36C3 - An ultrashort history of ultrafast imaging

36c3 preroll music

Herald: So who is excited about
photography or videography?

applaus
Herald: Yeah? The title of the talk kind

of gives us gives it away. OK. We bet we
are waiting for the last people to come in

and take a seat. Last time, raise your
hands if you have a free seat next to you.

Every one of you coming in, look for raised hands
and take your seat and then we will start.

Yeah, very good. OK. Looks like the doors
are finally closed. Okay, so the next talk

on the second day is about ultrafast
imaging. So many of you have done

videography or photography. Have thought
about exposure time, how fast you can do

your photography. And some of your might
have played with lasers and have built

blinky stuff with it or have done
scientific experiments and Caroline Will

now show us what happens if we take those
to combine them and take it to the

extreme. Caroline is working at DESY since
four years. She has not done her PhD and

is now working in a group for theoretical
fast modeling of inner workings of

molecules and atoms. She is doing a
computational work and working together

with experimentalists to verify their
observations, and now she is presenting

the inner mechanics of what she is doing
and how we can actually maybe photograph

molecules by their forming. Applause!

applause

Caroline: Great. Yeah. Thank you very much
for the introduction and thank you very

much for having me here. I'm excited to
see this room so full. So I'm going to

speak today about an ultrashort history of
ultrafast imaging. It's a really broad

topic. And I'm just gonna present some
highlights, some background. Before I

start, I'd like to give you a few more few
more words about myself. As we've already

heard, I work at DESY, this is the DESY
campus you see here and in the Center for

Free Electron Laser Science, circle in
orange. That's where I did my PhD. So this

whole campus is located in Hamburg. This
is probably also a familiar place to many

of you. And now this year we are in
Leipzig a bit further away for the 36th

Congress. So I'd like to start with a very
broad question. What is the goal of

ultrafast imaging? And we've heard already
that ultrafast imaging is related to

photography. Now, as many of you know,
when you take a picture, with a quite long

exposure time, you see just a blurry
image, for example, in this picture of a

bowl of water. We can hardly see anything.
It looks a bit foggy. But if we choose the

correct exposure time, which in this case
is 100 times shorter in the right picture

than in the left picture, then we see a
clear image and we can see dynamics

unfold. So we have here, a drop of water
that is bouncing back from the bowl and

also some ripples that are forming on the
surface of this bowl. This is only visible

because we chose the right exposure time.
And this is to me really the key of being

successful in ultrafast imaging to take a
clear picture of an object that is moving.

But it's not enough to say take just a
picture. So now imagine you're a sports

reporter. You get these two pictures and
you're supposed to write up what happened.

So it's complicated. So the top picture is
the start, the bottom pictures is the end.

Just from these two pictures, it's hard to
see. But if we see before picture, we can

see very complex dynamics unfold. There
are particles accelerating at high

velocity laughing coming in from the back. And even
particles we did not see in the first

picture at all somehow are very relevant
to our motion. And not only skiing races

are very dynamic, but most processes in
nature are also not static. This is true

for everything we see around ourselves,
but it's especially true for everything

that is quite small in the microcosm. And
in general, we can gain a lot more insight

from time resolved images. So from ultra
short movies. I'd like to show you the

very first ultrafast movie that was ever
taken. Or maybe even the first movie that

was taken at all. This guy, Eadweard
Muybridge lived in the 19th century. And

very shortly after the invention of a
photography method, he tried to answer the

question does a galloping horse ever lift
all of its feet off the ground? Why, it's

running. To us, it may seem like not so
important question, but in the 19th

century, the horse was the main method of
transportation, and horse races were very

popular. So there was a lot of interest in
studying the dynamics of a horse, and this

process is too fast to see with the naked
eye. But Muybridge implemented a stop

motion technique where the horse as it is
running, cuts some wires, that then

trigger photographs. And with this he was
able to take these twelve photographs of

the horse in motion. That was published
under this title in Stanford in the 19th

century. And we see very clearly in the
top row third picture and maybe also

second picture that indeed the horse lifts
all of its legs off the ground, which was

a new insight at that time. And when we
stitch all of these snapshots together, we

have an ultra fast movie of a horse
galloping, which might be seen as the

first movie that was ever made in the
history of mankind. Now, when I say

ultrafast today, I'm no longer thinking
about horses, but about smaller things and

faster things. But let's go there, very
gently. So the time scale that we are all

familiar with that we can see with the
naked eye is something of the order of

seconds. So, for example, the acceleration
of this cheetah, we can see with the naked

eye. Now, if we zoom in on this motion, we
see that there are muscles inside of the

animal that are contracting as it is
running. And this muscle contraction takes

place within milliseconds. So that's a
part of a thousand in one second. But we

can go even smaller than that to the
microsecond. So proteins inside of the

muscles or in any biologic matter fold and
unfold on a timescale of microseconds.

