Laser Expert Explains One Concept in 5 Levels of Difficulty

I'm Donna Strickland.

I'm a professor at the University of Waterloo.

I study lasers and, in particular,

I like really high intensity lasers.

So a laser is a way to get light to actually just

be a single color, going in a single direction

all of the waves peaking at the same time

so that the intensity can get very high.

Today I've been challenged to explain lasers

and high intensity lasers at five different levels.

From a child, to a teenager, to a college student,

to a graduate student and finally to a colleague of mine.

[upbeat music]

So I was told that maybe science is

one of your favorite subjects at school.

Is that right?

Yes.

Have you actually studied light yet?

Yes.

Okay what have you learned so far about light?

So we learned how to actually light up a light bulb.

Oh really?

Oh excellent.

Well I'm somebody who studies lasers.

So what do you think about lasers?

I don't know--

You haven't got to play with lasers.

So I brought one.

It's my friend's cat toy.

Do you ever use a laser as a cat toy?

No.

Well one of the fun things people do with lasers,

the cat will try to grab that dot.

I'm sure what you have at home is a flashlight.

I brought a cute little one.

So the question is, do you see any difference

between what a flashlight does and what a laser does?

Flashlight is a bigger shine and the laser is just a dot.

That's true, that laser's just a dot.

And so the other thing to notice though is that,

like if I shine it in your eyes and I'm sorry if I do!

But it seems awfully bright doesn't it?

And yet when, you know, you shine this down

and you put the laser, which one do you see easier?

[Harmoni] The laser.

The laser.

So which one do you think's more powerful?

The laser.

[Donna] And yet it's not.

Isn't that amazing?

Yes.

One of the things that lasers are great for

is that because it's a directed beam,

we can actually put that light where we want the light to go

and sometimes you maybe just want to see

something around the corner and you can't see it.

But with a laser, you can actually

and this is a smoky one so you can actually watch it go.

You see it actually bending the corner?

Yes.

And that's because the light will go through this glass

and, when it hits that corner, it has to bend.

And we actually send laser beams down glass fibers,

the size of your hair.

Yeah.

So this obviously is much bigger than our hair.

Right? Yes.

So this is just a demonstration.

If you have a laser like this, it actually

bends and comes out, I'm gonna point it to you

and you'll see it coming out the other end.

It hits these walls, it has to go around

and come out the other side.

You wanna play?

So is this the first time you've seen a laser

or have you gotten to play with lasers somewhere else?

I don't have a cat or--

You don't have a cat.

So you don't need a cat toy no.

Have you ever gone to a grocery store

and just scanned your objects over?

Yes.

Have you ever seen that maybe there's

a little bit of a red light when you do that?

Yes.

That's a laser.

We cut steel with them now.

We actually do surgery with lasers.

You know when some people have either scars

or birth marks that they don't want to see?

We can actually remove those with lasers now.

Never seen a laser light show?

When they light up the sky with

lasers, it's almost like fireworks.

Well I saw a shooting star before.

Oh you got to see a shooting star?

Well that's cool, that's nature giving you the show there.

So what do you think about lasers?

You think they're fun?

Yes they're very cool.

I like the one where you did the green one.

The green one.

And the next time you go to a grocery

store, take a peek at the red.

Okay. Okay.

[upbeat music]

Today we're here to talk about lasers.

So what do you think about lasers?

I think they're pretty cool.

They show up in a lot of my favorite books and movies.

Like Star Wars or just a bunch of

different sci-fi movies and books.

Do you know anything about lasers?

What makes a laser special kind of light.

All I really know is from sci-fi books and movies

and like the factory cutting lasers

that they use to cut steel and stuff.

Do you even know maybe how to make a laser

that it would be strong enough to cut steel?

No.

Okay. [laughing]

So one of the things about lasers is,

if you've ever seen a laser beam,

you know it's very directed.

Like what are they made out of?

What is a laser made out of?

Well really it's sort of the same thing as a light bulb.

Right?

So it's a beam?

So yeah it's a light bulb and there's a couple of mirrors.

Now the light bulb has to be a little bit special.

It has to be a type of material that can

store the energy in an excited state right?

It has to stay up there really energetic

and sit there for a while so that

when the light comes along,

it takes that energy and becomes stronger light

and then the mirror sends it back

and it does it again and again and again

and between these two mirrors, it makes

the light come out in a nice beam.

In a laser, it comes out as a single color.

They all come out with their waves at the same time.

