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.

Tailored flexible illusion coatings hide objects from detection

Antenna covered with copper patterned dielectric substrate creates a flexible metasurface that acts as an illusion coating, cloaking the antenna or making it appear to be something entirely different.Image: Zhihao Jiang/Penn State

By A'ndrea Elyse Messer

UNIVERSITY PARK, Pa. -- Developing the cloak of invisibility would be wonderful, but sometimes simply making an object appear to be something else will do the trick, according to Penn State electrical engineers.

"Previous attempts at cloaking using a single metasurface layer were restricted to very small-sized objects," said Zhi Hao Jiang, postdoctoral fellow in electrical engineering, Penn State. "Also, the act of cloaking would prevent an enclosed antenna or sensor from communicating with the outside world."

Jiang and Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of Electrical Engineering, developed a metamaterial coating with a negligible thickness that allows coated objects to function normally while appearing as something other than what they really are, or even completely disappearing. They report their research in Advanced Functional Materials.

The researchers employ what they call "illusion coatings," coatings made up of a thin flexible substrate with copper patterns designed to create the desired result. They can take a practical size metal antenna or sensor, coat it with the patterned film and when the device is probed by a radio frequency source, the scattering signature of the enclosed object will appear to be that of a prescribed dielectric material like silicon or Teflon. Conversely, with the proper pattern, they can coat a dielectric and it will scatter electromagnetic waves the same as if it were a metal object.

"The demonstrated illusion/cloaking coating is a lightweight two-dimensional metasurface, not a bulky three-dimensional metamaterial," said Werner.

The researchers take the object they want cloaked and surround it with a spacer, either air or foam. They then apply the ultrathin layer of dielectric with copper patterning designed for the wavelengths they wish to cloak. In this way, antennae and sensors could be made invisible or deceptive to remote inspection.

Another application of this material would be to protect objects from other emitting objects nearby while still allowing electromagnetic communication between them. This was not possible with the conventional transformation optics-based cloaking method because the cloaking mechanism electromagnetically blocked the cloaked object from the outside, but this new coating allows the object surrounded to continue working while being protected. In an array of antennae, for example, interference from the nearby antennas can be suppressed.

The metasurface coating consists of a series of copper, geometric patterns placed on a flexible substrate using standard lithographic methods currently used to create printed circuit boards. Each illusion coating must be designed for the specific application, but the designs are optimized mathematically. This method of manufacture is low cost and well established.

Another advantage of this method is that it works not only for direct hits by radio frequency waves incident normally on the coated object, but also continues to operate properly within a 20 degree field of view, making it a better angle-tolerant shield than previous attempts that employed bulky metamaterials. Currently, the metasurface coatings only work on narrow bands of the spectrum for any application, but can be adapted to work in other bands of the electromagnetic spectrum including the visible spectrum.

"We haven't tried expanding the bandwidth yet," said Werner. "But the theory suggests that it should be possible and it will probably require multiple layers with different patterns to do that."

Illusion coatings could be used for things other than hiding. They could enhance the way radio frequency ID tags work or could redistribute energy in different, controlled patterns making things more visible rather than less visible. The materials shielding ability can also be used to protect any type of equipment from stray or intentional electromagnetic interference.

The National Science Foundation supported this work through a Materials Research Science and Engineering Center at Penn State.

OT-Invisibility cloaks closer thanks to 'digital metamaterials'

    Now you see him … Eric Tastad/Flickr, CC BY-NC-SA

  

by Penny Orbell

http://phys.org/news/2014-09-invisibility-cloaks-closer-digital-metamaterials.html

The concept of "digital metamaterials" – a simple way of designing metamaterials with bizarre optical properties that could hasten the development of devices such as invisibility cloaks and superlenses – is reported in a paper published today in Nature Materials.
Metamaterials are artificially engineered out of microscopic subunits – such as glass, metal or plastic – arranged in a repeating fashion. Once assembled, these metamaterials possess unique properties, such as interacting with light in unusual ways, which aren't often seen in natural materials.

"The idea behind metamaterials is to mimic the way atoms interact with light, but with artificial structures much smaller than the wavelength of light itself," said Boris Kuhlmey, associate professor of photonics and optics at the University of Sydney.

"This way, optical properties are no longer restricted to those of the constituent materials, and can be designed almost arbitrarily."

The material world goes digital

The researchers of the Nature Materials paper, from the University of Pennsylvania, were inspired to develop digital metamaterials by the binary numeral system of Boolean algebra.

The binary system is used internally by most digital electronic devices, such as computers and smartphones. Complex digital devices have their digital information simply encoded as a string of 1s and 0s called "bits".

The proposed method for digital metamaterials is a simplified way of building metamaterials, yet still allows for complex and diverse properties to be achieved.

"The beauty of the new method is its simplicity," said Min Gu, professor of optoelectronics at Swinburne University of Technology.

Through the use of simulations in two-dimensional space, the researchers explored the possibility of creating metamaterials with only two specially chosen component parts, called metamaterial bits – analogous to the 1 and 0 "bits" of binary computer code. The arrangement of metamaterial bits represents the "digitising" of metamaterials.

In their study, the researchers chose to use nano-sized pieces of silver and silica (glass) as their repeating metamaterial bits. These are materials that interact with light in very different ways on an individual level. Once they were "digitised", the resulting metamaterial had its own unique properties, very different to those of its constituent parts.

"The components of the material work together to generate effects or give rise to phenomena that you wouldn't observe if they weren't arranged together in 3D (or in this case, 2D) space as an ordered assembly," said Tiffany Walsh, professor of bionanotechnology at Deakin University.

Sourcing material parts in order to achieve unusual properties of a metamaterial can be time consuming and expensive. This new way of thinking about the design of metamaterials may allow researchers to produce the optical properties they want from the metamaterial using only two component parts.

"What this [research] really does is put a new spin on the idea that with only two set materials arranged with the right portions – one metal, one insulator, here silver and silica – almost any optical property can be achieved," said Associate Professor Kuhlmey.

Professor Walsh said: "This is like the concept of turning sound waves from analog into digital – and they've pushed it into a new realm of physics.

"They've been able to take the permittivity – the response of the material when it's exposed to radiation – and digitised this. They've turned it into something that is more readily manipulated."

Waves and matter collide

One of the key applications for metamaterials lies in their ability to manipulate light.

"We already have knowledge about how to manipulate radiation (such as light) – we can use lenses, like a magnifying glass, for example, which focus light down on a spot; we can use mirrors to reflect light and change its direction," Professor Walsh said.

"But what these [metamaterials] can do is something more sophisticated: they're able to bend light, to scatter it, to manipulate it in unusual ways."

Using their digital method, the researchers showed that it is possible to create certain metamaterials with very low permittivity, which are rarely found in nature. Having control over these properties may open doors to more advanced technological applications, such as invisibility cloaking devices.

"It would be interesting in future to see if such a digital design method can facilitate the construction of optical, or invisibility, cloaks," said Professor Gu.

"With varying changes of silver/glass ratios (structured at the nanoscale) it is then in principle possible to make flat lenses and other tiny optical elements," Associate Professor Kuhlmey said.

"The authors […] showed in simulations that nano-patterned glass/silver structures can then bend light, which is also the principle behind invisibility cloaking."

He added that fabricating the proposed structures would be challenging but not impossible.

"[It would] require structuring glass and metal with a precision of a few atoms in thickness only – but thinking of metamaterials as binary structures may help devise new nano-patterning lithography (printing) techniques that take advantage of this," he said.

 Explore further: Array of perfect nanometer-scale spheres that can control the flow of visible light may find use in invisibility cloaks

Journal reference: Nature Materials