That's already a part in a million of a
second. Now going even smaller, to

nanoseconds there's certain dynamics that
take place within these proteins, for

example, of how they dissolve in water.
But the timescale that I'm interested in

today is the femtosecond. It's even faster
than that it's the timescale where

individual atoms move in molecules as
shown in this animation. Now a

femtosecond is very short. It's a part in
a million of a billion of a second, or as

we physicists like to call it, ten to the
minus 15 seconds because it's easier to

spell laughing to us. We can - to us - , we can go even
faster than that. The time scale of

electronic motion and in molecules would
be an attosecond. I'm just mentioning it

here because we don't stop at molecules,
but nature is even faster than that. But

for the purpose of this talk, I will
mainly focus on processes that take place

within the femtosecond. So within ten to
the minus fifteen seconds. Now, this time

scale is something that is not really
related to what we think about in everyday

life. But there are certain processes in
chemistry, biology and physics that are

really fundamental and that start at this
time scale. Just to give you an idea how

short a femtosecond is, the width of a
human hair is about 100 micrometer. It's

shown here in an electron microscopic
picture. And for light at the speed of

light, it takes only thirty femtoseconds
to cross the hair. So that's how fast a

femtosecond is. And even although this
timescale is so short, there are many

important processes that start here, I'd
like to mention, just two of them. The

first one is vision in our eyes and our
retina there sits a molecule called

rhodopsin, that is shown here to the left.
And when light hits rhodopsin, it starts

to isomorphise, which is a fancy word for
saying it changes its shape. And this

transmits, in the end, electrical impulses
to our brain, which enables us to see. And

this very first step of vision takes only
two hundred femtoseconds to complete. But

without it, vision would not be possible.
Another very fast process that is

fundamental in nature is photosynthesis,
where plants take light and CO2 and

convert it to other things, among them
oxygen. And the very first excitation

where light hits the plant and it starts
to make all this energy available. That

also takes less than one hundredth
femtoseconds to complete. So really the

fundamental questions of life lie at this
timescale. And I'd like to just mention

that all of these processes are not only
very fast, but they also take place in

very small objects, that are of a size of
a few atoms to nanometers, which makes it

also hard to observe because we cannot see
them with the naked eye or with standard

microscopes. Now, we've seen already that
it's important to choose the right

exposure time to get a clear image of
something that's moving, but the kind of

method that we need for taking such a
photograph of something that is moving

depends a lot on the timescale. So for
stuff that is moving within seconds or

fractions of a second, we can see that
with the naked eye, we can use cameras to

resolve faster motion, very much like
Muybridge did with the very first camera.

Today, of course, we can go much faster to
maybe a few microseconds. With very fancy

cameras called opto-electronic street
cameras - i won't go into detail here - we

can go down to picoseconds. So we are
already very close to the motion of

molecules, but we are not quite there yet.
The timescale that we want to investigate

is a femtosecond. So really a time
timescale of molecular motion and

electronics are not fast enough to reach
this timescale. So we need something new.

And fortunately, we can create light
pulses that serve as to say flashes, but

take snapshots of our moving molecules
with femtosecond time resolution and light

pulses can be made so short. So in the
following, I'm going to show you a bit

more detail on how we can use these ultra
short light pulses to take snapshots of

moving molecules. The first method that I
would like to briefly show you is X-Ray

diffraction, where we have an ultra short
pulse, an X-Ray pulse coming in. It hits a

sample shown here in the red bubbles.
That's essentially a molecule that that we

just place in the beam and it produces a
so-called diffraction pattern that we can

then record on a screen. Now, the whole
process is quite complicated. So I like to

just sketch the very basics of it. We see
here X-Ray radiation hitting a crystalline

sample here to the left and the sample is
excited, starts to radiate X-Ray back and

on the right we can see the X-Rays leaving
the sample again. They will interfere and

we can record this pattern on the screen.
So this is what we see here in this

visualization to the right. With this, we
can feed a reconstructionalgorithm that

allows us to transform back our
diffraction pattern that we've seen here

for for in this case a bio molecule. We
can reconstruct from that the image as it

was in real space. So this is some
protein, I believe. X-ray diffraction is

very nice for resolving small structures
with atomic detail. Another method how we

can take snapshots using ultra short
pulses, that I would like to briefly

introduce is absorption spectroscopy. Now
you may know that light contains several

colors. For example, you've surely have
held a prism in hand, and the prism can

break white light up into all the colors
of a rainbow, that we can see with the

eye. Now we can do the same with X-Ray
pulses. Then we cannot see the colors

anymore. So just let's just stick with a
prism here. When we place a molecule in

front of all these colors, the molecule
will block certain colors. That's quantum

mechanics. You just have to believe it or
learn about it in long studies. So the

molecule is placed in front of all these
colors. And to be right, the absorption

spectrum is recorded and the parts of the
spectrum that are very bright correspond

to the colors that have been blocked by
the molecule. And this is a very nice

technique to investigate ultra short
dynamics, because where these lines are

located is characteristic of the chemical
elements that we find in the molecule. For

example, if we use X-Ray radiation for
this specific molecule, that I've shown

here lysine, that's not so important which
molecule it is. We have three different

atoms in this molecule that are important
carbon, nitrogen and oxygen and they

absorb at very different colors so we can
keep them apart when we take the spectrum.

