal is to conceal a large object from being observed by terahertz radar in civilian or space communications.

So far, terahertz invisibility cloaks have been experimentally demonstrated in quasi-3D geometry based on both subwavelength building blocks and homogeneous uniaxial crystals.1, 2 These quasi-3D cloaks are also called ground plane or carpet cloaks, and they were proposed to overcome the difficulties of complete cloaks that require dielectric singularity, have high loss, and very narrow bandwidth.3, 4 One of the challenges that remains at terahertz frequencies is finding photo-curable dielectric materials with lower loss that allow for a broader response bandwidth. Indeed, the complete cloak that features 3D and broad bandwidth is the major challenge for the entire electromagnetic spectrum. Here we present experimental demonstrations of quasi-3D terahertz cloaks made from either 3D inhomogeneous metamaterials 1 or homogeneous 2 media.

The inhomogeneous cloak made from dielectric metamaterials was lithographically fabricated using a scalable Projection Microstereolithography (PμSL) process (see Figure 1). The triangular cloaking structure has a total thickness of 4.4mm, comprised of 220 layers of 20μm thickness. The distribution of the varying hole geometry can be clearly identified in the scanning electron microscope (SEM) images. The space underneath the bump is designated as the cloaked region. The cloak operates at a broad frequency range between 0.3 and 0.6THz, and is placed over an α-lactose monohydrate absorber with rectangular shape. The α-lactose monohydrate exhibits a resonant attenuation signature at 0.53THz due to the presence of collective vibrational transition modes. We measured the reflected terahertz wave in four cases: (I) a flat reflective surface, (II) exposed lactose on a reflective surface, (III) a control structure with a reflective bump placed on top of the lactose, and (IV) the cloaking structure placed on top of the lactose. The measurements were carried out in an angular-resolved reflection terahertz time domain spectroscopy (THz-TDS) system. The terahertz cloak in case (IV) conceals both the geometric and spectroscopic features of the lactose, which then closely resembles case (I), demonstrating the successful design of the cloak.

 

Figure 1. Inhomogeneous quasi-3D terahertz cloak. (a) Schematic diagram illustrating the projection micro-stereolithography system fabricating a 3D metamaterial cloaking device. The grayscale of individual pixels within each 85.2×85.2μm unit cell can be adjusted so the holes can be fabricated with sub-pixel precision. (b) Optical and scanning electron microscope images of the fabricated cloaking device. The gradual change in hole size near the bump can be clearly observed.1

We also demonstrated a large-scale terahertz invisibility cloak made from birefringent crystalline sapphire (see Figure 2). This homogeneous cloaking device features a large concealed volume, low loss, and broad bandwidth. In particular, it is capable of hiding objects with a dimension nearly an order of magnitude larger than that of its lithographic counterpart, but without involving complex and time-consuming cleanroom processing. The cloak was made from two 20mm thick high-purity sapphire prisms. The area beneath the cloak is 1.75mm tall, nearly ten times taller than the inhomogeneous counterpart. In addition, the useful bandwidth increased from 0.3-0.6THz to 0.2-1.0THz due to different design approaches and material absorption properties. The volume of the cloaking region is approximately 5% of the whole sample.

 

Figure 2. Measured cloaking effect of a homogeneous terahertz cloak with respect to the relative positions (x-axis) and the frequency (y-axis). The color represents the relative spectral amplitude of (a) cloaking, (b) flat surface reflection, and (c) reference with the same cloak lens. A schematic of the cloak design is shown in (d).2

We characterized the homogeneous cloak using the same terahertz spectroscopy system, except that no test sample was placed in the cloak region. Instead, we used the uncloaked transverse electric polarized beam profile as the reference, which exhibits significant beam splitting: see Figure2(c). The THz-TDS results indicated that the terahertz invisibility cloak successfully concealed both the geometric and spectroscopic signatures of the object, making it undetectable to the observer. As shown in Figure 2, the reflected transverse magnetic beam from the cloak (a) shows nearly the same profile as that reflected by a flat mirror (b). On the other hand, for the transverse electric beam, the detector received two largely separated beam profiles at the left and right side of the cloaking profile, shown in (c).

