Andrea Alù at TEDxAustin - On the Quest to Invisibility: Meta-materials and Cloaking
In his 1881 novella “The Invisible Man”, H. G. Wells describes a scientist who devoted all his life
to research in optics and eventually comes up with a practical way to make bodies invisible to
the human eye. Wells was not the first writer to talk about invisibility but with his fervent
imagination and clear descriptions of the involved optical processes he has fascinated
generations of readers, movie directors, and even many scientists… There is also that little bit of
voyeurism in all of us that gets excited at the idea of hiding behind an invisibility cloak and
observing what happens around us without being seen.
Human fascination for controlling and manipulating light is
definitely older than Wells, it is probably safe to say that it is as old as mankind.
What you see in this picture is the Lycurgus cup, a Roman glass vase realized over
1500 years before Wells. It is currently housed at the British Museum in London
and has a unique optical property. If you look at the cup when illuminated from
the back, the object is red, but when is illuminated from the front with light passing
through it, it actually looks green. Ancient Greeks and Romans had learned over
centuries of experiments, trial and error that by carefully melting tiny amounts of precious metals into glass
they could obtain such a surprising optical effect. If you looked at the vase under a microscope, you would be able
to see, dispersed here and there, tiny silver and gold alloys. The average size of these metallic nanoparticles is about
70 nanometers, 10 thousand times smaller than a single grain of sand. After centuries of study, we now know that
the specific material proportions, the size of these nanoparticles and the density with which they are embedded into
glass form the exact combination that can unlock this unique optical effect. It is quite amazing to think how these
artists a couple of millennia ago were able to realize these material tricks with simple tools and a lot of ingenuity.
Now let’s travel a few centuries later to Northern Europe. By that time, these same techniques had been further
mastered to realize the uniquely bright colors that we can admire in stained glasses decorating thousands of
churches. Also at these times, the artists working on these masterpieces did not know all the laws of optics that
govern these phenomena, but with hard work and amazing skills they were able to develop the precise combination
of metallic nanoparticles required to turn an ordinary glass into a marvelous piece of art. What those artists did not
imagine is that they would become the precursors of the modern scientists that today are unveiling the mysteries of
light interacting with matter, and that these stained glasses, as I will show you in a moment, are the ancestors of the
modern technology that may be able to realize Wells’ dream of an invisibility cloak.
Today, we are in a particularly exciting period in history, because with modern
nanotechnology tools we can control with extreme precision the shape, size,
orientation, composition, alignment and density of these nanoparticles to realize
optical effects that were believed impossible even only a few years ago. To give you
an idea of the modern ‘stained glasses’ that we are now able to produce, these are a
couple of microscope images of artificial materials recently produced in my lab.
What you see here are extremely thin layers of glass stacked on top of each other and
adorned with perfectly aligned tiny gold nanorods, even smaller than those found in
the Lycurgus cup. You may argue that these are not as nice looking as the stained
glasses we saw a minute ago, but I can assure you that they have far more reaching implications in the future of
optical devices and camera sensors. In the past ten years we have seen an unprecedented growth in the realization
and physical understanding of these nanomaterials. We have come to realize that by controlling the material
composition at the nanoscale, it may be possible to challenge rules and limitations that were for centuries
considered written in stone. This is essentially how a new field of science and technology has started, the field of
meta-materials. By their same definition, metamaterials are man-made materials with properties that transcend the
ones of natural materials.
As an example of how light can be tricked by metamaterials to do things we would
not expect, consider one of the most common optical phenomena, the refraction of
light at an interface between two materials. Refraction simply means that when an
optical beam enters a material, say water, from air, it changes the direction in
which it travels. This is actually the combined effect of billions of water molecules
interacting with the incoming light, which, as a result, gets bent. The denser the
material, the more bending we see. This phenomenon actually explains why a straw
in a glass of water looks broken. In 1968 a young Russian physicist published his
first scientific paper on a simple, but rather obscure theoretical question: what if
we were able to find a material with a negative index of refraction? The refractive
index is exactly what I just described, it tells us how much light is bent when it
enters a new material. It is ‘1’ for air, and it is larger than 1 for practically any other
material. Victor Veselago, this was the name of the scientist, asked himself what
optical effect he would get if this quantity got hypothetically negative. This is what he predicted: light would bend
the ‘wrong’ way. If we could find a negative-index material in liquid form, this is what our straw would look like.
Veselago’s paper didn’t receive much attention at the time of publication, nor in the following years… That’s not too
surprising: it was hard to believe such materials could exist and, even if they did, we wouldn’t know what to do
with them. Still, he spent the rest of his career looking for one, and his quest eventually ended 35 years later when a
group of scientists at UC San Diego was able to experimentally create the first example of a negative-index
metamaterial. 35 years, this is how long it can take for a challenging idea to go from dream to reality… Like the
images I showed you earlier, the composition, shape and arrangement of specifically designed ‘artificial molecules’
provided a new recipe to produce an effect that was considered impossible. What scientists had come to understand
during these 35 years was that, like water molecules bend light in the ‘usual’ way, properly designed metamolecules
can bend light in the opposite way.