But not only that, we can take the
spectrum at a later time when the molecule

has moved around a bit and we will see
that the colors, the position of the lines

have changed a tiny bit. So it's really
not much and I accelerated it already in

this visualization quite a bit. But with
experimental methods, we can resolve this.

And this allows us to then trace back to
how the molecule was moving in between

when we took these two snapshots. There
are many more methods that you can use to

take ultrafast images. So we call them
probe signals because we probe the

ultrafast motion of a molecule with such
an ultra short pulse. For example, we can

record photo electrons or we can record
fragments of a molecule and many more. But

I won't go into further detail here
because this is not an exhaustive list of

methods that we can use. I'd rather like
to show you how we can take molecular

movies so how we can combine all these
ultrashort pulses to in the end film a

molecule in action. Now we've already seen
in the movie of the horse that we need to

stitch several snapshots together and then
we have a full picture, full motion of a

molecule. So we just like to do the same,
but ten to the 15 times faster, should not

be too difficult, right? So we use our
ultra short pulse. First ultrasound parts

that we use as a trigger, parts that sets
off the motion and the molecule. This

defines us a certain time zero in our
experiment and makes it sort of repeatable

because we always start the same kind of
motion by giving it a small hit and now

it's just moving around. So we wait for a
certain time, a time delay and then come

in with a probe pulse. The probe pulse
takes a snapshot of a molecule. This goes

to some detector, goes to a kind of
complicated reconstruction method that we

just execute from our screen. And with
this, we reconstruct a snapshot of a

molecule. But this is only one snapshot
and we want a whole movie. So we need to

repeat this process over and over again by
shining and more and more probe pulses.

And this will create more and more
snapshots of a molecule. And in the end,

we could stitch all of these together and
we would arrive at the same image that you

see in the in the middle where the
molecules is happily moving around. There

is one little problem: The probe pulse
typically destroys the molecule. This is

very different. This is very different
from taking pictures of a horse. The horse

normally survives. laughting So the probe pulse
destroys the molecule. It just goes away.

So for each of these snapshots we need to
use a new molecule. So we typically have a

stream of samples that is falling from the
top to the bottom in our experiment. And

then we have to carefully align two pulses
a trigger pulse and a probe pulse that

come together and take a snapshot of this
molecule. And of course, we have to find a

method on how to make identical molecules
available in - Yeah - you see, there's a

lot of complications with doing these
experiments that I'm completely leaving

out here. So now we want to take a
molecular movie and we know that we want

to have ultra short pulses to do so. But I
didn't tell you yet what kind of light

source we need. So there are many light
sources all around us. We have here lights

from lamps. I have a light in my laser
pointer with light from the sun. But we

need quite specific light sources to take
these snapshots of molecular motion. We've

already established that we want
ultrashort pulses because else we cannot

resolve femtosecond dynamics, but for the
proper kind of wavelength that we need I

would like to quickly remind you of the
electromagnetic spectrum that you've

probably seen at some point in high
school. So, so light, as you see here in

the bottom picture is an electromagnetic
wave that comes in different wavelengths.

They can be quite long as in the case of
radio waves to the very left. Then we have

the region of visible light shown here as
the rainbow that we can perceive with our

eyes. And then we have wavelengths that
are too short to see with our eyes. First,

UV radiation, that gives us a tan in the
summer if we leave our house and then we

have X-ray radiation, soft and hard X-ray
radiation that have atomic wavelength. So

the wavelength is really on the order of
the size of an atom. So what kind of

wavelength do we need to study ultra short
dynamics - ultra fast dynamics? We can

first think about what kind of wavelength
we need when we want to construct an ultra

short pulse. I've drawn here two pulses to
the left, a slightly longer pulse to the

right, a shorter pulse. And now if you
think about squeezing the left parts

together such that it becomes shorter and
shorter, you see visually that the

wavelength also needs to shrink. So we
need shorter wavelengths for the shorter

the pulse we want to make. So this will be
located somewhere here in this region of

the electromagnetic spectrum. And another
important thing that we need to keep in

mind is if we want to take pictures by
X-ray diffraction, we are limited, so we

can only resolve structures that are about
the same size as the wavelength we used to

take our diffraction image. So if we want
to take a picture of something with atomic

resolution, our wavelength needs to be of
atomic size as well. And this places us in

the region of X-Rays drawn here, that have
a wavelength of less than a nanometer. So

we can establish that we want small
wavelengths in general. We have two

additional requirements that would just
touch upon very briefly. First, we need

very brilliant pulses because the pulses
are so short, we need to have a lot of

light in the short pulse. You can think
about taking a picture in a dark room with

a bad camera. You won't see anything. So
we need very bright flashes of light.

