[MUSIC]
Hi, and welcome to Coursera's
Nanotechnology: the Basics.
I'm Daniel Mittleman.
This is week four, and the topic of
week four is nanophotonics, which is the
intersection
of the world of nanoscience with the world
of photonics, which is the study of light.
In these series of lectures, we're going
to talk about
several different ways that, that
intersection leads to interesting
phenomena.
In the first lecture here, I want to give
a little bit
of an introduction about what is light,
and why is it that
light plus nano leads to something
interesting.
And what are a few of the situations where
that becomes really relevant.
So let's start out with what does light
have to do with nano objects.
Right?
Well in order to understand that we really
need to understand what's light.
So, this is the first picture that
illustrates what's light, light is an
electromagnetic wave.
Its an isolating electric field and
magnetic field that propagates through
space.
And there are many different forms of
light.
We can
talk about radio waves, which have a very
long wavelength.
We can talk about gamma rays, which have a
very short wavelength.
And all of these various other terms, that
fall in between which
you've heard of, which chained are
simply electromagnetic waves with
different wavelengths.
The visible region of the spectrum is
here.
It's sort of in the middle of this
picture.
That's the tiny slice you can actually see
with your eye.
And that corresponds to wavelengths
between 400 and 700 nanometers.
So, if that's the wavelength of light,
then that immediately
raises the question, what has light got to
do with nanoparticles?
Because there's clearly a size mismatch
between a
nanoparticle and a light wave, in the same
kind of ratio that there's a size mismatch
between a skier and the mountain she's
skiing on.
Right?
A nanoparticle is a 100 times smaller than
the wavelength of visible light.
So then what does light got to do with
nanomaterials?
It seems like a really logical question to
ask.
So, in order to understand that, let's
first
start out by talking about what is light.
The properties of light are governed by
Maxwell's equations, which were first
written down in the 1860s and are still
considered exactly correct today.
they fit very nicely on a coffee mug.
but we're not going to try and solve these
equations.
It's a little bit beyond the scope of the
class.
Instead I'm simply going to jump straight
to the answer.
And here's the answer.
This is really what Maxwell's equations
tell us that
light is.
Light is actually, not just one wave, but
two.
It's an electric field, which is shown
here in red,
and a magnetic field, which is shown here
in blue.
They oscillate at the same frequency, they
propagate together through space.
There's always both of them, never just
one.
So in order to talk about what is light we
need some
terminology which will appear throughout
the entire week of lectures this week.
So let's start defining some terms.
The first and most important one is the
wavelength.
The wavelength of the light is symbolized
by the Greek letter lambda and you will
see that in every lecture this week so
please get use to lambda being wavelength.
What we mean by the wavelength is simply
the distance from
the peak of one oscillation to the peak,
the next peak over.
So it's simply the distance in space that
a wave oscillates.
The oscillation distance, the peak-to-peak
distance.
As I've mentioned, for visible light that
corresponds to somewhere
between around 400 and 700 nanometers,
roughly speaking.
For infrared, it's larger.
For ultraviolet, it's smaller, and so on.
The other important quantity here is the
frequency, which
has also got a frequency, that's the Greek
letter nu.
And the frequency is simply, if you sit at
a point in space, and
the wave goes by you, how many times do
you see a cycle per second?
How many cycles per second?
So that's the frequency of a wave.
So those two numbers turn out to be
related by this simple
equation here, which is a good one to
remember.
Lambda times nu equals c, c is the speed
of the wave.
If the speed of the wave is a constant
number, then lambda,
the frequency times the wavelength has to
be equal to that constant.
So if the frequency goes up, the
wavelength has
to go down, such that their product
remains constant.
Of course, for light waves in empty waves,
the speed of the wave is a
fixed constant of the universe, it never
changes, and that's a good number to know.
That's the
speed of light in empty space.
That's really the speed limit of the
universe.
Nothing, no energy, no information, no
objects
can ever travel faster than that speed.
Okay, so what are some other properties of
light?
Well clearly light carries energy.
We know this because if we stand outside
on a sunny day we warm up.
That's energy being carried from the sun
to our skin by a light wave.
to talk about the energy in a light wave,
we have really two different ways to do
this.
Number one is to talk about the energy of
a single photon.
And, a photon of course is the quantum
version of what a light wave is.
It's the smallest fundamental unit of a
light wave, if you like.
It's the indivisible quantum, of a light
wave.
and the energy of a single photon is
simple h nu.
Nu is the frequency that I just talked
about and
h is Planck's constant, which is another
fundamental universal constant.
So, if you have a larger
frequency light wave, that corresponds to
a
smaller wavelength, and a higher photon
energy, right?