Every wave peaks at the same time

which then makes it a giant wave

and it's this giant wave that

has a tremendous amount of power.

So it can do something like cutting steel.

But when you cut steel, or if you were cutting this floor,

it's gray, it actually will absorb the light.

That's why the light, you don't see it because the

light isn't bouncing back or through it.

So I like to use demos to explain how my laser works.

So I've brought basically a hammer and a nail.

I'm sure you've probably hammered a nail

before into a piece of wood but the question is,

if you asked yourself why it is we hit the big end

and it's the tiny end that we put on the piece of wood.

We could never pick up a hammer and hit the sharp end

and hope this would go in to the piece of wood.

Because it needs to be centered so

it can more easily just go in.

That's right.

All the force that we apply here, goes all the way through

but then it can only basically come out

when it contacts the wood in that one tiny spot.

And so sometimes it's the force that you push something with

but sometimes it is that force per unit area.

But sometimes it's not even the force per unit area

'cos, you know, push down as hard as you can on that

and see if we can get it pushed in.

See it doesn't really work does it?

So the laser needs ultimately many things.

It needs to be centered, it needs time to actually

penetrate and then it needs--

Well it depends.

So if you're cutting steel,

you need to have the nail.

You need to have it concentrated all of the light,

not going in all directions but you

need it into as small a point as possible.

And for that we use a lens.

For a lens, the light's coming down as a column,

you put in a lens, it all focuses it down in the same way

as the nail and then it starts cutting that steel.

Okay so that force per unit area--

So it's kind of like a magnifying glass?

Like it magnifies the light down to a point?

Exactly.

You know sometimes you want to have all of your energy

not just in a small area but in the small volume

and so one of the other dimensions is time or length.

But with light, time and length are the same thing

because light always travels at the speed of light.

Exactly.

But if you send a one second long light pulse

out into the skies there, the beginning of the pulse

is actually two thirds of the way to the moon.

It's 300,000 kilometers long.

So now, if you talk about light being

concentrated, that doesn't seem very concentrated.

The type of lasers I play with in my lab

will be no thicker than this piece of paper.

So we take that energy that might be

in three hundred thousand kilometers and we

squeeze it all the way down into just this

piece of paper and actually the beams

are more the size of this piece of paper

and so, in my lab, pieces of paper like this

would be flying through the sky but we can't see them.

'Cos they don't come in our eyes, they fly by us

and they're infra red.

There'd be like little concentrated beams of light

just flying everywhere? Exactly.

And so now, if we have light like

that that we wanna machine with.

I bring this funnel and so, if we had a lens here

and the light was coming down--

It funnels into--

Into a spot.

So here would be my light coming from my laser

and it would just be coming down, down, down, down, down.

Hit a lens and it would have to focus down.

But now, all the light started out with this big of a spread

so concentrated this much.

Eventually it would be here, more concentrated.

But by the end, right at the focal spot,

that's when I get all of my light, all of the energy has

been squeezed down into fitting inside this piece of paper

and that's why I say that I built a laser hammer.

Because when this hits a piece of glass,

it just smacks hose electrons right off the atoms

and there's nothing else for them

to do, they have to fly away.

So can you tell me what you learned

and maybe about the focusing of the light?

Well what I learned, lasers they aren't like particles.

They're more like a super concentrated beam of light

that can be any color.

They get really concentrated and that's what makes them

lasers and that's why they cut things and break things

because they just move the electrons out of the way.

So do you think that lasers are fun enough

to talk with your friends about or?

Of course I'm gonna have to share

something about my experience.

Learning about lasers with an expert like you.

[upbeat music]

[Donna] So you're a college student?

Yes.

[Donna] And what's your major?

I'm an engineering physics major with a minor in math.

I'm in the three, two program for biomedical engineering.

Excellent.

I got an engineering physics degree.

There you go, something in common.

We're here today talking about lasers.

So have you have had much exposure to lasers yet at school?

Not yet.

I'm really hoping that we will.

I think it's super interesting just the field

in general because I really do enjoy examining

all the calculations and being able to do

a little bit more of the math side of physics.

Okay.

As opposed to the experimental side and seeing things?

Okay so I'm much more, I like to see the things happening.

So then the question is, what's so special

about making a light that's intense enough

to actually possibly blow things up?

Certainly we can blow atoms up with the laser hammer

and when the laser light comes in

and just smacks the electrons right off the atom.

And so the question really is how do you make that?

