 
[MUSIC PLAYING]
 
So I'm George Eleftheriades
from the University of Toronto.
And today, I would
like to share with you
our excitement over the past
few years with metamaterials
and how we can use them for
the extreme manipulation
of electromagnetic
waves, including
how to make invisibility cloaks.
So, in order to
explain all of this,
I need to tell you a little bit
about electromagnetic waves.
Now, if you don't
not know it, they
are actually all us
even in this room.
So, these are electric
and magnetic [INAUDIBLE]
that propagate an empty
space at the speed of light--
an astonishing 300,000
kilometers per second.
Most of these waves
are actually invisible,
but yet they are
used in every day
wireless devices, such as
smart phones, WIFI routers,
wireless keys, and
even microwave ovens.
So these are radio waves
and microwaves-- but also
visible light and X-rays are a
form of electromagnetic waves.
So what's the difference
between all these waves?
Well, picture a pebble
falling in the water--
then this will create
ripples-- waves.
So having this wave
picture in mind,
we can tell you
what's the difference.
So you see, there is a
characteristic distance
between two successive
peaks in these wave motion,
which is called the wave length.
Now, the wave length is
measured in centimeters
for radio waves and microwaves.
But for light, it's
much, much shorter--
like a thousandth of
half a millimeter.
So that's the difference.
Now, on the other
hand, metamaterials
is a new field in which we
try to make-- and engineer
materials-- materials that
do not exist in nature,
but which have unusual
electromagnetic properties--
such as, say, negative
index of refraction.
So how do they look like?
Well, I'll show you here a
collection of metamaterials--
the first two on the left are
for microwaves and radio waves,
and the one on the right
is for manipulating light.
So it has a much
finer structure.
But anyway, this is just
some boring science.
The question is, what
can we do with them?
Well, we can manipulate
electromagnetic waves
in extreme ways.
We can make light
bend negatively,
and I will explain what this is.
We can make lenses that
have entirely flat surfaces
and yet they can focus light.
We can use these
lenses to see details
with a microscope
that was thought
to be impossible before--
very tiny details.
And finally, they
can even be used
for making invisibility cloaks.
OK, so, let's start
this discussion
by examining one of the most
fundamental phenomena-- when
light interacts with
matter, such as glass.
So, if you have a light beam
impinging on a piece of glass
obliquely, then the light
bends, and this bending
is called refraction.
 
Now, refraction is responsible
for making eye glasses
and magnifying
glasses, and the amount
of light bending--
the amount of bending
depends on a property called
the index of refraction.
So, for glass, the index
of refraction is 1.6.
For diamonds, it's 2.4, so
much more bending for diamond,
and that's why it's so
much more shiny than glass.
So, the law really that connects
the angle of incidence--
the angle of refraction--
and the index of refraction
is called [? Snell ?] slope and
was established 400 years ago--
more than 400 years ago.
And yet, it's only recently--
in the past 10 to 15 years
that we found ways
to extend [INAUDIBLE]
and bend light much
more extremely.
Why?
Because today, we can
engineer metamaterials
with a negative
index of refraction.
So this was not-- this is
our unusual electromagnetic
property that I
was talking about.
So let's see what
this means-- when
we have a light beam impinging
on a piece of glass--
these [INAUDIBLE]
material there.
Well, as I mentioned before,
light will bend slightly.
Why?
Because light slows
down in glass.
But if we make
this glass to have
a negative index of
refraction, look what happens.
Light bends much more
extremely-- almost
bends backwards and
falls back to itself.
So with this extreme
way of bending light,
we can envision new
and exciting lenses,
such as these
[? Veselago/Pendry ?] lens,
which is just a [? slat, ?] so
it has completely flat surfaces
and yet it can focus light,
unlike any other lens that you
are familiar with.
So this is a really cool lens.
How does it look like?
Well, this is a lens
that we have constructed
in my lab a few
years back and can
be used to focus microwaves--
electromagnetic waves.
So, actually, you see the
focusing that takes place here.
Actually, these spots
are so tiny that we
can extend the resolution
of optical devices,
and this is shown in
the next slide here.
So what do we mean by that?
So, do you think you can
take a magnifying glass
and look at molecules and atoms?
No.
Why not?
Because they are so much smaller
than the wavelength of light.
But with this, the
[? Veselago ?] negative index
lenses, at least in
principle you can do that.
You can go and look at very
tiny objects and distances
between the objects, so this
is called super resolution.
And here is how it works.
So you see on the
right-- oh, sorry--
you see on the right here a
collection of objects, which
are very closely spaced--
so sub-wavelength space.
You tried to look
at them with a lens,
and you just see
a blurred image,
so you cannot tell them apart.
But with one of
these super lenses,
you can actually resolve
them very nicely.
So, of course, you can use
this for applications, ,
such as for improving resolution
from magnetic resonance imaging
apparatus, and that's
what we showed here.
There is a coil
next to a human leg,
and as you can see
on the top panel--
the further leg
cannot be imaged,
but when we insert one of these
super lenses in between the two
legs, then the second
leg is visible as well.
So these are applications
that microwaves,
but we can extend this
to visible light as well.
And this is an example
of a super microscope
that we have recently
constructed in my lap.
And what you see
here is actually
images of two tiny apertures.
So, on the bottom
panel here, you
see what happens when you try to
resolve this two tiny apertures
that are very closely spaced
with a conventional microscope.
So here they are
just barely resolved.
This is the minimum distance
that you can resolve them.
It's called the
Rayleigh distance,
and it's about half wavelength.
If just space these
apertures closer together,
you cannot tell them apart.
