Report
On
Invisible Cloak

Submitted To: Prepared By:
Mrs. Parul Pathak Ankita Aggarwal
Mr. Ganesh Mishra Laxmi Pooja

Contents

* Inroduction

* What is Invisible Cloak

* What makes it so different?

* Meta Materials

* How does it works? * About the cloak * How close are scientists to creating a real-life invisibility cloak? * References

Invisible Cloak

Introduction

Invisibility has long been employed in works of science fiction and fantasy, from 'cloaking devices' on spaceships in the various Star Trek series to Harry Potter’s magic cloak. But physicists are beginning to think they can actually make devices with just these properties.
To achieve the feat of 'cloaking' an object, they have developed what are known as metamaterials, some of which can bend electromagnetic radiation, such as light, around an object, giving the appearance that it isn’t there at all.
The first examples only worked with long-wavelength radiation such as microwaves.
One small device that made small objects invisible to near-infrared radiation and worked in three dimensions was unveiled by physicists from the UK and Germany earlier 2015.
Admit it. You'd love to own an invisibility cloak. Utter an embarrassing faux pas at a party? Just throw on your magical garment and vanish from the snooty gaze of your fellow partygoers. Want to hear what your boss is really saying about you? Stroll right into his or her office and get the goods.
Such fantastic fashion accessories have become ridiculously standard in the world of science fiction and fantasy. Everyone, from boy wizards to intergalactic safari hunters, has at least one invisible blouse in their wardrobe, but what about us poor saps in the real world?

What is Invisible Cloak?

Its creators claimed there was nothing stopping them from scaling their invention up to hide larger objects from visible light – although others had pointed out a flaw in their design.
Now, researchers at Boston University and Tufts University claim that they have come up with an invisibility cloak that works within the terahertz band – the radiation between infrared and radio wavelengths – but could be modified to work with visible light. Intriguingly, it is made out of silk.
“There’ve been many high tech approaches to cloaking and the basic idea behind these is to take light and have it pass around something as if it isn’t there, often using high-tech or exotic materials,” said John Howell, a professor of physics at the University of Rochester. Forgoing the specialized components, Howell and graduate student Joseph Choi developed a combination of four standard lenses that keeps the object hidden as the viewer moves up to several degrees away from the optimal viewing position.
“This is the first device that we know of that can do three-dimensional, continuously multidirectional cloaking, which works for transmitting rays in the visible spectrum,” said Choi, a PhD student at Rochester’s Institute of Optics.
Many cloaking designs work fine when you look at an object straight on, but if you move your viewpoint even a little, the object becomes visible, explains Howell. Choi added that previous cloaking devices can also cause the background to shift drastically, making it obvious that the cloaking device is present.

What makes it so different?

Such invisibility cloaks rely on metamaterials, which are a class of material engineered to produce properties that don’t occur naturally.
Light is electromagnetic radiation, made up of perpendicular vibrations of electric and magnetic fields. Natural materials usually only affect the electric component – this is what is behind the optics that we’re all familiar with such as ordinary refraction.
But metamaterials can affect the magnetic component too, expanding the range of interactions that are possible.
The metamaterials used in attempts to make invisibility cloaks are made up of a lattice with the spacing between elements less than the wavelength of the light we wish to ‘bend’.
The silk-based cloak recently announced uses'split-ring resonators' – concentric pairs of rings with splits at opposite ends. 10,000 gold resonators were initially attached to a one-centimetre-square piece of silk.
As silk is not rejected by the human body, it is thought that they could be used to coat internal organs so that surgeons can easily see what lies behind them.
Another use for metamaterials, potentially with greater scientific applications, is in building a superlens.
Ordinary lenses are restricted by their “diffraction limit”. As David R Smith of the University of California, San Diego explained in Physics World, this means that “the best resolution that is possible corresponds to about half of the incident wavelength of the light that is used to produce the image”.
In 2000, Sir John Pendry of Imperial College London suggested that a metamaterial with a negative refractive index might get around problems such as wave decay and allow imaging of objects only nanometers in size.
Among the first practical applications would likely be using metamaterial lenses to view live viruses and maybe even bits of DNA. In 2005, a thin slab of silver was used to image objects just 60nm across – just over one hundredth the size of a red blood cell.

Meta Materials

The meta-skin takes its name from meta-materials, which are composites that have properties not found in nature and that can manipulate electromagnetic waves. By stretching and flexing the polymer meta-skin, 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," said researchers from Iowa State University.The researchers wanted to prove that electromagnetic waves - perhaps even the shorter wavelengths of visible light - can be suppressed with flexible, tunable liquid-metal technologies.
They came up with rows of split ring resonators embedded inside layers of silicone sheets. The electric resonators are filled with galinstan, a metal alloy that is 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 millimetres and a thickness of half a millimetre.
They have a 1 millimetre 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 per cent in the frequency range of 8 to 10 gigahertz, researchers said.
When objects are wrapped in the meta-skin, the radar waves are suppressed in all incident directions and observation angles.
"Therefore, this meta-skin technology is different from traditional stealth technologies that often only reduce the backscattering, ie, the power reflected back to a probing radar," the researchers said.
The meta-skin could in future coat the surface of the next generation of stealth aircraft, said lead author Jiming Song, professor at Iowa State University.
The researchers are even hoping to develop a cloak of invisibility using the technology in future.
"The long-term goal is to shrink the size of these devices," said Liang Dong, from Iowa State University. Then hopefully we can do this with higher-frequency electromagnetic waves such as visible or infrared light," Dong said.

