Image name: Spectral invisibility cloaking
Caption: A broadband wave illuminates an object,
which reflects green light in the shown example,
making the object detectable by an observer
monitoring the wave. A spectral invisibility
cloak transforms the blocked color (green) into
other colors of the wave's spectrum. The wave
propagates unaltered through the object, without
"seeing its color" and the cloak subsequently
reverses the previous transformation, making the
object invisible to the observer.
Image Credit: Luis Romero Cortés and José Azaña,
Institut National de la Recherche Scientifique
approach to invisibility cloaking
could be used to
transmissions and advance sensing,
telecommunications and other applications
Researchers and engineers
have long sought ways to conceal objects by manipulating how light
interacts with them.
A new study (Full-field
broadband Invisibility through Reversible Wave Frequency-spectrum
Control) offers the first demonstration of invisibility
cloaking based on the manipulation of the frequency (color) of light
waves as they pass through an object, a fundamentally new approach
that overcomes critical shortcomings of existing cloaking
The approach could be applicable to securing data transmitted over
fiber optic lines and also help improve technologies for sensing,
telecommunications and information processing, researchers say.
theoretically, could be extended to make 3D objects invisible from
a significant step in
the development of practical invisibility cloaking technologies.
Most current cloaking
devices can fully conceal the object of interest only when the
object is illuminated with just one color of light.
However, sunlight and
most other light sources are broadband, meaning that they contain
The new device, called a
spectral invisibility cloak, is designed to completely hide
arbitrary objects under broadband illumination.
The spectral cloak operates by selectively transferring energy from
certain colors of the light wave to other colors. After the wave has
passed through the object, the device restores the light to its
the new approach in Optica, The Optical Society's journal
for high impact research.
"Our work represents
a breakthrough in the quest for invisibility cloaking," said
José Azaña, National Institute of Scientific Research (INRS),
"We have made a
target object fully invisible to observation under realistic
broadband illumination by propagating the illumination wave
through the object with no detectable distortion, exactly as if
the object and cloak were not present."
When viewing an object, what you are really seeing is the way in
which the object modifies the energy of the light waves that
interact with it.
Most solutions for
invisibility cloaking involve altering the paths that light follows
so that waves propagate around, rather than through, an object.
Other approaches, called
"temporal cloaking," tamper with the propagation speed of the light
such that the object is temporarily concealed as it passes through
the light beam during a prescribed length of time.
In either approach, different colors of an incoming light wave must
follow different paths as they travel through the cloaking device,
thus taking different amounts of time to reach their destination.
This alteration of the
wave's temporal profile can make it apparent to observers that
something is not as it should be.
cloaking solutions rely on altering the propagation path of the
illumination around the object to be concealed; this way,
different colors take different amounts of time to traverse the
cloak, resulting in easily detectable distortion that gives away
the presence of the cloak," said Luis Romero Cortés, National
Institute of Scientific Research (INRS).
solution avoids this problem by allowing the wave to propagate
through the target object, rather than around it, while still
avoiding any interaction between the wave and the object."
José Azaña and his team accomplished this by developing a
method to rearrange different colors of broadband light so that the
light wave propagates through the object without actually "seeing"
To do this, the cloaking
device first shifts the colors toward regions of the spectrum that
will not be affected by propagation through the object.
For example, if the
object reflects green light, then light in the green portion of the
spectrum might be shifted to blue so that there would be no green
light for it to reflect.
Then, once the wave has
cleared the object, the cloaking device reverses the shift,
reconstructing the wave in its original state.
The team demonstrated their approach by concealing an optical
filter, which is a device that absorbs light in a prescribed set of
colors while allowing other colors of light to pass through, that
they illuminated with a short pulse of laser light.
The cloaking device was constructed from two pairs of two
commercially available electro-optical components.
component is a dispersive optical fiber, which forces the
different colors of a broadband wave to travel at different
The second is a
temporal phase modulator, which modifies the optical
frequency of light depending on when the wave passes through
One pair of these
components was placed in front of the optical filter while the other
pair was placed behind it.
The experiment confirmed that the device was able to transform the
light waves in the range of frequencies that would have been
absorbed by the optical filter, then completely reverse the process
as the light wave exited the filter on the other side, making it
look as though the laser pulse had propagated through a
cloaking to use
While the new design would need further development before it could
be translated into a Harry Potter-style, wearable
invisibility cloak, the demonstrated spectral cloaking device could
be useful for a range of security goals.
For example, current
telecommunication systems use
broadband waves as data signals to
transfer and process information.
Spectral cloaking could
be used to selectively determine which operations are applied to a
light wave and which are "made invisible" to it over certain periods
This could prevent an
eavesdropper from gathering information by probing a fiber optic
network with broadband light.
The overall concept of reversible, user-defined spectral energy
redistribution could also find applications beyond invisibility
For example, selectively
removing and subsequently reinstating colors in the broadband waves
that are used as telecommunication data signals could allow more
data to be transmitted over a given link, helping to alleviate
logjams as data demands continue to grow.
Or, the technique could
be used to minimize some key problems in today's broadband
telecommunication links, for example by reorganizing the signal
energy spectrum to make it less vulnerable to dispersion, nonlinear
phenomena and other undesired effects that impair data signals.
While the researchers demonstrated spectral cloaking when the object
was illuminated from only one spatial direction, Azaña said it
should be possible to extend the concept to make an object invisible
under illumination from every direction.
The team plans to
continue their research toward this goal.
In the meantime, the team
is also working to advance practical applications for
single-direction spectral cloaking in one-dimensional wave systems,
such as for fiber optics based applications.
L. Romero Cortés, M.
Seghilani, R. Maram, J. Azaña - "Full-field
broadband Invisibility through Reversible Wave
Frequency-spectrum Control" - Optica, 5, 779-786
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