This video was sponsored by brilliant.org.
Lasers have transformed science.
Today, they're used in all manner of research from supercold atoms to astrophysics to biophysics.
In fact the most recent Nobel Prize in Physics was awarded
"for groundbreaking inventions in the field of laser physics".
What's unique about them is the fact that they produce light of one wavelength, which is coherent.
That means that the crests of laser light waves are like marching soldiers
all moving together with the same speed and same constant separation from one another.
This allows scientists to do really cool things
and the more powerful the laser,
the cooler the things we can do.
Modern research lasers are really impressive pieces of kit.
Okay, not quite that impress-
This is the Central Laser Facility in Oxfordshire in the UK.
The facility is used for a variety of research projects,
including recreating the conditions inside stars
and even imaging the motions of individual electrons in chemical reactions.
They have a variety of lasers,
but in particular they have a hugely powerful one called Vulcan.
Vulcan outputs so much energy that for a tiny fraction of a second,
it's more than ten thousand times more powerful than the entire national grid.
In fact it's so powerful
that the beam in Vulcan would melt the components that make up the laser
if the scientists who designed it hadn't used a clever trick.
This trick is what allows modern research lasers to be as powerful as they are
and it's what won the most recent Nobel Prize in Physics.
In this video, we're going to talk about how it works.
We'll come back to this laser and how it's so powerful later.
Promise.
To begin with, all lasers work by a process called stimulated emission.
In fact that's where the word laser comes from.
It's an acronym for "light amplification by stimulated emission of radiation".
Stimulated emission works by carefully selecting a material
known as a gain medium, which amplifies an initial pulse of light.
In this material individual atoms can be in a ground state or in an excited state.
There are no in-betweens.
An atom is either excited or it's not.
If the atom is excited
then it can jump down to the ground state
by emitting a flash of light called a photon.
This flash of light has a very specific wavelength associated with it,
unique to the material the atom is made of.
And so it will be the same for every atom of that gain medium
that jumps from the excited state to the ground state.
Lasers work because of an effect where an atom in an excited state
is much more likely to jump down to the ground state
and so emits that specific flash of light
when an identical flash of light passes nearby.
This process then allows for a small initial flash of light
to pass through the gain medium and be amplified several times
by passing several excited atoms,
causing them to jump down to the ground state
and emit their identical flashes of light,
thus becoming a really strong signal.
If you put a mirror at either end of the gain medium
then you can repeat this process as many times as you like,
if you keep raising atoms into excited states
by for example an electric field,
and then release the light at one end.
That gives you a pulse of laser light.
But it's only the first part of a modern research laser like Vulcan.
Because while this initial laser signal is weak,
it can be amplified through other components.
But, there's a problem.
As I said before,
if you try and amplify this signal up to the power output of Vulcan or the Death Star,
then the density of energy that you're putting through the optical components
like lenses and amplifiers is enough to damage those components.
So you just have to find a Nobel Prize-winning way around it.
That way is called chirped pulse amplification.
And this is how it works.
The reason that the laser beam would melt the glass is
because it would deliver an incredible amount of energy in a very short time.
When I say a very short time,
I mean about 200 femtoseconds
or 0.2 million millionths of a second.
Chirped pulse amplification works by, first,
stretching the laser pulse to a million times its original length.
So it now lasts a few nanoseconds.
This stretched signal is then amplified, increasing the energy in the pulse
until the energy density of the pulse is just less than the threshold for damaging the components in the laser.
Then, just before the beam hits its target, in the final part of the laser,
the pulse is compressed back to its original shorter length in time.
The amount of energy in the pulse is still the same, but it's now over a shorter period of time,
so the energy density is much much higher.
So you deliver to the target an energy density that is way higher than your optical components could tolerate.
Think of how the Death Star fires.
If it fired its main beam from within the space station, it would damage itself.
So instead, it combines several lower power laser beams into one massively powerful beam
just before it hits the target.
Except, while the Death Star combines and focuses beams
which are spread out in space (pun intended),
modern research lasers combine beams spread out in time.