But with our super
microscope, you
see we can tell them
apart very nicely
well below this
fundamental Rayleigh limit.
So this is very exciting,
because we can take-- now--
microscopes, increase
their resolution
for a number of applications
for biology, medicine, material
science, and so on.
OK, so, this brings me
to the last application,
which is this invisibility
cloaking business.
So, invisibility has aroused
mankind-- the imagination
of humans-- since the Antiquity.
So, now, for example, in
ancient Greek mythology,
there was this invisibility
cloak-- rather,
invisibility cap
[INAUDIBLE] which
supposedly was made
out of dog skin.
I don't know why.
And according to
legend, the princess
had to wear it after she slain
the monster Gorgon to escape
the wrath of her sisters.
So, and, of course,
in most modern times,
invisibility has been the
main theme for novels--
such as HG Wells' classic
novel "The Invisible Man"--
Harry Potter movies, and--
what I grew up, actually--
"Star Trek."
So the cloak-- the star
ships of the Romulans.
But seriously, how can we
make an visibility cloak?
Well, look at this top picture
on the left here accordingly.
So, let's say we want to make
this sphere become invisible.
So the idea is
this-- you surround
this sphere with a
metamaterial shell,
and then when a light
beam comes from-- impinges
from this sphere-- then
the metamaterial shell
takes the constituent rays,
averts them around the object,
and then they are
combined on the other side
to their original trajectories.
So, for all practical purposes,
the object becomes invisible.
So, this is very nice
actually, and you
see how-- how the
metamaterial extremely
manipulates light and
electromagnetic waves.
Now, in my lab, we have
experimented with this idea.
And you see here-- a source--
is like our pebble in the water,
creating this
cylindrical waves--
next to a metallic cylinder.
So the waves cannot penetrate
through the cylinder.
You see, they just create a
shadow region at the back.
But when the cylinder is wrapped
with this metamaterial shell,
then the cylindrical waves
are restored completely as
if there was nothing there.
If you imagine--
how the waves will
look like if the
object was not there,
that's what you would see.
So, practically,
this is invisibility.
Now, this is very
nice, and I would
say it's the standard way
of making invisibility--
if I can use that-- invisibility
cloaks in the past, maybe, four
or five years.
But there is a problem, though.
You see, this metamaterial
cloak has to be quite thick,
and it becomes thicker
and thicker the larger
the object you want to cloak.
So if you want to
cloak yourself,
you will need another
thick layer of metamaterial
around you.
So not very practical.
So to solve this problem, we
have experimented recently--
my lab-- with another idea of
making invisibility cloaks,
which we call active metasurface
[INAUDIBLE] [? I quote ?]
so many terms.
But let's see how this works.
I think it's quite simple,
actually, to understand.
So how do we see an object?
When light impinges from an
object, then light scatters.
But it scatters-- it
reflects from the object--
all over the space, so some
of this scattered light
reaches our eyes, and
that's how we see it.
So if we have a
way to cancel out
this scattered fields--
these scattered waves,
then only the ambient
light will remain,
and so the object
will look invisible.
That's really the idea.
How can we achieve that, though?
So, there is a phenomenon called
destructive interference, which
probably you learned
in high school.
So, when you take
two waves and they
are the negative of each
other and you add them,
then they cancel.
And so, we can use this
cancellation phenomenon
to make a cloak.
And that's what
we did basically.
You take the object--
you surround it now
with a thing surface
consisting of antennas--
each antenna locally
radiate a wave that cancels
the local scattered
field-- wave.
So, therefore, for all
practical purposes,
when you cancel the
scattered waves,
the object again
becomes invisible.
So this is how it looks
like schematically.
We take the cylinder--
a metallic cylinder--
and we surround
it with antennas--
these tiny loops here that
you see are antennas--
and this will create
an invisibility cloak.
So this is how it looks like
in the lab-- on the left,
you see the metallic
cylinder surrounded
by these tiny antennas held
in place with little Styrofoam
blocks.
Each antenna is connected to
a cable-- these cables are
connected to electronics,
as you see here.
And then, these antennas
now radiate and make
the object look invisible.
So, does this work?
Well, it works really,
really well actually.
So, you see, a wave
comes from the left
and hits on the
cylinder, and then
it scatters all over the place.
And at the back, it
creates this shadow region.
But when we switch on our
active cloak, look what happens.
The waves just pass
right through it
as if there was nothing there.
So the metallic
cylinder literally
becomes transparent--
completely invisible.
So, I can show you
here on animation--
these are the actual measured
fields when the waves hit
the cylinder without the
presence of the cloak.
And you see that they just
scattered all over the place,
and they created a shadow
region at the back.
But when we switch on the cloak,
they just pass right through it
as if nothing were there.
So we-- and remember, this
is a very thin cloak, so--
and its thickness doesn't depend
on the size of the object.
It doesn't depend on
the shape of the object.
It doesn't depend on the
composition of the object.
So we think that's a very
practical way for making
invisibility cloaks, which has
a very bright future-- or shall
we say an invisible future?
So I would like to
conclude by saying
that the work of metamaterials
is truly fascinating.
That's what keeps us going.
It can offer exciting
opportunities-- sometimes
making science fiction
becoming reality.
Many people in the world
are working on this field.
They are inventing new
ways to manipulate light
and making new
practical applications.
I think the collective
goal of this community
is that one day these
applications can
find their way in
some practical way
in managing our everyday lives.
So, I would like to
finish by acknowledging
my present and
former Ph.D. Students
whose names are shown
here, and really
whose work has been
featured in this talk.
Thank you very much.
 