The photon energy is proportional to its
frequency.
Another way to talk about the energy
carried
by a light wave as a more macroscopic
quantity.
This is the intensity of a light wave.
It's the total power falling on the
surface area.
So it's the energy per area per time.
And energy per time is power, right?
So the units are watts per square meter.
Really the
correct terminology is not intensity but
irradiance, that's
the correct term, but people are often
sloppy
and say intensity when they mean
irradiance, so
we just have to get used to that.
Some good examples to just give you some
scale of numbers.
If you stand outside on a sunny day and
the sun is straight
overhead, then the intensity of the light
is approximately a kilowatt per square
meter.
The world's most powerful lasers are much
bigger numbers
than that, they're more like 10 to the
20th watts
per square meter, and that's obviously
something you
would not want to stand in front of.
Okay, so how can we make light talk to
objects on the nano scale?
Well, one clear way that we can think
about this
is by simply focusing the down to a tiny
spot.
Right?
And by putting a lens in front of a laser
beam, we focus that laser beam down to a
tiny spot.
Right, or by using a magnifying glass in
front of the sun.
We get a little spot of light.
So how big is the spot that we can make?
Right?
So, let's think about that.
If we take a lens which has a certain
diameter
D, and the light wave completely fills the
lens, right?
And then a focal length f, so at one
focal length distance away, we will get a
tiny spot.
So, how big is the spot?
Well, the calculation's a little
sophisticated, a little
complicated, so let's just jump to the
answer.
The answer is that the spot size is twice
the f number, times the wavelength.
And here's the f number.
The f number
is simply the ratio of the focal length,
divided by the diameter.
So, it's a property of the lens.
So, the important thing to realize here is
that, that's
the spot, that limits how small a spot
size can be.
And the reason is because it's very hard
to make
a lens with an f number smaller than a
half.
That's really tough.
So that means that the spot size is,
practically speaking,
are never going to be much smaller than
the wavelength.
Okay, you cant maybe get a little smaller
but not a lot smaller.
And that limit is called
the defraction limit.
The defraction limit is, how small can you
focus light?
and, and, if it's really limited by the
diffraction of waves.
Now diffraction is a phenomena that is a
general property of any wave, not just
light waves.
and it's really what causes the limiting,
the, what
limits how small we can focus a light wave
down.
This is why, for example, when you look
through a microscope, you can see cells,
because cells are bigger than the
wavelength
of the light you're using to see them.
But you can't
see nanoparticles in an optical
microscope, because they're smaller.
So you can't see things smaller than the
wavelength, roughly speaking.
Diffraction is a bit of a hard concept to
grasp, so here's a little video that will
illustrate a
diffraction phenomenon to show you, kind
of, what goes on
when you try and put light through a tiny
aperture.
Diffraction is a phenomenon that is
exhibited by all waves, including light
waves.
And basically, diffraction is
what happens when a light wave encounters
some sort of obstruction or aperture
or scattering site, and then after the
scatterer the waves that scatter from
different parts of the object will
interfere constructively and destructively
with each other
downstream to create some sort of
complicated
pattern, that's known as a diffraction
pattern.
So this is illustrated in the picture
here, where
our diffracting object is actually a very
simple object.
It's just a slit,
which is a pair of metal razor blades
that can be moved closer together and
further apart.
And what you're seeing now, is a video of
what that slit looks like,
without the laser beam on it, just so you
can picture what's going on here.
So, what I have, is the red laser beam and
the green laser beam pointed together,
onto this slit and as
I open and close the slit, you can see the
diffraction pattern changing, because the
diffraction pattern depends on the opening
size of that slit.
If I open it a lot, then you'll see
essentially no diffraction pattern because
the light goes straight through.
And if I close it down, so that it's a
very small slit,
then you could see a pattern of light and
dark stripes on the wall.
And if you look closely, you'll notice
that those light and
dark stripes are spaced differently for
the green and for the red.
Because green light has a shorter
wavelength than red light,
it's diffraction pattern is different,
because the diffraction pattern depends
on the wavelength of the light.
Now that you've seen that diffraction can
lead to complicated light patterns
when you put light through a small
aperture, maybe you can begin
to appreciate why it's really hard to,
force light through, into a
spot that's much smaller than the
wavelength, because things get pretty
complex.
As an aside, it's worth pointing out, that
there are
many images we've showed you in this class
already of nanostructures.
And you now know that those could have not
have possibly
been taken, those images, could not have
possibly be taken in
an optical microscope, and now you know
why.
Because their, the objects you're looking
at
are smaller than the wavelength of visible
light.
instead we use typically an electron
microscope to image nanostructures
and the reason that works is because
electrons behave like waves.
