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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
Emerging Technologies
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.
Emerging Technologies
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."
Emerging Technologies
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.
Emerging Technologies
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
Emerging Technologies
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.
Emerging Technologies
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
Emerging Technologies
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.
Emerging Technologies
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.
Emerging Technologies
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.
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