This straightforward cloaking approach greatly reduces the complexity in design and fabrication of the metamaterial-based cloaks. More importantly, the initial work in the visible regime confirmed that this is a promising way toward practical macroscopic cloaking devices with frequency and incidence angle robustness.

In conclusion, we demonstrated that quasi-3D invisibility cloaks in the terahertz regime, comprised of either homogeneous or inhomogeneous media, can completely conceal both the geometrical and spectroscopic signatures of a rectangular absorber placed beneath them. Our next steps in this direction will explore innovative approaches toward a complete 3D cloak for terahertz wavelengths. The keys will be the transformation optics design, unique metamaterial building blocks, and novel fabrication processing.

The authors thank Y. Bao, C. T. Stuart, and Y. Yang for outstanding contributions in this work. We acknowledge financial support from the U.S. National Science Foundation and the China National Natural Science Foundation.

---

Wei Cao

Oklahoma State University

Stillwater, OK

Fan Zhou, Cheng Sun

Northwestern University

Evanston, IL

Jianqiang Gu, Dachuan Liang, Jiaguang Han

Tianjin University

Tianjin, China

Shuang Zhang

University of Birmingham

Birmingham, United Kingdom

Weili Zhang
Oklahoma State University
Stillwater, OK
and
Tianjin University
Tianjin, China

---

References:

1. F. Zhou, Y. J. Bao, W. Cao, C. T. Stuart, J. Gu, W. Zhang, C. Sun, Hiding a realistic object using a broadband terahertz invisibility cloak, Sci. Rep. 1, p. 78, 2011.

2. D. Liang, J. Gu, J. Han, Y. Yang, S. Zhang, W. Zhang, Robust large dimension terahertz cloaking, Adv. Mater. 24, p. 916, 2012.

3. J. S. Li, J. B. Pendry, Hiding under the carpet: a new strategy for cloaking, Phys. Rev. Lett. 101, p. 203901, 2008. doi:10.1103/PhysRevLett.101.203901

4. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, J. B. Starr, D. R. Smith, Metamaterial electromagnetic cloak at microwave frequencies, Science 314, p. 977, 2006.

Improving terahertz optics with efficient broadband antireflection coatings

Thin strips of chromium form an efficient antireflection coating for terahertz light and can be applied to a broad range of surfaces. Credit: A*STAR Institute of Materials Research and Engineering

http://phys.org/news/2014-05-terahertz-optics-efficient-broadband-antireflection.html#jCp

Antireflection coatings are familiar from their use in everyday optical devices, such as glasses and lenses. They can increase the amount of light that passes through optical instruments by reducing the fraction of light reflected (and hence lost) at surfaces. Antireflection coatings have applications beyond visible light: for instance, in the infrared and terahertz regimes they are useful for chemical sensing and imaging applications, such as those employed at airport security checks.

Now, Jing Hua Teng from the A*STAR Institute of Materials Research and Engineering and colleagues from the A*STAR Institute of Microelectronics and Osaka University, Japan, have developed ultrathin antireflection coatings for terahertz waves that can be applied to almost any surface. "Their fabrication is very straightforward, as it takes only one step of photolithography, metal deposition and lift-off," explains Teng.

Antireflection coatings are usually based on interference effects, which requires them to be at least as thick as the wavelength of light. This is practical for visible light, with wavelengths in the range of hundreds of nanometers. However, it is a serious limitation for infrared or terahertz radiation, which has much longer wavelengths of the order of hundreds of microns. Moreover, as these coatings are often functional only over narrow frequency ranges, they do not operate across the broad ranges needed for terahertz sensing applications.

The research team developed antireflection coatings based on metamaterials, which are metallic structures that are much smaller than the wavelength used. These structures completely alter the optical properties of a material in a predetermined way, enabling the generation of a much broader range of optical effects than those that occur naturally. One application of the unusual optical effects they produce is invisibility cloaks.

In the new design for metamaterial surfaces developed by the researchers, thin strips of chromium are fabricated on a silicon surface to form a grating (see image). Silicon, being flexible, is a typical material for terahertz optics. When terahertz light passes through the stripes and into the silicon, its phase is changed in the same way as for the much thicker coatings based on interference effects; this suppresses surface reflection.

These metasurfaces have the advantage that they can function across an unprecedentedly wide frequency range, namely 0.06 to 3 terahertz. The flexibility of the coatings for other wavelengths and applications also enhances their commercial appeal, comments Teng. "The beauty of this method is that it is very flexible and can be easily adapted to other metals and substrates."