Another requirement is we need coherent
laser light. So we cannot just use any

light, but it needs to have certain
properties like laser light.

Unfortunately, the lasers that you can buy
commercially do not operate in the region

of the electromagnetic spectrum that we
are interested in. So we need to come up

with something new. And I will show you
how we can generate ultra short pulses

both in the laboratory where we can
generate pulses that are very short and

extend up to maybe the soft X-ray region.
And another method to generate ultra short

pulses is at free electron laser sources,
where we can go really to the hard X-Ray

regime. But first I'd like to go to the
laboratory. So in the laboratory, it's

possible to generate an ultrashort pulse by
using a process that's called high

harmonic generation. In high harmonic
generation we start off of a high

intensity pulse, that's a red pulse coming
in from the left, which which is focused

in a gas cell. And from there, it
generates new frequencies of light. So the

light that comes out is no longer red, but
it's violet, blue. We cannot see it with

the naked eye. So that's an artist's
impression of how high harmonic generation

works. Before going into more detail about
why this method is so good at producing

ultra short pulses, I'd like to mention
that this is only possible because we have

the high intensity driving pulses, the red
laser pulses available. This goes back to

work by Donna Strickland and Gerard
Mourou, who were awarded the Nobel Prize

in the year 2018 in physics for this work
that has been done in the 80s. Now we're

coming to the only equation of his talk,
which is this equation that relates the

energy width and the time duration of a
ultra short pulse. By the law of fourier

limits we cannot have pulses that are very
short in time and at the same time very

narrow in energy. But we need to choose
one. So if we want to have policies that

are very short in time like the pulse that
I've shown here on the bottom, that is

actually only two hundred fifty
attoseconds long, so even shorter than a

femtosecond, then we need to have a very
broad width in energy. And this means

combining a lot of different colors inside
of this pulse. And this is what makes high

harmonic generation so efficient at
creating ultra short pulses, because the

spectrum that the colors that come out of
high harmonic generation are shown here

and they really span a long width. So we
get a lot of different colors with about

the same intensity. And you can think of
it like putting them all back together

into one attosecond pulse. That is very
short in time. This method has really made

a big breakthrough in the generation of
ultra short laser pulses we see here a

plot of a time duration of laser pulses
versus the year, and we see that since the

invention of the laser, here in the mid
60s, there was a first technological

progress and shorter and shorter pulses
could be generated. But then in the 80s,

there was a limit that had been reached of
about five femtoseconds, I believe. And we

could not really go farther than that and
only with high harmonic generation, that

sets in here shortly before the year 2000,
we were able to generate pulses that are

of a femtosecond duration. So that really
touch the timescale of molecular motion.

The current world record is a pulse, that
is only 43 attoseconds long, established

in the year 2017. So that's really the
timescale of electrons and we can do all

sorts of nice experiments with it where we
directly observe electronic motion in

atoms and molecules. This is all very
nice, but it has one limitation: We cannot

go to hard X-rays, at least not right now.
So high harmonic generation cannot produce

the kind of very short wavelengths that we
need in order to to do X-ray diffraction

experiments with atomic resolution. So if
we want to have ultra short pulses that

have X-Ray wavelengths, we need to build
right now very complex, very big machines,

the so-called free electon lasers. Now,
this would be a specific light source that

can produce ultra short pulses with X-ray
wavelengths in itself. The X-Ray

wavelengths is not so new. We know how to
take X-ray images for about one hundred

and thirty years and already in the 50s.
Rosalind Franklin, who is looking at a

microscope here, was able to take a
picture of DNA, an X-ray diffraction

pattern of a DNA double helix that was
successful in revealing the double helix

structure of our genetic code. But this is
not a time resolved measurement. So think

of it as you have a molecule that is in
crystalline form, so it's not moving

around and we can just take an X-ray image
of it, it's not going anywhere. But if we

want - if we want to take a picture of
something that is moving, we need to have

very short pulses. But we still need the
same number of what we call photons, light

particles. Or think of it as we need more
brilliant X-ray flashes of light than we

could obtain before. And there was very
nice technological development in the past