This is how our journey to invisibility has essentially started. With a few colleagues we realized that if we can trick
light to go in the wrong direction using metamaterials, we could think of even more exotic effects!… Invisibility and
cloaking represent today the most exciting phenomena so far achieved with metamaterials. The possibility of
realizing this effect has spurred the imagination of scientists and lay people, connecting metamaterials with
something that we had so far only dreamed in novels and movies. In the last eight years, several proposals have
been made to apply metamaterials to invisibility. How would it work? Well, when a beam of light hits an object, it is
reflected and scattered around by its surface. This is essentially how we see the object, by collecting a portion of
these scattered waves. If we were able to avoid the interaction between light and the object and eliminate these
scattered waves, then the incoming beam would essentially go undisturbed through the object, making it invisible
to anyone around it. Notice that the challenge here is not only to eliminate reflections, this is what stealth
technology already does on military planes. What we want to achieve is much more challenging, suppress any
interaction between the object and light to eliminate even the shadow and making it completely undetectable.
One idea is to use metamaterials to carefully ‘bend’ light rays right around an object, like a form of mirage. My
colleagues and I have worked on a different approach and proved in 2005 that properly designed metamaterials can
be made to scatter a form of ‘negative’ light. If we manage to balance the positive scattering from the object and the
negative scattering from the metamaterial, the overall effect would be to cancel the scattered wave and produce an
invisible object. The wave would just go through without scattering, and you would not even see its shadow. After
we proposed the idea and started working on an experiment, we discovered that Wells had already figured it all
out. In his novella he essentially describes the same idea in lay terms: Griffin, the crazy scientist, based his discovery
on a method to change a body's refractive index to the one of air, so that it scatters no light. Putting it in Wells’ own
words, Griffin devised ‘a method by which it would be possible, without changing any other property of matter to
lower the refractive index of a substance so far as all practical purposes are concerned. Either a body absorbs light,
or it reflects or refracts it, or does all these things. If it neither reflects nor refracts nor absorbs light, it cannot of itself
be visible’. When I first read this, I found quite amazing that a writer from the 19th century was able to imagine
these difficult concepts and describe them in such simple yet powerful words?
Last year our group at the University of Texas at Austin was able to show for
the first time invisibility of a three-dimensional object. Instead of targeting the
visible spectrum, we worked with radio-waves. They are governed by the same
physical laws as light, but they make the experiment easier because they are
longer. We took a cylinder, over half-foot long, and covered it with a
metamaterial cloak that was carefully designed to have an electromagnetic
response that is the exact opposite of the one of cylinder. We achieved this
effect by inserting carefully designed metallic plates in a ceramic material, a
little bit like the stained glasses I showed you a few minutes ago. Our
measurements proved that total transparency of an object is possible, for
different angles and observer’s positions, even very close, or right behind the
object. To understand better what this looks like, in this animation you see a radio-wave hitting the original cylinder
from left to right. As you see, the wave is strongly distorted and perturbed by the presence of the cylinder. It looks
like it is bouncing off its surface. This interaction is essentially what makes us see the object. Once we add our cloak,
the wave goes through it as if the object is not there, and the cylinder becomes invisible to radio waves! I can tell you
that this is essentially what we were able to measure in our laboratory. If you were to sit right behind the cloaked
cylinder you would see the wave coming through it towards you, without being able to notice that there is an
obstacle in between. For all practical purposes the cloaked cylinder has no shadow and is invisible to radars! Not
quite human eyes yet, but the concept is essentially the same.
We are now working to apply this technology to larger and more complicated
shapes; to collections of objects, and to different frequencies. We are not only
thinking to the obvious applications, like defense and camouflaging, but also to
a variety of other fields of practical interest. Imagine if we could make invisible
antennas that can receive signals without being detected. Isn’t this the modern
way of hiding behind an invisibility cloak and seeing without being seen? These
invisible antennas would also not interfere with each other on a crowded roof or
a tower station. We can apply a similar idea to near-field microscopes,
drastically improving bio-medical sensors and measurements. We are also
thinking of ways to apply this technology to improve the efficiency of light
absorption for green-energy applications, to make optical nanotags for unique biological identification, and to
conceive optical nanodevices for the next generation of ultrafast, optical
computers.
If you don’t care of these technological advances, and are just dreaming of getting
a cloak of invisibility sooner rather than later, I have to warn you that Wells’ story
doesn’t end so well: Griffin successfully carries out the experiment on himself, but
he fails at his attempt to reverse the procedure and stays invisible forever. He gets
betrayed by his best friend that doesn’t keep his secret. So Griffin murders him
and begins a ‘reign of terror’. I am confident that the future of metamaterials is
much brighter than Griffin’s story. I like to think that our metamaterials are the
new stained glasses of the 21st century –just a bit less colorful than the old ones. In our ongoing fascination and
pursuit to change the way light interacts with materials, we are getting closer to bringing fiction to reality and we
have shown that, by thinking out of the box, it is possible to overcome established limitations of science and
technology. At the end of the day, this is only a 10 year-old field of study….