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Metamaterials cloaks promise invisible armies, protection from earthquakes & tsunamis

A team of researchers at Moscow’s National University of Science and Technology (NUST MISIS) have come up with a unique metamaterial which can make combat vehicles invisible, the authoritative scientific journal Physical Review wrote.


Metamaterials are artificially structured materials designed to control and manipulate physical phenomena such as light and other electromagnetic waves, sound waves and seismic waves in unconventional ways, resulting in exotic behavior that’s not found in nature. Military is interested in metamaterials primarily for cloaking, to make their platforms, weapons and persons invisible from electro-optic sensors, radars and sonars.


They are predicted to be able to protect the building from earthquakes by bending seismic waves around it. Similarly, tsunami waves could be bent around towns, and sound waves could be bent around a room to make it soundproof.

Promise of Metamaterials

“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 from the University of Sydney. This way, their properties are derived from both the inherent properties from their base materials as well as the way they are assembled, such as the design of their shape, geometry, size, orientation and arrangement. Thus optical properties are no longer restricted to those of the constituent materials, and can be designed almost arbitrarily.


Metamaterial-enabled devices have a wide range of applications in the RF, THz, IR, and visible spectrum.

Invisibility Cloaks

Metamaterial cloaking is used to building devices that can hide something, so that a defined region of space is invisibly isolated from passing electromagnetic fields (or sound waves).


Radar Cloaks

In 2006 researchers demonstrated it was possible to absorb or direct electromagnetic waves around an object through a coating and make it “invisible”. However, it only worked on microwaves and in two dimensions. Moreover previous cloaking efforts required materials as much as 10 times thicker than the wavelength being dodged. Missile guidance and marine radar wavelengths measure roughly 3 centimeters; that would require about a foot of coating.

Russian Scientists  develop metamaterial to Make Combat Vehicles Invisible

“The experimental part of our research was the creation of a one-of-a-kind metamaterial consisting of a small flat grid of the so-called meta-molecules cut out from a solid piece of ordinary steel,” the project’s director Alexei Basharin was quoted as saying by the NUST MISIS press service. The NUST MISIS team worked closely with colleagues from the University of Crete, Greece


Basharin said that thanks to the special shape and configuration of these cells the scientists managed to obtain metamaterial with absolutely unique properties. This metamaterial can be used to make supersensitive sensors to detect explosives and chemical weapons.


“An addition of a nonlinear semiconductor will turn the metamaterial into an adjustable screen for stealth technologies, which make fighting vehicles less visible in radio, infrared and other bands,” the NUST MISIS press service said in a statement. The newly obtained metamaterial can also become a vital element of the latest types of lasers and serve the basis for quantum computers.


Ultra-thin Dielectric metasurface cloak of University of California-San Diego

Boubacar Kante, a professor at the University of California-San Diego and his colleagues, have developed the first effective “dielectric metasurface cloak” based on a new material consisting of a layer of Teflon substrate with tiny ceramic cylinders embedded into it.


Kante said his material requires thickness of only 1/10 of the wavelength. Hiding from that same 3 cm wavelength would thus only require about a 3 mm coat. Different thicknesses (thinner) could be used for electromagnetic waves as small as those of visible light (which ranges from about 400 to 700 nanometers.) The cloak would be useful for Unmanned Aerial Vehicles and other planes, ships and anything else interested in dodging radar could have a use for it.


IOWA Engineers develop flexible skin that traps radar waves, cloaks objects

Iowa State University engineers have developed a new flexible, stretchable and tunable “metaskin” that uses rows of small, liquidmetal devices to cloak an object from radar. By stretching and flexing the polymer metaskin, it can be tuned to reduce the reflection of a wide range of radar frequencies.


“It is believed that the present meta-skin technology will find many applications in electromagnetic frequency tuning, shielding and scattering suppression,” the lead authors Liang Dong, associate professor; and Jiming Song, professor wrote in journal Scientific Reports.


Metaskin is composed of rows of split ring resonators embedded inside layers of silicone sheets. The electric resonators are filled with galinstan, a metal alloy that’s liquid at room temperature and less toxic than other liquid metals such as mercury. Those resonators are small rings with an outer radius of 2.5 millimeters and a thickness of half a millimeter. They have a 1 millimeter gap, essentially creating a small, curved segment of liquid wire.


The rings create electric inductors and the gaps create electric capacitors. Together they create a resonator that can trap and suppress radar waves at a certain frequency. Stretching the meta-skin changes the size of the liquid metal rings inside and changes the frequency the devices suppress.


Tests showed radar suppression was about 75 percent in the frequency range of 8 to 10 gigahertz, according to the paper. When objects are wrapped in the meta-skin, the radar waves are suppressed in all incident directions and observation angles.


