Emerging Technologies

Emerging Technologies #14Two Research Groups Create Active Invisibility Cloaks

Emerging Technologies

Table of Contents

Section 1 – Executive Summary....................................................................................................................... 3

Section 2 – Related News & Articles............................................................................................................... 4

Article 1. New 'active' invisibility cloak design 'drastically reduces' visibility 4

Article 2. New invisibility cloak ‘thinner, lighter and cleverer’ than previous designs 6

Article 3. Two research groups create active invisibility cloaks 7

Article 4. New invisibility cloak combines metamaterials and fancy electronics to be

thinner, lighter, more invisible 8

Article 5. Could this invisibility cloak be the military's best ally? Device uses

electromagnetism to make objects invisible to radar systems 9

Article 6. Greek Researcher in Canada Creates New Invisibility Cloak 10

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Section 1 – Executive Summary

Two Research Groups Create Active Invisibility Cloaks

University of Toronto researchers have demonstrated an invisibility cloak that hides objects within an

electromagnetic field, rather than swaddling it in meta-materials as other approaches require.

Instead of covering an object completely in an opaque cloak that then mimics the appearance of

empty air, the technique developed by university engineering Prof. George Eleftheriades and

Ph.D. candidate Michael Selvanayagam makes objects invisible using the ability of

electromagnetic fields to redirect or scatter waves of energy.

The approach is similar to that of 'stealth' aircraft whose skin is made of material that absorbs the

energy from radar systems and deflects the rest away from the radar detectors that sent them.

Rather than scattering radio waves passively due to the shape of its exterior, however, the Toronto pair's

'cloak' deflects energy using an electromagnetic field projected by antennas that surround the object

being hidden.

Most of the proposals in a long list of 'invisibility cloaks' announced during the past few years actually

conceal objects by covering them with an opaque blanket, which becomes 'invisible' by displaying an

image of what the space it occupies would look like if neither the cloak nor the object it concealed were

present.

An invisibility cloak concealing an adolescent wizard hiding in a corner, for example, would display an

image of the walls behind it in an effort to fool observers into thinking there was no young wizard

present to block their view of the empty corner.

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Section 2 – Related News & Articles

Article 1. New 'active' invisibility cloak design 'drastically reduces' visibility

Source: http://www.independent.co.uk/life-style/gadgets-and-tech/news/new-active-invisibility-cloak-

design-drastically-reduces-visibility-8934474.html (12/11/2013)

A new type of “active” invisibility cloak that could operate over a broad range of frequencies has

been developed by researchers at the University of Texas in Austin.

By employing a “superconducting thin film” that is electrically powered the cloak could overcome the

limitations of current “passive” designs.

Scientists have previously created small-scale invisibility cloaks that work only in response to very

limited types of light. The researchers at the University of Texas give the example of an object that is

made invisible to red light, but becomes bright blue as a result, “increasing its overall visibility”.

"Our active cloak is a completely new concept and design, aimed at beating the limits of [current cloaks]

and we show that it indeed does," Professor Andrea Alù, a lead author on the study, told the BBC.

"If you want to make an object transparent at all angles and over broad bandwidths, this is a good

solution […] We are looking into realising this technology at the moment, but we are still at the early

stages."

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The Austin team began their research by surveying current designs, concluding that achieving complete

invisibility is “impossible” using current designs that rely on “passive” metamaterials.

Metamaterials are manmade and have physical properties unknown in nature. They redirect types of

radiation so that they bend around an object and make it invisible. However, they can only be ‘set’ to

work at specific frequencies at any one time, and can actually become more visible to other portions of

the spectrum.

“When you add material around an object to cloak it, you can't avoid the fact that you are adding matter,

and that this matter still responds to electromagnetic waves," said Professor Alù.

The solution proposed by the University of Texas team is to create “active” invisibility cloaks that

are electrically powered, dispersing small amounts of electrical current across a metamaterial

surface to effectively cloak a range of frequencies “orders of magnitude broader” than current

designs.

The technology proposed in the paper would also allow cloaks to be thinner and lighter than current

designs, opening forward the possibility of invisibility being deployed outside of the lab.

Whilst active invisibility cloaks of the type proposed in the paper have yet to be built, the research of

Professor Alù’s team has been greeted as a tentative step forward for the technology’s development.