50 years or so, where we were able to go
from the X-ray tube to newer light sources

called Synchrotron, and today, free
electro lasers that always increase the

peak brilliance in an exponential way. So
we can take really brilliant, really

bright X-ray flashes right now. I cannot
go into the details of all of that, but I

found a very nice talk from two years ago,
but actually explains everything from

Synchrotron to FELs still available online
if you're interested in this work. And as

always, if something is failing scaling
exponentially, most of you will be

familiar with Moore's Law, that tells us
about the exponential scaling of

transistors. If something grows this fast,
it really opens up a new series of

experiments of new technological
applications that no one has thought of

before. And the same is true with free
electron lasers. So I'm going to focus

just on the most brilliant light sources
for X-rays. Right now, the free electron

lasers that are at the top right here of
this graph have been around for maybe 10

years or so. I cannot go into a lot of
detail on how to generate ultrashort

pulses with X-Rays. So I'd like to
give you just a very broad picture of how

this works. First, we need a bunch of
electrons, that is accelerated to

relativistic speed. This sounds very easy,
but is actually part of a two kilometer

long accelerator, that we have to build
and maintain. Now we have this bunch here

of electrons shown in red and it's really
fast and now we can bring it into

something that is called an undulator.
That's a series of alternating magnets,

shown here in green on blue for the
alternating magnets. And you may remember,

that when we put an electron, that is as a
charged particle, into a magnetic field,

the Lorence force will drive it away. And
if you have alternating magnets, then the

electron will go on a sort of wiggly path
in this undulator. And the electron is a

charged particle as it is wiggling around
wherever it turns around, it will emit

radiation, that happens to be in the X-ray
region of the electromagnetic spectrum,

which is exactly what we want. We can
watch this little movie here to see a

better picture. So this is the undulating
seeing from the side. We now go inside of

the undulator. We have a series of
alternating magnets. Now the electron

bunch shows up and you see the wiggly
motion as it passes the different magnets.

And you see the bright X-Ray flash that is
formed and gets stronger and stronger as

the electron bunch passes the undulator. So
we need several of these magnet pairs to

in the end, get the very bright X-Ray
flash. And at the end of the undulator we

dump the electron, we don't really need
this electron bunch anymore and continue

with a very bright X-Ray flash. This whole
process is a bit stochastic in nature, but

it's amplifying itself in because of the
undulator. This is why the longer the

undulator is, the more bright X-Ray
flashes we can generate. This whole thing

is kind of complicated to build, it's a
very complex machine. So right now there

are only very few free electron lasers in
the world. First one in California called

LCLS 1, currently being upgraded to LCLS
2. There are several in Europe. There's

one in Switzerland, in Italy and Hamburg.
So there's a Flash that does not operate

in the hot X-ray regime, but was kind of
first free electron laser. That's the most

recent addition to the free electron laser
zoo. It's the European XFEL also located

in Hamburg. And then we have some of these
light sources in Asia, in Korea, South

Korea, Japan, and one currently under
construction in Shanghai. I'd like to show

you a bit more details about the European
X-ray free electron laser, because it's

closest to us, and at least closest to
where I work. So the European XFEL is a

three point four kilometer long machine
that is funded by in total 12 countries,

So Germany and Russia paying the most and
then the other 10 countries also providing

to the construction and maintenance costs.
This machine starts at the DESY campus,

but as shown here to the right of the
picture. And then we have first an

accelerator line for the electrons that
it's already one point seven kilometers

long and where we add electrons reach
their relativistic speed. Then the

undulate comes in, so the range of magnets
where we X-Ray flashes are produced. The x

X-Ray flashes then cross the border to
Schleswig-Holstein, laughter shown here, on the

other side in a new federal state. They
reach the experimental hall. We have in

total six experimental end stations at the
European XFEL that provide different

instrumentation, depending on which kind
of system you want to study, you need

slightly different instruments. And it's
not only for taking molecular movies, but

the XFEL is used, among others, for
material science, for the imaging of bio

molecules, for femtosecond chemistry, all
sorts of things. So really wide range of

applications. It's right now the the
fastest such light source can take twenty

seven thousand flashes per second, which
is great because every flash is one

picture. So if we want to take a lot of
snapshots, if you want to generate a lot

of data in a short time, it's great to
have as many flashes per second as

possible. And as you can imagine, it's
kind of expensive since there are so few

free electron lasers in the world to take
measurements there. The complete price tag

for constructing this machine, it took
eight years and cost one point two billion

euros, which may seem a lot, but it's the
same amount that we spend on concert halls

in Hamburg. loud laughter applause So kind of comparable. Now,
when you factor in maintenance and so on,

I think a minute of X-Ray beam at such an
XFEL cost several thousands of tens of

thousands of euros in the end. So getting
measurement time is complicated and there

are committees that select the most
fruitful approaches and so on. So in order

to not to waste or do taxpayers money.
With this, I'd like to make a small

comparison of the light sources that I've
introduced now. So I introduced the

laboratory light sources and the XFEL
light source. In general, in the

laboratory we can generate very short
pulses of less than 100 attoseconds by now

and in the XFEL we are limited to
something about 10 femtoseconds right now.