 Explore further: Nonlinear optical materials convert terahertz radiation into infrared light

More information: Ding, L., Wu, Q. Y. S., Song, J. F., Serita, K., Tonouchi, M. & Teng, J. H. "Perfect broadband terahertz antireflection by deep-subwavelength, thin, lamellar metallic gratings." Advanced Optical Materials 1, 910–914 (2013). dx.doi.org/10.1002/adom.201300321

Department of Defense funds terahertz-range metamaterials research

http://phys.org/wire-news/161698553/department-of-defense-funds-terahertz-range-metamaterials-resear.html

Metamaterials research having potential applications in high-speed data transmission, medical imaging and other kinds of imaging and remote sensing is the focus of a U.S. Department of Defense project funded for five years at $7.5 million.

Penn State is part of this six-member Multi-University Research Initiative by the Air Force Office of Scientific Research. The project is led by Mark Cappelli, professor of mechanical engineering, Stanford University. Also collaborating with Stanford are the University of Texas at Austin, Tufts University, UCLA and the University of Washington.

Penn State researchers will focus on the fundamental science necessary to develop plasma photonic crystals and plasma-embedded metamaterials that operate in the terahertz range. Terahertz is the region of the electromagnetic spectrum that lies between far infrared and microwave, and is a nonionizing frequency invisible to the human eye. This regime is already being used in airport surveillance and astronomy.

The researchers will generate the plasmas inside holes in the metamaterial arrays using radio frequency excitation with the entire device encapsulated in an inert gas. Using micro-lens arrays, focused lasers will generate very dense, highly ionized plasma arrays. Unlike the metal structures of typical metamaterials, researchers can control a plasma's dielectric properties by varying the plasma density. Plasmas afford the possibility of controlling metamaterials at high bandwidth. This will enable such applications as antennas with beam steering, filter devices, multiplexers, phase shifters and electro-optical modulators.

Researchers at Penn State will be the primary team charged to develop a new class of low-loss dielectric resonators and multilayer low temperature co-fired ceramics to replace the usual metallic split-ring resonators found in traditional metamaterial structures. Metamaterials are artificial structures with sub-wavelength features that can interact with electromagnetic waves in a manner unlike that of natural materials. Long-term goals of metamaterials research include invisibility cloaking devices and perfect lenses to capture short-range light waves for fine detail light microscopy.

The principal investigators at Penn State are Clive Randall, professor of materials science and engineering, and Michael Lanagan, professor of engineering science and mechanics. The Penn State team members are pioneers in the development of dielectric materials and leaders in the long-running Center for Dielectric Studies, an industry supported research center that recently was renewed with technical new opportunities with North Carolina State University as the NSF I/UCRC Center for Dielectrics and Piezoelectrics.

Provided by Pennsylvania State University

This Phys.org Science News Wire page contains a press release issued by an organization mentioned above and is provided to you “as is” with little or no review from Phys.Org staff.

The first large-scale invisibility cloak that hides objects from visible light

James Plafke

http://www.extremetech.com/extreme/179973-the-first-large-scale-invisibility-cloak-that-hides-objects-from-visible-light
Among all of the sci-fi tech we see in movies — space and time travel, shrink rays, weaponized lasers — the invisibility cloak always seemed like the one piece of sci-fi technology that researchers could never create. Oddly, though, in recent times it has been thrust into the forefront of in-development science fiction technology. Now researchers at the University of Central Florida have managed to create a large-scale invisibility cloak that masks the spectrum of visible light. This is significant, as invisibility cloaking has previously only been possible for very specific wavelengths of radiation (say, microwaves). Visible light, which covers a broad swath of terahertz-level frequencies, has so far proven very hard to mask.

The real-life invisibility cloak is generally not what you picture when you hear the term. Generally, you picture a Harry Potter-style robe that completely erases your visual presence from an environment. In more “realistic” movies (relative to casting magic spells from a wand, at least), invisibility cloaks bend light around an object, making it look as if it’s covered in a liquid mirror. In real life, invisibility cloaks don’t come remotely close to the movies; for instance, they often tend to be solid objects that simply play perspective orreflective tricks on the eye. Now, researchers at a certain writer’s alma mater, University of Central Florida, have created a cloak that actually bends and masks visible light using a fishnet-type of metamaterial.