How does it works?

In order to both cloak an object and leave the background undisturbed, the researchers determined the lens type and power needed, as well as the precise distance to separate the four lenses. To test their device, they placed the cloaked object in front of a grid background. As they looked through the lenses and changed their viewing angle by moving from side to side, the grid shifted accordingly as if the cloaking device was not there. There was no discontinuity in the grid lines behind the cloaked object, compared to the background, and the grid sizes (magnification) matched.
The Rochester Cloak can be scaled up as large as the size of the lenses, allowing fairly large objects to be cloaked. And, unlike some other devices, it’s broadband so it works for the whole visible spectrum of light, rather than only for specific frequencies.
Their simple configuration improves on other cloaking devices, but it’s not perfect. “This cloak bends light and sends it through the center of the device, so the on-axis region cannot be blocked or cloaked,” said Choi. This means that the cloaked region is shaped like a doughnut. He added that they have slightly more complicated designs that solve the problem. Also, the cloak has edge effects, but these can be reduced when sufficiently large lenses are used.
While their device is not quite like Harry Potter’s invisibility cloak, Howell had some thoughts about potential applications, including using cloaking to effectively let a surgeon “look through his hands to what he is actually operating on,” he said. The same principles could be applied to a truck to allow drivers to see through blind spots on their vehicles. Howell became interested in creating simple cloaking devices with off-the-shelf materials while working on a holiday project with his children.

About the cloak

The cloak is covered with “nanoantennas made of tiny gold blocks of different sizes that can counteract that distortion, making it seem to an observer like the light is coming from a flat surface.”
The cloak is 80 nanometers thick, which is a bonus since the study authorssay prior attempts have been too bulky and therefore difficult to scale up. Still, the current cloak only covers a very tiny object, so there’s a long way to go before people can make themselves invisible or hard to see.
Other research teams are also looking into the creation of an invisibility cloak. In early July, University of California, San Diego researchers designed their own early version. “Invisibility may seem like magic at first, but its underlying concepts are familiar to everyone. All it requires is a clever manipulation of our perception,” Boubacar Kanté, a professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering said in a statement about his work. “Full invisibility still seems beyond reach today, but it might become a reality in the near future thanks to recent progress in cloaking devices.”
How close are scientists to creating a real-life invisibility cloak?
Bottom of Form
If you could have any superpower, what would it be? The ability to fly? Walk through walls? Super-strength? No doubt, many of us would be tempted by invisibility. Just think of it: sneaking in and out of rooms undetected, listening in on conversations like a fly on a wall. Humans have been captivated by the idea of invisibility for ages, and this fascination has permeated popular culture, from Harry Potter's magic cloak to Frodo's mystical ring, which renders him undetectable. But how close are we to creating an actual invisibility cloak?
"Invisibility may seem like magic at first, but its underlying concepts are familiar to everyone. All it requires is a clever manipulation of our perception," says Boubacar Kanté, a professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering.
Rendering something invisible without the use of wizardry requires changing how light reacts when it hits an object. The reason we can see things at all is because light bounces off of them and into our eyes. If we can prevent this, or change how light reacts around an object, we could change how it looks. No natural material can manipulate light in this way, but researchers are busy concocting their own "meta-materials" made of tiny electronics that have special properties to reroute light and "cloak" an object from our view.
For example, last year, researchers at UC Berkeley created an "ultra thin invisibility cloak" that, when activated, can make a 3D object appear entirely flat, like a mirror. It's made out of microscopic gold nanoantennas just 80 nanometers thick (as Jesse Emspak at Live Science notes, "an average strand of human hair is about 100,000 nanometers wide"), which means it's flexible and could be wrapped around an object, like a blanket. The gold nanoantennas can be "tuned" to make the material look like something else, which has big implications for disguise in the military. "You could cover a tank with it and make it look like a bicycle," the study's director, Xiang Zhang, told the Los Angeles Times.
The catch? The material has to be "tuned" to a specific background, meaning that if I'm standing in front of a forest, this cloak would need to be tuned to match the trees behind me, and would only really work if I stayed in one place. That's not particularly practical for anyone hoping to wander the halls of Hogwarts, but Zhang says making the cloak adaptive is the next step. Also, the prototype is incredibly small, just the size of a few cells. But the research offers a proof of concept and could be scaled up.

Meanwhile, at the University of Rochester, researchers have found a way to use lenses to bend light so that it avoids an object, making it disappear. This is a more low-tech cloaking method, but it could do things like let a surgeon "look through his hands to what he is actually operating on," saidJohn Howell, a professor of physics at the university.

References: * www.google.com * http://www.rochester.edu/newscenter/watch-rochester-cloak-uses-ordinary-lenses-to-hide-objects-across-continuous-range-of-angles-70592/ * http://time.com/4042506/invisibility-cloak/ * http://www.indiatimes.com/news/world/scientists-just-created-the-world-s-first-invisibility-cloak-252112.html * http://science.howstuffworks.com/invisibility-cloak.htm