Optical Cloaks

3D invisibility “skin” cloak at Berkley

Berkeley researchers have devised an ultra-thin invisibility “skin” cloak that can conform to the shape of an object and hide it from detection with visible light. Although this cloak is only microscopic in size, the principles behind the technology should enable it to be scaled-up to conceal macroscopic items as well.


They demonstrated a metasurface cloak made from an ultrathin 80 nanometers thick layer of gold nanoantennas that was wrapped around a three-dimensional object about the size of a few biological cells and arbitrarily shaped with multiple bumps and dents. The surface of the skin cloak was meta-engineered to reroute reflected light waves so that the object was rendered invisible to optical detection when the cloak is activated.


When the cloak is turned “on,” the bump-shaped object being illuminated in the center white spot disappears from view. The object reappears when the cloak is turned “off.” This is the first time a 3D object of arbitrary shape has been cloaked from visible light.


Invisibility in diffusive light scattering media

In 2014, scientists demonstrated good cloaking performance in murky water, demonstrating that an object shrouded in fog can disappear completely when appropriately coated with metamaterial. This is due to the random scattering of light, such as that which occurs in clouds, fog, milk, frosted glass, etc., combined with the properties of the metatmaterial coating. When light is diffused, a thin coat of metamaterial around an object can make it essentially invisible under a range of lighting conditions.


While metamaterials may not yet be making objects invisible to the eye; they could be used to redirect other kinds of waves, including mechanical waves such as sound and ocean waves. Ong points to the possibility of using what has been learnt in reconfiguring the geometry of materials to divert tsunamis from strategic buildings. French researchers earlier this year, for example, diverted seismic waves around specially placed holes in the ground, reflecting the waves backward.


Acoustic Cloak

In 2014 researchers created a 3D acoustic cloak from stacked plastic sheets dotted with repeating patterns of holes. The pyramidal geometry of the stack and the hole placement provide the effect.


Prof Wegener works on cloaking, but his aim is not to make things invisible. He wants to hide them from physical forces, and last year his lab produced a honeycomb-like material that made an object beneath it unfeelable. This particular metamaterial was a solid lattice that acts like a fluid in certain ways, deflecting pressure around its hidden cargo.


Now the tiny, hidden cylinder was very small in that case (less than 1mm) but related work by Prof Wegener’s team was picked up by French physicists and engineers, who showed that a careful pattern of drilled holes could divert damaging earthquake vibrations.


But an invisibility cloak needn’t be a sinister tool of war. Vanderbilt’s Valentine suggests architectural usage. “You could use this technology to hide supporting columns from sight, making a space feel completely open,” he said. Other potential uses include rendering parts of an aircraft invisible for pilots to see below the cockpit, or to rid drivers of the blind spot in a car.


Metamaterials could also absorb and emit light with extremely high efficiency — for example in a high-resolution ultrasound — or redirect light over a very small distance. This, says Anthony Vicari of Lux Research, “could be used to improve fibre optical communications networks, or even for optical communications within microchips for faster computing.”


Super lenses

Researchers at Michigan Technological University in continuation of work done by Durdu Güney, a professor of electrical and computer engineering have found a way to pass light waves to pass through the lens without getting absorbed. They utilized metamaterials based on thin silver films tweaked at the subwavelength scale so that light waves pass through instead of reflecting off the metal.


“Aluminum and silver are the best choices so far in the visible light spectrum, not just for a perfect lens but all metamaterials,” Güney says, explaining that metamaterials have been successfully created with these metals, although they still tend to absorb light waves. “Loss—or the undesired absorption of light—is good in solar cells, but bad in a lens because it deteriorates the waves,” he explains.


Optical Computer Networks

In 2012, the Berkeley Nanosciences and Nanoengineering Institute published a paper with South Korean scientists describing a metamaterial-based electro-optical modulator made from a sheet of graphene just a single atom thick that was able to switch lightwaves at terahertz frequencies.


More recently, a group at City College of New York, led by the physicist Vinod Menon, demonstrated light emission from ultrafast-switching LEDs based on metamaterials. Together, such innovations could make possible optical computer networks far faster than today’s gigabit networks.


Digital Metamaterials

Cristian Della Giovampaola and Nader Engheta from the University of Pennsylvania propose using just two subunits with opposing properties. Called “metamaterial bits,” these are like the 1s and 0s in a binary code. They used nano-sized pieces of silver and silica, which interact with light in very different ways: One’s a metal, the other’s an insulator.


The duo used a computer simulation to create layered structures that constitute bytes of increased functionality and complexity. Once they were “digitized,” the resulting material had its own unique properties, distinct from its constituent subunits.

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