The metamaterial fishnet is composed of metal and dielectric composite films, created using a nanotransfer printing method. The films are layered in such a way as to create a fishnet-like pattern, which in turn allows the control of visible-spectrum light. [Research paper: DOI: 10.1002/adom.201470019]

The printed metamaterial sample is small — about 0.6 square inches (or four square centimeters) — but thanks to the fact that it’s a printing process, the UCF team feels it can print the material on a larger-scale for more practical applications, such as for use on fighter jets.

Debashis Chanda, a UCF assistant professor who led the project, noted that while invisibility cloaks won’t be on store shelves anytime soon (or ever), the team has been contacted by multiple companies looking to help fund more research on the matter. One of the interested parties is Lockheed Martin, so there is some high-profile interest behind the tech. For now, though, you shouldn’t salivate at the thought of being able to infiltrate the Monday morning meeting at work every week to find out if your coworkers are actually doing anything productive in there.

Do-it-yourself invisibility with 3-D printing

             This is Yaroslav Urzhumov. Credit: Duke University
http://phys.org/news/2013-05-do-it-yourself-invisibility-d.html
Seven years ago, Duke University engineers demonstrated the first working invisibility cloak in complex laboratory experiments. Now it appears creating a simple cloak has become a lot simpler.

"I would argue that essentially anyone who can spend a couple thousand dollars on a non-industry grade 3-D printer can literally make a plastic cloak overnight," said Yaroslav Urzhumov, assistant research professor in electrical and computer engineering at Duke's Pratt School of Engineering.

Three-dimensional printing, technically known as stereolithographic fabrication, has become increasingly popular, not only among industry, but for personal use. It involves a moving nozzle guided by a computer program laying down successive thin layers of a material—usually a polymer plastic—until a three-dimensional object is produced.

Urzhumov said that producing a cloak in this fashion is inexpensive and easy. He and his team made a small one at Duke which looks like a Frisbee™ disc made out of Swiss cheese. Algorithms determined the location, size and shape of the holes to deflect microwave beams. The fabrication process takes from three to seven hours.

The results of Urzhumov's experiments were published online in the journal Optics Letters, and the team's research was supported by the U.S. Army Research Office through a Multidisciplinary University Research Initiative grant.

Just like the 2006 cloak, the newer version deflects microwave beams, but researchers feel confident that in the not-so-distant future, the cloak can work for higher wavelengths, including visible light.

"We believe this approach is a way towards optical cloaking, including visible and infrared," Urzhumov said. "And nanotechnology is available to make these cloaks from transparent polymers or glass. The properties of transparent polymers and glasses are not that different from what we have in our polymer at microwave frequencies."

The disk-like cloak has an open area in its center where the researchers placed an opaque object. When microwave beams were aimed at the object through the side of the disk, the cloak made it appear that the object was not there.

"The design of the cloak eliminates the 'shadow' that would be cast, and suppresses the scattering from the object that would be expected," said Urzhumov. "In effect, the bright, highly reflective object, like a metal cylinder, is made invisible. The microwaves are carefully guided by a thin dielectric shell and then re-radiated back into free space on the shadow side of the cloak."

Urzhumov said that theoretically, the technique can be used to create much larger devices.

"Computer simulations make me believe that it is possible to create a similar polymer-based cloaking layer as thin as one inch wrapped around a massive object several meters in diameter," he said. "I have run some simulations that seem to confirm this point."

 Explore further: Invisibility cloak research moves forward at MTU

More information: "Thin Low-Loss Dielectric Coatings for Free-Space Cloaking," Y. Urzhumov, et al. Optics Letters. Online May 3, 2013. DOI 10.1364/OL.38.001606

Thin, Active Invisibility Cloak Demonstrated For First Time

http://www.photonicsonline.com/doc/thin-active-invisibility-cloak-demonstrated-for-first-time-0001

Invisibility cloaking is no longer the stuff of science fiction: two researchers in The Edward S. Rogers Sr. Department of Electrical & Computer Engineering have demonstrated an effective invisibility cloak that is thin, scalable and adaptive to different types and sizes of objects.