Back in the 70s and into the 80s, I know that was

a long time ago for you, we had big energy lasers

and we had short pulse lasers, we

couldn't have big energy, short pulse lasers

and actually it was my supervisor and I

that figured the way around that and we got

something called chirped pulse amplification.

Have you heard of chirped pulse amplification by any chance?

Vaguely.

Well I brought a little bit of a prop

to explain how chirped pulse amplification works.

Our short pulses are made of different colors.

So I've got a colored slinky here.

We probably could've called it stretched pulse amplification

but that's kind of boring so we used the word chirped.

The word chirped comes around because birds chirp.

When the birds are singing, the notes are actually

changing audio frequency with time and that's a chirp.

The point is is that when all the light is

squeezed together like this, it's a short pulse.

And that's when it's a hammer because all

of the light is now concentrated and you can imagine if this

was coming along and also using a lens to focus it small,

then all of that light at the focal spot, concentrate.

And so that was the laser hammer.

So we can't have that in the laser.

So the question is what could we do?

The fact that it's different colors

and different colors because of dispersion,

travel different speeds inside material.

So we used a long fiber, 1.4 kilometers of fiber

but, in fiber, the red colors really haven't got

that much in common with the glass atoms and so they

spend very little time interacting and they travel fast.

The red is going to start traveling faster than the green,

faster than the blue and, as you travel down the fiber,

next thing you know, you have a long pulse

and it's chirped from red at the

beginning to blue at the back.

And so the frequencies go hoo!

Like this okay?

So this is a chirped pulse and now it's a long pulse.

And so, first this is what we did, we chirped it,

we stretched it, then we can safely amplify it

because it's not all concentrated

and, after we amplify it, then we use something called

a compressor and we put all of the colors back together

and it was back being a short pulse but a high energy pulse.

And then we really had what I like to call a laser hammer.

When this laser pulse goes inside,

it smacks those electrons right off the atom.

So the laser hammer that you were describing with

other types of lasers and the one in

the chirped is it still the same premise?

Well a lot of lasers and when lasers

first came along, they were only single color.

Your cat toy, would only be a single color,

probably a red one and so that's just one color.

And one color means that it has to

really be there for the whole time.

One color is one wave length of light

and so it's just one wave that goes on and on and on.

If you want a short pulse, you

actually have to have all the colors.

And if you can imagine that one point in time,

and I like to say it's like a conductor of an orchestra.

When you're listening to an orchestra warm up, they

sound terrible, they're all playing their own notes.

But when the conductor conducts them, they all play

different notes but, together, it's beautiful music.

So we have something in the laser called a mode locker

and it's like the conductor and it says go now.

And all the colors will start together but

some colors are long wave length and others are shorter.

So next thing you know, you have peaks

meeting valleys and they cancel each other.

And the more colors you can bring in, the faster

that happens and the shorter the pulse you can make.

What's the pendant?

The necklace is something that

was designed for my Nobel prize.

It's sold at the Nobel museum and it is a chirped pulse.

So we've been talking a lot about lasers and applications,

what have you learned about chirped pulses?

I learned that it all stretches, which is super cool,

because red moves the fastest

and so it kinda tugs around blue.

It really threw me off just how fast.

It's hard to imagine things happening that fast.

And I also learned how many of the things

that I know are lasers.

Like how many of the things that I've been looking for,

like, the answers to, it is in lasers.

[upbeat music]

[Donna] So I understand you're in grad school.

Where?

At NYU.

And what are you studying?

I'm studying soft matter physics,

which involves the physics of squishy stuff.

We make microswimmers in the laboratory

and we drive them with a laser.

And what kind of laser do you use?

We use a 10 watt laser, it's a fiber laser.

Do you know a lot about lasers?

Or just about the laser that you use.

Not a lot, just a little bit.

Okay.

So this is about high intensity lasers.

Not only how do you make them,

but what was really stopping them being made

in both cases is non linear optics.

We wanna do something that requires

a huge photon density application,

and so that's how come we came up with

chirped pulse amplification,

so that we could stretch the pulse,

safely amplify it, then compress it at the end,

and then we're ready to do whatever we want at the end.

So what do you think the main difference is

between the continuous wave laser that you have

that runs at 10 watts and a chirped pulse amplifier?

I feel like the continuous laser

delivers power at a continuous rate,

whereas you want all that power to be delivered

in a really, really short time with your amplification.

And so we get the power with a lot less energy

'cause its power is energy per unit time.

