On the quest to invisibility - metamaterials and cloaking |Andrea Alú | TEDxAustin

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From the Lycurgus cup to H. G. Wells's "Invisible Man," there has always been a fine line between fiction and reality when talking about the possibility to manipulate light and obtain invisibility.

Realizing the world's first cloak of invisibility for a three-dimensional object, Engineering Professor Andrea Alú explains how the discovery of metamaterials is pushing technology beyond conventional limits, producing vastly new opportunities beyond what nature can offer.
This emerging technology offers extensive applications in bioscience, energy, defense, and plenty more we can only imagine from here.

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)