Translator: Alessandro Brizzi Reviewer: Denise RQ
In his 1881 novella "The Invisible Man,"
Herbert George Wells described the scientist
who devoted all his life to research in optics.
And eventually, he comes up with a way
to make objects, or bodies, invisible to the human eye.
Wells was not the first person to write about invisibility,
but with his fervent imagination,
and his detailed descriptions of the involved optical processes,
he was able to fascinate generations
of readers, movie directors, and even many scientists.
There is also that little bit of voyeurism in all of us
that likes, get us exited, by thinking of hiding behind an invisibility cloak
and look around us without being seen.
Human fascination for controlling and manipulating light
is actually much older than Wells,
probably is safe to say that is as old as mankind.
What you see in this picture is the Lycurgus cup,
a Roman glass vase that is dated 1,500 years older than Wells.
It is housed at the British Museum, in London and has a unique optical effect:
if you look at the cup when it's illuminated from the back,
it looks red;
but when it is illuminated from the front,
with light passing through it, it actually looks green.
Ancient Greeks and Romans had learned
over centuries of experiments of trial and error
that if they would carefully melt
tiny proportions of precious metals into glass,
they could achieve such surprising optical effect.
If you looked at the glass under a microscope,
you would be able to see tiny alloys of silver and gold.
These metallic nanoparticles are as small as 70 nanometers,
that means 10,000 times smaller than a single grain of sand.
We now know, after centuries of studies
that actually, the exact material proportions,
the size, and the density of these nanoparticles
are the exact combination
that can unlock this unique optical effect.
It is quite amazing
to think how the artists of a couple of millennia ago
were able to come up with these precise material tricks
to realize this optical effect.
They had very simple tools, actually,
and a lot of ingenuity probably to get to that.
Let's travel a few centuries later to modern Northern Europe.
These same techniques were further mastered by the artists of those times
to realize the uniquely bright colors that we can admire in the stained glasses
decorating thousand of churches all over Europe.
Probably all over the world, actually.
Also at those times, the artists working on these masterpieces
were not really aware of all the laws of optics
that govern these phenomena.
But with unique hard-work and amazing skills,
they were able to find the right recipe
to turn an ordinary glass into a beautiful piece of art.
Again, they were using these nanoparticles to realize these effects.
It is pretty amazing to think, also,
how these artists were doing at those times, with the tools they had.
And one thing we can be sure of is that these artists could not imagine
how they could become the precursors of the modern scientists
that are currently unveiling the mysteries of light interacting with matter.
And that these stained glasses that I'm showing you,
as I will show you in a moment in more details,
are actually the ancestors of the modern technology
that may be able to realize Wells's dream of an invisibility cloak.
Today, we are in a particularly exciting period in history
because with modern nanotechnology tools,
we can actually precisely control
the size, shape, orientation, position, alignment of all these nanoparticles,
and we can realize optical effects
that were considered impossible even just a few years ago.
Just to give you an idea
of the type of artificial nanomaterials that we can currently make,
or the modern stained glasses that we are realizing,
these are a couple of microscope images that we recently realized in my lab.
What you see are extremely thin layers of glass,
stacked on top of each other and adorned by perfectly aligned tiny, gold nanorods,
even smaller than the ones present in the Lycurgus cup.
You may be thinking now
that these do not look as nice looking as the stained glasses I showed you before,
but I can tell you that they have far more reaching implications
in the future of applied optics and camera sensors.
The past ten years have seen an unprecedented growth
in the realization and in the physical understanding
of how materials operate at the nanoscale,
and we have come to realize
that if you can really control these nanomaterials at the nanoscale,
we may be able to challenge rules and limitations
that have been considered written in stone, for centuries.
This is how a new field of science and technology has effectively started
the field of metamaterials, or materials, man-made
that have properties that can go far beyond, or transcend,
the ones of natural materials.
Just to give you an example
of how these metamaterials can really trick light,
consider one of the most basic phenomena, in optics:
maybe you're already familiar with it ...
The phenomenon of refraction of light at an interface between two materials.
Refraction means that when a beam of light enters a new material
say, water from air,
It actually gets bent, it changes the direction in which it travels.
That's the collective or combined effect of all the water molecules
that are interacting with the impinging light
and, as result, bends it.
This same phenomenon explains also
why, if you look at a straw in a glass of water,
it actually looks broken at the interface with water.
In 1968, a young Russian physicist wrote his first theoretical paper
on a simple but rather obscure theoretical question.
He asked himself what would happen
if we could hypothetically find a material with a negative index of refraction.
Well, the index of refraction is essentially what I just described you:
is a quantity that measures how much light gets bent
when it enters a material.
Victor Veselago, this was the name of the scientist, at that time,
wondered what would happen if this quantity becomes negative.
It is usually one for air;
actually, larger than one for any other practical material.
But he has this curiosity.
And what he found in his paper is actually, light gets bent the other way.
If we could find such material in liquid form,
this is how our straw would look like.
At the time of publication of this paper,
Veselago's work didn't received a lot of attention
and to be honest, even in the years later, almost no one read it.
The reason it's not too surprising:
scientists at that time didn't think such materials could exist.
And even if they would, we wouldn't know what to do with them.
However, Veselago continued working on this topic, for many many years,
and along all his career,
and his quest, eventually, ended 35 years later,
when a group at the University of California in San Diego
was able to experimentally realize for the first time,
a negative index metamaterial.
Thirty-five years.
That's how long it can take
for a fascinating idea to go from dream to reality.
Like the images I showed you earlier,
what these scientists had figured out is that, by carefully controlling
the composition, shape, and arrangement of artificial molecules,
they could achieve this effect that was considered impossible.