So we aren't depositing much energy in comparison.

Can I just ask, 'cause you are using

the thermal process of it heating up,

but have you ever had the opportunity

to use laser tweezers?

I have, yeah.

We use optical tweezers to trap particles in solution.

And twirl the motors or not twirl the motors?

No, I haven't worked with that.

You haven't?

Okay.

So I was always curious, how much further,

like what higher power can we go now?

So chirped pulse amplification took us sort of from,

we were at 10 to the 12, but when I was working

the 10 to the 12 sort of sat on a football sized field.

It was a kilojoule laser with a nanosecond pulse.

And we brought it down to something we call

tabletop terawatt.

So it was the same terawatt but now it was one joule

and one picosecond, so it could fit on a basic optical bench

like you would have in your lab.

We were able to take that up to,

I think the record is right around

somewhere between 10 to the 22

and 10 to the 23 watts per square centimeter.

So then in going forward, one of the holy grails

is can we reach 10 to the 29 watts per square centimeter?

So we still have six orders.

So we've gone from 10 to the 12 to 10 to the 23.

So we've done 11 orders,

so you think six isn't so much harder.

I have to tell you, over time, it's rolling over.

We need another Nobel Prize winning idea.

But if we get out there, that's where,

if you focus the intensity,

the energy in that volume is enough to break the vacuum.

We could probably use this to drive chemical reactions

at a very, very specific spot.

Like if we want to target just a single cell in the body.

Yes.

And maybe what, do pump-probe spectroscopy

and watch the cell?

Or to ionize it?

I mean I was thinking more on the lines of if we want to,

let's say destroy one cell,

like a tumor cell or something like that.

So that the neighboring areas are not affected

but just the cell burns.

I don't know if people are working on that

because I'm not so much in the medical field,

but I should look into that and see if that's a possibility.

So after hearing about high intensity lasers,

can you think about next time you go back to the lab

and you're wondering how to do something in the lab

with lasers, can you see how short pulses might help you?

I think short pulses might help in my experiment

in the sense that if I drive my swimmers

with a continuous wave as opposed to a pulsed wave,

maybe a continuous wave would heat up the sample too much

and a pulse laser would deliver power

exactly where I need it so that I could

run my experiment longer.

That's true.

Thank you very much.

Thank you, Donna.

It was really nice to meet you.

Hello Donna, good to see you.

Good to see you Mike, nice to have you here with me.

So we go way back.

1991, the year I got married,

I moved across the country, left my husband in New Jersey,

to work with you in Livermore.

I remember very much and how hard it was

to convince you to travel across the country

and work at the lab.

And stay there.

And stay there.

[laughing]

I couldn't convince you to stay.

You could not convince me to stay, no.

But you were there long enough to make a big impression

and get some good work done.

And I've been talking sort of,

started with sort of what is a laser

through linear optics, non linear optics,

high intensity laser physics, and saying that you know,

we're trying to get to that Schwinger limit

of 10 to the 29 watts per square centimeter.

We're about somewhere just shy of 10 to the 23

I think at this point.

But even if we get to 100 petawatts

and focus that down to a wavelength,

we're not at 10 to the 29.

So you're hoping to build the biggest laser.

Yes.

At Rochester.

But we're still not gonna get to the Schwinger limit,

isn't that right?

So just a little bit of back history again.

After you demonstrated CPA, I was intrigued by

how powerful could we make lasers?

And then how lasers are wonderful

because they allow you to take energy

and compress it in space and time.

So having high P power is something,

it's been a motivator for me for a long time.

And so we at the university are making proposals

to build two 25 petawatt lasers, maybe 30 petawatt lasers.

We'll use them to be able to get combined power

of over 10 to the 24 watts per square center.

Still far from the Schwinger limit.

But we have a trick.

We're going to use one of these petawatt lasers

to make an electron beam.

And this electron beam will be relativistic.

We actually think we could make electron beam,

maybe even 100s of GeV up to TeV,

which would be another Nobel Prize if we did that.

That's right, if you can do that, go for it.

And then we will shine that laser onto that electron beam,

and the electron's rest frame, we beat the Schwinger limit.

Okay, but that's kind of cheating.

That's not getting to 10 to the 29.

If you could do it with 100% efficiency

that's all we need.

That's why I wanna cheat.

That's the Nobel Prize winning idea though

if we can come up with it.

Because again, if we're able to do this,

with ways I can see today.

Right?