And, the wavelength of those electrons can
be
much smaller than the wavelength of
visible light.
So we can use electrons to image objects
that are smaller than a few hundred nano-
[INAUDIBLE].
Okay, so can we overcome this diffraction
limit?
Can we force light down through a tiny
hole in order to be able to image
nanostructures.
Right?
That's a really good question to ask, and
there's
a lot of people thinking about how to do
that.
Sort of the simplest way you might think
about doing that is simply
just what we did before, take our light
and focus it down with
the lens, but now at the focus, let's put
an opaque screen, a
piece of metal, with the sub wavelength
aperture, a little hole in it.
Okay?
And see if we can force the light through
that little hole.
So, interestingly this does work, although
typically you don't use
a flat piece of metal with a hole in it,
you use a different geometry which is say,
for example
an optical fiber, which has been coated by
a metal coating.
And then that tapers down to a tiny spot
tiny tip and then at
the tip you have a little bitty opening
where the light can leak out.
And if that opening is small, smaller than
the wavelength,
and you hold that tip very close to the
surface
you want to image, again on the nanometer
scale close, then
you can make images of that surface which
are with
a resolution, a spatial resolution that is
smaller then the wavelength.
Because the size of the hole is what
determines your resolution, not the
wavelength of the light.
So this can be done, it works its a
technique known
as near field scanning optical microscopy
usually abbreviated NSOM, like this, and
here you're seeing a couple of examples.
This is an NSOM image showing individual
molecules in florescence.
And this is an NSOM image showing the
light emission from a smaller laser
which is not coming uniformly from
everywhere
on the laser, they are clearly hot spots.
So there's, clearly NSOM is a powerful
technique for being able to image
things with light, of visible wavelengths,
on a scale smaller than the wavelength.
The best, the world's record that I'm
aware of,
the best resolution for NSOM is about
lambda over 30.
More typical is probably lambda over 10.
So, NSOM works well for imaging below the
wavelength, but you're really
not going to get below it, I mean, you
can't do lambda over 1,000.
So we still can't image individual
nanoparticles using NSOM.
So, what else is interesting about nano
plus photons?
Well, I want to talk about three
situations in which that combination is
important.
Number one is that the sensing of things
on the nano scale is often transduced.
The signals are often transduced with
light.
And we've seen examples of this before.
Here's one example, which is the example
of
atomic force microscopy, where the signals
are let,
read out by bouncing a laser off of the
structure that has a nano component to it.
So, that's one example of transducing a
signal from the nano
scale to a place where we can read it
using light.
And that, there are numerous examples of
that.
a second idea, is another one we've
encountered before which is that the
optical properties of materials often
change
when we make those materials nano sized.
And the conical example of this that
we've discussed on several occasions in
this class
is the idea of quantum confinement, where
if you take a semiconductor nano particle
and
go from a big one to a small one, then you
shift the color the
emission, because you've changed the
electronic structure,
the electronic level spacings, of the
material by
virtue of having, by making it small.
That causes the level spacing to change,
and
therefore the color of the light to
change.
And we will talk in more detail about this
in subsequent lectures this week.
A third idea, which we will also address
in some
detail this week, is a little bit
different from that one.
Instead of changing the properties of the
material, what we're doing is
structuring the material, in order to
change the way it interacts with light.
So the important idea here is that when
you structure a material on the nano
scale,
you can change the interaction with light
even if the intrinsic material properties
are unchanged.
And let me give you a thought experiment
to, to illustrate what I mean here.
So let's supposed you're looking at a
window, ordinary window pane.
And let's suppose that we replace the pane
of glass with a
thinner one and then a thinner one and
then a thinner one.
The question is, does the view out the
window change?
And the answer is, if you make the glass
thin
enough, on the hundreds of nanometers
scale, then indeed the view
does change and that's not because the
properties of the glass have changed,
it's because of the way that
that nanostructure interacts with light
has changed.
And we'll talk about this in subsequent
lectures this week.
So the big ideas for today's lecture are,
first of all
that light has a characteristic length
scale, which is the wavelength.
Please get used to that symbol lambda
because it
will, you will see it in every lecture
this week.
which is much larger than a nanometer.
On the other hand despite
that the interaction of light with
nanomaterials is interesting for three
reasons.
First of all, nano-enabled sensing is
often enabled by transduction with light.
Number two, in certain types of materials
when you make them
nano, their properties really change,
including
their way they interact with light.
And number three, even if their properties
don't change, by structuring them on the
nano scale, we can really create new ways
for that material to interact with light,
which can give us really new and exciting
possibilities for, for devices and
components and structures.
Thanks for listening.