Along all these 35 years,
the scientists in many countries had come to understand
that by controlling materials at the nanoscale
and realizing artificial metamolecules,
they could bend light the wrong way
like water molecules bend light in the usual way.
This is essentially how our journey to invisibility has started.
With few of my colleagues, we realized
that if we could trick light to go the other way,
we could even try more exotic effects.
Invisibility and cloaking represent today
one of the most exciting applications of metamaterials
that we have achieved so far.
Just the possibility of achieving this effect
has spurred the imagination of scientists and laypeople all over the world,
and has connected this field of technology
with something that has been in our dreams, just in books, or novels.
So, in the past eight years,
there have been a lot of suggestions and different proposals
to associate metamaterials with invisibility.
How would that work?
We have to understand a little bit how we see materials:
so, when a light beam excites or hits a material,
actually its surface reflects and scatters around
all the waves that are interacting with it.
And our eyes can pick up a portion of the scattered waves,
and essentially let us see the object.
If we were able to, somehow avoid this interaction,
between light and the object,
or cancel all these scattered waves,
then essentially, the object would become invisible.
Notice, this is different
than just trying to eliminate the reflections from an object.
That's what stealth technology already does in military airplanes, for instance.
What we want to achieve is much more challenging.
We want to eliminate the whole scattered waves around the object
including the shadow on the back of the object.
So to make the object completely undetectable.
One idea to realize this effect was to take a metamaterial cloak,
put it around the object,
and carefully bend the light rays all around
so that they wouldn't interact with the object.
With few of my colleagues back in 2005, we actually proposed a different approach,
and we realized if we could design a metamaterial
that would scatter a form of negative light,
opposite to the one of the object,
then, by properly balancing the positive light scattered from the object
and the negative light, scattered from the metamaterial,
we may be able to cancel the whole scattered wave
and have the light just go through the object
without being detectable.
After we came up with the idea, and we started working on an experiment,
we actually realized that Wells had already figured it all out.
(Laughter)
In his novel, he actually describes a very similar effect in lay terms:
Griffin, the crazy scientist working on this experiment,
had realized that if he could lower
the refractive index of a body to the one of air,
it would scatter no light.
Saying it in Wells's 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 it does all these things.
If it neither reflects nor refracts nor absorbs light,
it cannot of itself be visible."
Isn't it amazing?
When I first read this passage
I was thinking how an author, a writer of the 19th century,
could come up with such difficult concepts
and also explain them in such simple but yet so powerful words.
Last year, my group at the University of Texas at Austin,
was able to realize for the first time invisibility
on a three-dimensional object.
Instead of working with light or visible spectrum,
we worked with radio waves.
They are longer so they made the experiment easier,
but they follow the same physical laws as light.
We took a cylinder that is half a foot long,
and we covered it with a metamaterial cloak that was carefully designed
to have the exact opposite response, an electromagnetic response,
of the cylinder we were targeting.
We achieved this effect
by carefully inserting metallic plates in a ceramic shell,
much like the images I showed you earlier.
And our experiment proved that total transparency of an object is possible
for all angles of observation, for all the positions of the observer,
even near the surface of the object, or right behind it.
Just to understand how this looks like,
this animation shows you a radio wave that is hitting the original cylinder,
without a cloak.
As you see, the radio wave, when it hits the cylinder,
gets reflected and bounces off the surface of the cylinder.
That is actually how our eyes can see objects,
by collecting these deflections and scattering.
Once we put the metamaterial cloak around it,
this is actually what we were able to observe experimentally,
something very similar to this.
The wave would just go through the object, without interfering with it,
and there is even no shadow on the back of the object.
If you were to sit right behind the cloaked cylinder and look through it,
you would see a radio wave coming towards you
as if there is nothing in between.
By all practical purposes, the object is invisible to radars.
Not quite human eyes yet, but it's essentially the same physics.
We are now working on trying to extend this concept to larger objects,
collections of objects, and even different frequencies.
We are not only thinking
to the obvious defense, or camouflaging type of applications,
but we are also thinking of other fields of practical interest.
Imagine for instance if we could realize invisible antennas,
that could receive a signal without being detected.
Isn't it the modern 21st century a way of hiding behind an invisibility cloak
and looking around without being seen?
Also, these invisible antennas can be not interfering with each other
in a crowded environment, like on top of the roof of a building.
Similar concepts may be also applied to near-field microscopy,
to improve biomedical measurements
by being able to go very close to an object, with our microscope tip,
and see very tiny details of this object, without interfering with our measurement.
We have also suggested that we could use these ideas
to improve the absorption efficiency for green energy applications,
to realize optical nanotechs for biomedical identification,
and even to realize nanodevices
that could be used for the next generation of ultra fast optical computers.
Here it is.
If you don't care about all these technological advances,
and you are just dreaming of getting an invisibility cloak
sooner rather than later,
I understand you, but I have to warn you that Griffin's story doesn't end so well.
Actually, Griffin manages to apply the procedure on himself,
he gets invisible,
but he doesn't manage to reverse the procedure,
so he stays invisible forever.
His best friend betrays him, and reveals his secret all around,
so Griffin decides to kill him, and he starts a reign of terror.
I'm confident that the future of metamaterials
is brighter than Griffin's story.
(Laughter)
I actually like to think
that metamaterials are the new stained glasses of the 21st century;
a little bit less colorful that the old ones, as we know.
In our ongoing pursue and fascination
to trick and manipulate light with materials,
I think we are getting closer to bring a little bit of fiction into reality,
and we have shown that by thinking a little out of the box,
we have been able to overcome some fundamental limitations
of modern science and technology.
It's interesting that at the end of the day,
this is just a ten-years-old field of study.
Thank you.
(Applause)