I can see us doing it today,

by just exploiting what we already know

and taking it to a limit,

then that would be a real motivator, I think,

to be able to even take these techniques further.

Cheating's not the right exact word

as taking advantage of relativity.

Take advantage of all physics not just optical physics.

So that's why we wanna do it this way.

We have to get into entertainment.

When is the high intensity laser gonna

get into entertainment so then there's real money?

Yes, yes, well we have Star Trek,

we had the photon torpedoes.

I always thought they actually knew what you were doing,

there's a photon torpedo.

Have you ever seen that in--

No.

[Michael] Oh.

I don't like science fiction.

Oh no, Star Trek had photon torpedoes

and they showed bursts of light about this long.

It was a CPA, it was a few nanosecond pulse.

Didn't say how much energy it carried,

and you could see it.

I don't know what it was gathering off of

but you could see it, so it was a great thing.

So we can either cheat by doing laser accelerating

and going into that rest frame.

Yes.

And that's, like you said, we're stuck a little bit,

we're not up to that kind of acceleration,

so that would be a Nobel Prize winning possible idea.

Absolutely.

Or we need, going around and giving my talks now,

I show how we're plateauing.

I showed how there was a plateau, CPA raises it up,

but we're sort of starting to plateau again

and we need another Nobel Prize winning idea.

And so do you think it's on the horizon?

Do you see anything out there that really says,

oh yeah that's a good way to go?

'Cos we're gonna have to get out to the x-rays right?

We can't stay in the visual.

So actually there's a potential ways

of doing it with optical or near optical radiation.

And there's been a lot of work done in the defense

department and so on, how do I combine laser beams together?

Okay.

And make them act as one coherent source.

So one of the things that we'll be doing

with our two petawatts, we're going to see

if we can actually combine them into a 50.

If you can do that, you can begin

to imagine doing this with many lasers.

Many petawatt lasers of the scale we're talking about.

So one could possibly see an exawatt from that.

People have been able to combine 10s of lasers together

for a coherent source so you have to be able

to phase lock them, you have to be able to make

their phases be exactly connected

and related and be able, as they propagate through all

the different optical components, whatever it may be--

And right across the beam.

Don't you think that's gonna be the challenge?

'Cos it's not like our beams are

as perfect as we like to think they are.

So that's right so you have to have the aperture size,

you have to have them phase

locked across the entire aperture.

Which will be a great challenge

and people have done it, again, with small lasers.

The lasers we'll be trying to do is about a 40cm aperture.

So we'll begin to start looking at this.

And actually wavelength control and then be able

to adaptive optics other ways which

you can control the uniformity of the phase

is something that's been developed in lots of ways now.

For defense applications, for science applications.

So we will do our best to

make use of all these technologies.

Lasers I think have progressed so much.

Just like the semi conductor did, 'cos there's such a market

for it, there were so many different applications for that.

Okay so there's a lot of us working around

the world on these high intensity lasers and so,

what do you think the real fun is?

What do you see the real excitement being?

I remember when

laser was first demonstrated 1960,

what could we do with this?

We already got light.

Now we can't live without lasers.

My cell phone, that is in my

pocket, has billions of transistors.

How is that made?

With lasers.

All of the greatest circuitry is done with lasers.

Actually now it's using x-rays,

made from laser heated matter.

That came out of the laser fusion program.

So it is amazing, the parallels.

And optics is used everywhere.

We're gonna possibly take over from CERN,

we'll just do high energy physics with lasers,

we're looking at gravity waves with lasers,

we wanna do black holes with lasers,

we wanna machine with lasers,

we wanna do medicine with lasers.

It's everywhere.

And now, with the Nobel prize, people are hearing

more about it so they know lasers are everywhere.

[Michael] I couldn't agree with you more

and you winning the Nobel prize has

been an inspiration to lots of people.

Only three women have won the Nobel prize in physics

and only one educated in the United States.

You.

There you go.

And I use that every place.

Okay.

And only one Canadian.

There you go!

[laughing]

[upbeat music]

Today was fun, I got to explain the work I do at all levels.

It's always fun for me to talk to elementary school

students because they bring such enthusiasm.

With a student who's already started to learn optics,

to a graduate student and finally my own colleague

where we can really get into a huge conversation

about what's the future of this field.

Electronics was the technology for the 20th century

and it brought us the transistor.

Electrons don't move nearly as fast as light and so,

trust me photonics will take us

where we wanna go in this century.