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Article 2. New invisibility cloak ‘thinner, lighter and cleverer’ than previous designs

Source: http://metro.co.uk/2013/11/12/new-invisibility-cloak-thinner-lighter-and-cleverer-than-previous-

designs-4183955/ (12/11/2013)

It may not be a Harry Potter invisibility cloak, but

scientists have claimed a brand new design is the

next best thing.

Their latest device conceals objects from different

light frequencies. It’s also able to make items even

more visible than they usually are. Unfortunately, the

cloak is unable to achieve complete invisibility,

something scientists from the University of Texas say

is ‘impossible’.

Prof Andrea Alu, who led the study, told BBC News: ‘Our active cloak is a completely new concept and

design, aimed at beating the limits of [current cloaks] and we show that it indeed does.

‘If you want to make an object transparent at all angles and over broad bandwidths, this is a good

solution. ‘We are looking into realising this technology at the moment, but we are still at the early stages.’

Even though the technology might not be suitable for aspiring wizards, it could be used in biomedicine

and military technology.

Explaining how the cloak could make an object more visible than before, she added: ‘For example,

you might make a cloak that makes an object invisible to red light. But if you were illuminated by

white light, which contains all colours, you would actually look bright blue, and therefore stand

out more.’

Their design would also make cloaks thinner and light

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Article 3. Two research groups create active invisibility cloaks

Source: http://www.gizmag.com/active-invisibility-cloak/29770/ (14/11/2013)

Sometimes everything can seem to happen

at once. The new game in town is active

invisibility cloaks (AIC), which use

electronics and antennas to generate a

cloaking field to hide an object. Two types of

active cloaks have just been revealed (excuse

the pun). While being impressive feats of

technology, such cloaks could easily be

defeated in practice.

Earlier this week, a research paper from Andrea Alu's group at the University of Texas demonstrated that

any passive invisibility cloak (e.g., one just depending on the properties of metamaterials) would not only

fail to achieve invisibility under broadband illumination (for example, white light), but would actually

cause the cloaked object to stand out more strongly than when it is uncloaked.

A natural question to ask is if there are any other possibilities. In fact, there are. An active cloak uses

sensors, electronics, and antennas to actively generate an electric field near the surface of the cloak that

interferes destructively with the radiation scattered from the cloak and contents. This electric field will

be called a cloaking field. In the past few days, Professor George Eleftheriades' EE research group at the

University of Toronto has revealed a working model of an active cloak, while Prof. Alu's group has

worked out a new approach for making active cloaks.

In essence, if the cloak transmits the exact opposite of the light being scattered from the cloak, it will

appear as if there is no object there. Even the shadow behind the cloak, which results from the

illumination that is scattered from the front of the cloak, is removed. Such a cloak is as broadband as the

sensors, electronics, and antennas can handle.

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Article 4. New invisibility cloak combines metamaterials and fancy electronics to be

thinner, lighter, more invisible

Source: http://www.extremetech.com/extreme/170709-new-invisibility-cloak-combines-metamaterials-

and-fancy-electronics-to-be-thinner-lighter-more-invisible (11/11/2013)

A researcher at the University of Texas

at Austin has devised an invisibility

cloak that could work over a broad

range of frequencies, including visible

light and microwaves. This is a

significant upgrade from current

invisibility cloaks that only cloak a very

specific frequency — say, a few hertz in

the microwave band — and, more importantly, actually make cloaked objects more visible to other

frequencies. The UT Austin cloak would achieve this goal by being active and electrically powered, rather

than dumb and passive like existing invisibility cloaks.

As you probably know, the last few years have seen a lot of research into invisibility cloaks. These cloaks

are mostly based on metamaterials — special, man-made materials that bend radiation in ways that

shouldn’t technically be possible, allowing for cloaking devices that bend radiation around an object,

hiding it from view. The problem with these cloaks is that metamaterials are tuned to a very specific

frequency — so, while that specific frequency (say, a thin band of microwaves) passes around the object,

every other frequency scatters off the cloak. In a beautiful twist of irony, most invisibility cloaks actually

create more scattered light, making the cloaked object stand out more than if it was just standing there

uncloaked.