Professor George Eleftheriades and PhD student Michael Selvanayagam have designed and tested a new approach to cloaking—by surrounding an object with small antennas that collectively radiate an electromagnetic field. The radiated field cancels out any waves scattering off the cloaked object. Their paper ‘Experimental demonstration of active electromagnetic cloaking’ appears today in the journal Physical Review X.

“We’ve taken an electrical engineering approach, but that’s what we are excited about,” says Eleftheriades. “It’s very practical.”

Picture a mailbox sitting on the street. When light hits the mailbox and bounces back into your eyes, you see the mailbox. When radio waves hit the mailbox and bounce back to your radar detector, you detect the mailbox. Eleftheriades and Selvanyagam’s system wraps the mailbox in a layer of tiny antennas that radiate a field away from the box, cancelling out any waves that would bounce back. In this way, the mailbox becomes undetectable to radar.

“We’ve demonstrated a different way of doing it,” says Eleftheriades. “It’s very simple: instead of surrounding what you’re trying to cloak with a thick metamaterial shell, we surround it with one layer of tiny antennas, and this layer radiates back a field that cancels the reflections from the object.”

Their experimental demonstration effectively cloaked a metal cylinder from radio waves using one layer of loop antennas. The system can be scaled up to cloak larger objects using more loops, and Eleftheriades says the loops could become printed and flat, like a blanket or skin. Currently the antenna loops must be manually attuned to the electromagnetic frequency they need to cancel, but in future they could function both as sensors and active antennas, adjusting to different waves in real time, much like the technology behind noise-cancelling headphones.

Work on developing a functional invisibility cloak began around 2006, but early systems were necessarily large and clunky—if you wanted to cloak a car, for example, in practice you would have to completely envelop the vehicle in many layers of metamaterials in order to effectively “shield” it from electromagnetic radiation. The sheer size and inflexibility of the approach makes it impractical for real-world uses.  Earlier attempts to make thin cloaks were not adaptive and active, and could work only for specific small objects.

Beyond obvious applications, such as hiding military vehicles or conducting surveillance operations, this cloaking technology could eliminate obstacles—for example, structures interrupting signals from cellular base stations could be cloaked to allow signals to pass by freely. The system can also alter the signature of a cloaked object, making it appear bigger, smaller, or even shifting it in space. And though their tests showed the cloaking system works with radio waves, re-tuning it to work with Terahertz (T-rays) or light waves could use the same principle as the necessary antenna technology matures.

“There are more applications for radio than for light,” says Eleftheriades. “It’s just a matter of technology—you can use the same principle for light, and the corresponding antenna technology is a very hot area of research.”

Abstract-Realization of an all-dielectric zero-index optical metamaterial

 

                                                     a, IFCs of air and a low-index metamaterial, illustrating angularly
                                                     selective transmission due to conservation of the wave vector parallel
                                                     to the surface. b, Simulated angle- and wavelength-dependent transmittance
                                                     of the fabricated struture.

Parikshit Moitra,^{1, 5}Yuanmu Yang,^{1, 5}Zachary Anderson,² Ivan I. Kravchenko,³Dayrl P. Briggs³ Jason Valentine⁴
http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2013.214.html

Metamaterials offer unprecedented flexibility for manipulating the optical properties of matter, including the ability to access negative index^{1, 2, 3, 4}, ultrahigh index⁵ and chiral optical properties^{6, 7, 8}. Recently, metamaterials with near-zero refractive index have attracted much attention^{9, 10, 11, 12, 13}. Light inside such materials experiences no spatial phase change and extremely large phase velocity, properties that can be applied for realizing directional emission^{14, 15, 16}, tunnelling waveguides¹⁷, large-area single-mode devices¹⁸ and electromagnetic cloaks¹⁹. However, at optical frequencies, the previously demonstrated zero- or negative-refractive-index metamaterials have required the use of metallic inclusions, leading to large ohmic loss, a serious impediment to device applications^{20, 21}. Here, we experimentally demonstrate an impedance-matched zero-index metamaterial at optical frequencies based on purely dielectric constituents. Formed from stacked silicon-rod unit cells, the metamaterial has a nearly isotropic low-index response for transverse-magnetic polarized light, leading to angular selectivity of transmission and directive emission from quantum dots placed within the material.