In terms of brilliance the XFELs can go to
much more bright pulses, simply because

they are bigger machines and high harmonic
generation in itself is a kind of

inefficient process. In terms of
wavelength X-Ray free electron lasers

enable us to reach these very short
wavelengths with X-Rays, that we need to

get atomic resolution of defractive
images. In the laboratory we are a bit

more limited to maybe the soft X-ray
region. There's another important thing to

keep in mind when we do experiments,
that's the control of pulse parameters. So

is every pulse that comes out of my
machine the same as the one that came out

of my machine before. And since the XFEL
produces pulses by what is in the end, a

stochastic process, that's not really the
case. So the control of possible

parameters is not really given. This is
much better in the laboratory. And in

terms of cost and availability, it would
of course, be nice if we could do more

experiments in the lab. Then at the XFEL
simply because we XFEL ls so expensive to

build and maintain and we have so few of
them in the world. And you can see this

tunnel here. It stretches for two
kilometers or so, all packed with very

expensive equipment. So I'd like to show
you a brief example of what we can learn

in ultrafast science. So this is a
theoretical work that we did in our group.

So no experimental data, but still nice to
see. This is concerned with an organic

solar cell. So we all know solar cells.
They convert sunlight to electric energy

that we can use in our devices. The nice
thing about organic solar cells is that

they are foldable, very lightweight, and
we can produce them cheaply. The way that

such a solar cell works is we have light
shining in and at the bottom of the solar

cell there sits an electrode that collects
all the charges and creates an electric

current. Now light creates a charge that
somehow needs to travel down there to this

electrode and in fact, many of these
charges. So the important thing where we

build such an organic solar cell is that
we need a way to efficiently transport

these charges. And we can do so by putting
polymers inside. A polymer is just a

molecule that is made up of two different
or two or more different smaller

molecules. And one such polymer, which
should be very efficient at transporting

these charges is BT-1T, that is shown here
of a name is not so important, it's an

abbreviation. Because in BT-1T when we
create a charge at one end of a molecule

here at the top, it travels very quickly
to the other side of a molecule and you

can imagine stacking several of these
BT-1T or especially of the Ts together,

putting it in this material. And then we
have a very efficient flow of energy in

our organic solar cell. So what we did was
we calculated the ultrafast charge

migration in BT-1T, shown here to the
right. The pink thing is the charge

density that was created by an initial
ionization of the molecule. And now I show

you the movie, how this charge is moving
around in a molecule so you can see

individual atoms moving, the whole
molecules vibrating a bit. And the charge,

if you look closely, is locating on the
right half of a molecule within about 250

femtoseconds. Now, we cannot observe this
charge migration directly by looking at

this pink charge density that I've drawn
here, because it's at least for us, not

experimentally observable directly. So we
need an indirect measurement, an X-Ray

absorption spectroscopy that I showed you
in the beginning could be such a

measurement. Because in the X-Ray
absorption spectrum of BT-1T that I've

shown here in the bottom left, we see
distinct peaks depending on where the

charge is located. Initially the charge is
located at the top sulfur atom here and

this molecule and we will see a peek at
this color. Once the charge moves away to

the bottom of a molecule to the other
half, we will see a peak at the place

where nothing is right now because the
charge is not there. But if I start this

movie, we will again see very fast charge
transfer. So within about two hundred

femtoseconds, the charge goes from one end
to the molecule to the other end of a

molecule. And it would be really nice to
see this in action in the future XFEL

experiment. But the process is very long.
You need to apply for time at an XFEL. You

need to evaluate all the data. So maybe a
couple of years from now we will have the

data available. Right now we are stuck
with this movie, that we calculated. Now,

towards the end of my talk, I'd like to go
beyond the molecular movie. So I've shown

you now how to generate the light pulses
and an example of what we can study with

these light pulses. But this is not all we
can do: So when you think of a chemical

reaction, you might remember high school
chemistry or something like this, which is

always foaming and exploding and nobody
really knows what is going on. So a

chemical reaction quite naturally involves
molecular dynamics, for example, the

decomposition of a molecule to go from
here, from the left side to the right

side, the molecules will somehow need to
rearrange so all the atoms will have moved

quite a bit. We've seen already how we can
trigger these chemical reactions or these

molecular motion that was part of a
molecular movie. But it would be really

cool if we could control the reaction with
light. So the way to do this, it's not

currently something that is possible, but
maybe in the near future, would be to

implement a sort of optimisation feedback
loop. So we would record the fragments of

our reaction, send it to an optimization
routine that will also be quite

complicated and will need to take into
account the whole theory of how light and

matter, interact and so on. And this
optimization routine would then generate a

new sequence of ultra short pulses and
with this feedback loop, it might be

possible to find the right pulses to
control chemical reactions, taking into

account the quantum nature of this motion
and so on. Right now this is not possible.