According to Andrea Alù at UT Austin, this is a fundamental issue of passive invisibility cloaks, and the

only way to get around it is to use cloaks fashioned out of active, electrically active materials. It might

change in the future with more advanced passive metamaterials, but for now active designs are the way

forward. Research into active invisibility cloaks is currently being carried out by multiple groups, but

none have yet been built.

Alù’s proposed design consists of a conventional metamaterial base, but with CMOS negative

impedance converters (NICs) placed at the corner of each metamaterial square (top image). A NIC

is an interesting electronic component that adds negative resistance to a circuit, injecting energy rather

than consuming it. NICs are not widely used as we’re not entirely sure how to use them. Alù seems to

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propose that by interspersing NICs (which must be powered) with the metamaterial, multiple

frequencies can be cloaked. In the image above, you can see a standard metamaterial cloak (blue), vs

Alù’s metamaterial-and-NIC cloak (green). Alù’s proposed cloak is invisible over a large range of

frequencies, while a standard passive cloak is only invisible for a small range, and more visible than non-

cloaked devices in other ranges.

From our own experience with writing about invisibility cloaks on ExtremeTech, we’d have to agree that

active designs make more sense. Where passive cloaks have all been incredibly bulky and not all that

effective, an active cloak can be thinner, more flexible, and capable of cloaking a much wider range of

frequencies. Given our mastery of CMOS, and the utterly insane things that we can do with computer

chips, it seems foolhardy to not pursue active, electronic invisibility cloaks.

Article 5. Could this invisibility cloak be the military's best ally? Device uses

electromagnetism to make objects invisible to radar systems

Source: http://www.dailymail.co.uk/sciencetech/article-2505168/Could-invisibility-cloak-militarys-best-

ally.html (13/11/2013)

Engineers have surrounded an object with small antennas that collectively radiate an electromagnetic

field, cancelling out waves scattered off it

The University of Toronto researchers believe their innovation could be used to hide military vehicles

and to conduct surveillance operations

As the technology advances, it could also be adapted to make objects invisible to the human eye

too

Scientists are currently competing to create the best invisibility cloak, as imagined in films such as Harry

Potter and Star Trek.

As yet, no-one has managed to replicate a flexible cloak as worn by the boy wizard, or the cloaking device

used by the Kingons to make their ships invisible to another starship's sensors, but two Canadian

scientists have created an invisibility cloak that they say is thin and adaptive to different types and sizes

of objects.

Unlike other recent cloaks that rely on planes of glass simply bending light in a way that renders small

objects temporarily invisible, the researchers have taken an electrical engineering approach, which

makes objects undetectable to radar.

They said as the technology advances, it could also be adapted to make things invisible to the human eye

too.

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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, rendering the object

invisible to radar, explained the two researchers at the University of Toronto.

‘We've taken an electrical engineering approach, but that's what we are excited about," said Professor

Eleftheriades. ‘It's very practical.’

When light hits an object, such as a postbox and bounces back into a person’s eyes, they can see it and

similarly, when radio waves hit the object, they bounce back to a radar detector and reveal it is a postbox,

in a certain location.

The scientists’ 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

Article 6. Greek Researcher in Canada Creates New Invisibility Cloak

Source: http://canada.greekreporter.com/2013/11/13/greek-researcher-in-canada-creates-new-

invisibility-cloak/ (13/11/2013)

New invisibility cloaks that make objects disappear at various frequencies of the electromagnetic

spectrum, are continuously coming to light. The new achievement is the creation of Greek researcher,

George Eleftheriades, professor at the University of Toronto. Eleftheriades designed and implemented a

new type of thin electromagnetic cloak, which can be adapted in order to make objects of different types

and sizes “disappear” in the field of radio waves.

Eleftheriades’ research group uses a new method of invisibility. Specifically, they surround an object with

small antennas, which emit an electromagnetic field, neutralizing the waves reflected from the object,

making it invisible.

The researchers conducted the experiment with a metal cylinder, making it disappear from the radio

waves. As Eleftheriades said, this technology can be used on a larger scale, making larger objects

invisible, using more antennas.

Invisibility cloaks, beyond their obvious military uses, could have further practical applications. They

could be used for objects that intervene and interrupt the transmission of signals from the mobile

telecommunication stations.

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George Eleftheriades is considered as a pioneer and an expert in the field of metamaterials. He has been

honored with several awards for his work, and in 2009 he became a fellow of the Royal Society of

Sciences Canada.