And while the Death Star does this through a focusing dish, a bit like a lens,
research lasers accomplish the same thing through another standard piece of kit.
Diffraction gratings break up lights,
diffracting - that is basically deflecting -
different wavelengths of light by different amounts.
Red light gets deflected the most, and blue light the least.
A bit like a prism.
Except, remember how at the start of the video I said that light from a laser is all the same wavelength?
So if you put it through a diffraction grating, it's all going to be diffracted by the same amount, right?
Well, not quite.
Because of a variety of physical processes that literally have whole textbooks written about them,
laser light actually occupies a very narrow band of wavelengths, not just one number.
So when you put it through a diffraction grating, it does spread out in space ever so slightly.
The fact that it gets spread out in space is crucial to the trick of chirped pulse amplification.
Because, if you use a diffraction grating,
and normally a reflecting diffraction grating is used,
position it at an angle with some lenses,
another grating and a mirror,
you can create a pulse stretcher.
The key part here is on the right.
Notice how the longer wavelength - red - light travels a shorter distance
than the shorter wavelength - blue - light.
Because all light travels at the same speed,
this means that while the red and blue light arrived at the same time in the laser pulse,
when they leave the system, the red light does so slightly ahead of the blue light.
the duration of the pulse of light has increased
and its colours spread out like a rainbow in time.
At this point you can amplify your signal,
which Vulcan does through neodymium glass amplifiers,
and then when you're done,
compress the pulse of light back to its original short duration
using a similar setup of diffraction gratings.
But this time blue light has a shorter path to travel and the red light travels a longer path.
So you undo the stretching process
and you're left with a short pulse of extremely amplified laser light.
This whole process is extremely delicate and needs to be carefully monitored,
which explains why Vulcan is such a huge piece of kit.
But it allows it to generate incredible energy densities.
In 2005 it won a Guinness World Record for being the most powerful focused laser system in the world,
generating a petawatt pulse.
That's a billion million watts.
And when I was there, they let me fire it.
I'm about to fire one of the most powerful lasers in the world.
And it's touchscreen.
Commence primary ignition.
That may have been a bit anticlimactic,
but that was very cool for me.
So modern research lasers are only possible because of chirped pulse amplification:
stretching out your pulse of laser light before amplifying it,
and then re-compressing it back to a short, incredibly intense pulse.
By doing this, we can access incredibly high energy densities,
even bringing the physics of faraway stars to Earth for a fraction of a second.
I was assured by the scientists at the research facility however,
that we are still a very very long way off the construction of this particular technological terror.
Yeesh.
In this video I talked about how lasers are only possible because of a quantum phenomenon.
That means that Star Wars, cat videos, and I don't know,
life-changing medical science
is only possible because of the work of physicists at the start of the 20th century.
I was lucky enough to study quantum mechanics in depth in my Physics degree, and it was awesome.
One of the best aspects of my degree was the fact that the teaching was based around solving problems.
Every week I would have several sheets of questions to answer based on material covered in the lectures.
Personally, I don't really understand something until I've tried my hand at some questions
and gotten to grips with the ideas.
You don't have to commit to a degree to do this, however.
In fact, you can learn by solving problems in quantum theory along with loads of other topics on brilliant.org.
I'm so happy that Brilliant have sponsored a bunch of my science videos,
because their philosophy of learning through solving problems,
and in particular making sure you know it's okay to make mistakes,
because that's how you learn,
is exactly how I think people should learn.
Brilliant is freemium,
meaning that parts of the website are free,
so you absolutely should check those out,
and parts like their curated courses are accessible with a premium subscription.
If you like the sound of one of those, then go to brilliant.org/SimonClark
and the first 200 people to do so will get 20% off their subscription.
Thank you so much for watching the video and for making it all the way to the end.
Thanks also to the team at the NLF for inviting me to film,
that's Helen, Emily and Carrie,
thank you for showing me around.
There is another video coming out of that trip on a different topic, which I won't spoil just yet.
That'll be coming out in a couple of weeks, so I'm really really excited about that one.
I got to interview someone very exciting.
So yeah, that'll be on the channel in a couple of weeks.
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