First, because the whole process of how we
can generate these ultra short pulses is

not so well controlled that we could
actually implement it in such a loop. And

also the step optimization routine is more
complex than it looks like here in this

picture. So this is something that people
are working on at the moment, but this

would be something like the ultra fast
wishlist for next Christmas, not this

Christmas. So we've succeeded in taking a
molecular movie, but we would also like to

be able to direct a molecular movie. So to
go beyond just watching nature, but

controlling nature because this is what
humans like to best, laughing fortunately or

unfortunately, it depends. So I'd like to
just show you that this is really an ultra

fast developing field. There's lots of new
research papers every day, every week

coming in, studying all sorts of systems.
When you just take a quick and dirty

metric of how important ultrafast science
is this is the number of articles per year

that mentioned ultrafast in Google
Scholar, it's exponentially growing. At

the same time, the number of total
publications in Google Scholar is more or

less constant, so the blue line here
outgrows the green line considerably since

about ten years. So what remains to be
done? We've seen that we have light

sources available to generate ultra short
pulses, but as always, when you have

better machines, bigger machines, you can
take more fancy experiments. So it would

be really nice to develop both lab based
sources and free electron laser sources so

that we can take more, more interesting,
more complex experiments. Another

important challenge, that's what people in
my research group where I work are working

on is to improve theoretical calculations
because I did not go into a lot of detail

on how to calculate these things, but it's
essentially quantum mechanics and quantum

mechanics skales very unfavorably. So
going from a very small molecule like the

glycene molecule here to something like a
protein is not doable. Simply, it cannot

compute this with quantum mechanics. So we
need all sorts of new methodology to -

Yeah - to better describe larger systems.
We would in general like to study not only

small molecules and not only take movies
of small molecules, but really study large

systems like this is the FMO complex, that
is a central in photosynthesis or solid

states that are here shown in this crystal
structure simply because this is more

interesting for biological, chemical
applications. And finally, as I've shown

you, it would be cool to directly control
chemical reactions with light. So to find

a way how to replace this mess with a
clean light pulse. With this, I'm at the

end. I'd like to quickly summarize
femtodynamics, really fundamental in

biology, chemistry and physics. So more or
less, the origin of life is on this

timescale. We can take molecular movies
with ultrashort laser pulses and we can

generate these pulses in the laboratory or
add free electron lasers with different

characteristics. And we would like to not
only understand these ultra fast

phenomena, but we would also like to be
able to control them in the future. With

this, I'd like to thank you for your
attention and thank the supporting

institutions here that funded my PhD work.

applause

Herald: Well, that was an interesting
talk. I enjoyed it very much. I guess this

will spark some questions. If you want to
ask Caroline a question, please line up

behind the microphones. We have three in
the isles between the seats if you want to

leave, please do so in the door here in
the front. And until we get questions from

the audience, do we have questions from
the Internet?

Signal angel: Yes. Big fat random user is
curious about the design of the X-Ray

detector. Do you have any information on
that?

Caroline: That's also very complex. I'm
not a big expert in detectors. At this

point, I really recommend watching the
talk from two years ago, that explains a

lot more about the X-Ray detectors. So
what I know about the X-Ray detector is

that it's very complicated to process all
the data because when you have 27000

flashes of light, it produces, I think
terabytes of data within seconds and you

need to somehow be able to store them and
analyze them. So there is also a lot of

technology involved in the design of these
detectors.

Herald: Thank you. So the first question
from microphone 2 in the middle.

Microphone 2: So my question is.
Herald: Please go close to the microphone.

Microphone 2: My question is regarding the
synchronization of the detector units when

you're pointing to free electron laser so
you can achieve this in synchronization.

Caroline: This is also very complicated.
It's easier to do in the lab. So you're

talking about the synchronization of
essentially the first pulse and the second

pulse. Right. So in the lab, you typically
generate the second pulse from part of the

first pulse. So you have a very natural
alignment, at least in time of these two

pulses. The X-ray free electron lasers
have special timing tools that allow you

to find out how much is the time delay
between your two pulses. But it's true

that this is complicated to achieve and
this limits the experimental time

resolution to something that is even
larger than the time duration of the

pulses.
Herald: So now next question from

microphone number 3.
Microphone 3: Yes, i remember in the

beginning, you explained that your
measuring method usually destroys your

molecules. That's a bit of a contradiction
to your idea to control. laughing

Caroline: In principle, yes. But, so in
the case of control, we would like to use

a second pulse, that does not destroy the
molecule. But for example, at least

destroys it in a controlled way, for
example. loud laughter So there's a difference between

just blowing up your molecule and breaking
apart a certain part, but yet that we are

interested in. And that's what we would
like to do in the control case. So we

would like to be able to to control, for
example, the fragmentation of a molecule

such that we only get the important part
out and everything else just goes away.

Microphone3: Thank you.
Herald: So then another question for

microphone 2 in the middle.
Microphone 2: So thank you for the talk. I

was interested in how large structures or
molecules can you imagine with this lab

contributions and with this XFEL thing?
Caroline: Sorry. Can you repeat?

Microphone 2: So how large molecules can
you imagine in this laboratory with this

high harmonic measures?
Caroline: So how large is not really the

fundamental problem? People have taken
snapshots of viruses or bigger bio

molecules. If you want to - the problem is
rather how small can we get? So yeah, to

take pictures of a very small molecule.
Currently we cannot take a picture of an

individual small molecule, but what people
do is they create crystals of a small

molecule, sticking several of them
together and then taking images of this

whole crystal for single particles, I
think right now about the scale of a virus

nanometers.
Microphone 2: Thank you.

Herald: OK. Do we have another question
from the Internet?

Signal Angel: We have. So this is
concerning your permanent destruction of

forces, I guess. How do you isolate single
atoms and molecules for analysing between

the different exposures?
Caroline: Yes, excellent question. So

molecules can be made available in the gas
phase by - so if you have them in a solid

somewhere and you heat that up, they will
evaporate from that surface. This is how

you can get them in the gas phase. This,
of course, assumes that you have a

molecule, that is actually stable in the
gas space, which is not true for all

molecules. And then the, the hard thing is
to align all three things. So the pump

pulse, the probe pulse and the molecule
all need to be there at the same time.

There are people doing whole PhD theses on
how to design gas nozzles that can provide

this stream of molecules.
Signal Angel: So you basically really

having a stream coming from a nozzle?
Caroline: Yes.

Signal Angel: It's a very thin stream, I
guess.

Caroline: Yes.
Signal Angel: Then you're exposing it like

in a regular interval.
Caroline: And of course you try to hit as

many molecules as possible. So this is
especially important when you do pictures

of crystallized molecules because
crystallizing these molecules is a lot of

work. You don't want to waste like 99
percent that just fall away and you never

take snapshots of them.
Signal Angel: Thanks.

Herald: So another question from
microphone 3.

Microphone 3: How do you construct this
movie? I mean, for every pulse to have a

new molecule and for every molecule is
oriented differently in space and has

different oscillation modes. How do
correlate them? I mean, in the movie, I

mean, every molecule is different than the
previous one.

Caroline: Yes. Excellent question. That's,
So first, what people can do is align

molecules. So especially molecules that
are more or less linear. You can force

them to be oriented in a certain way. And
then there's also a bit of a secret in the

trigger pulse that first sets off this
motion. For example, if this trigger pulse

is a very strong proto ionization, then
this will kill off any sorts of

vibrational states that you have had
before in the molecule. So in this sense,

the trigger parts really defines a time
zero, that should be reproducible for any

molecule that shows up in the stream and
the rest is statistics.

Microphone 3: Thank you.
Herald: So there's another question on

microphone 3.
Microphone 3: Are there any pre pulses or

ghosts, You need to get rid of?
Caroline: Sorry. Again.

Microphone 3: You have to control pre
pulses or ghosts during this effect for

measurement.
Caroline: That I'm not really sure of,

since I'm not really conducting
experiments, but probably.

Herald: And another one from the middle,
from microphone 2 please.

Microphone 2: I suppose if you apply for
experimentation time at the XFEL laser,

you have to submit very detailed plans and
time lines and everything. And you will

get the time window for your experiment, I
guess. what's going to happen if you're

not completely finished within that time
window? Are they easy possibilities to

extend the time or are they do they just
say, well, you had your three weeks,

you're out apply in 2026?
Caroline: Yeah, I think it's a regular

case, that you're not finished with your
experiments by the time your beam time

ends. That's how it usually goes. It's
also unfortunatly not free weeks, but it's

rather like 60 hours delivered in five
shifts of twelve hours. So, yeah, you

write a very detailed proposal of what you
would like to do. Submit it to a panel of

experts, both scientists and technicians.
So they decide, is it interesting enough

from a scientific point of view and is it
feasible from a technical point of view?

And then once you are there, you more or
less set up your experiment and do as much

as you can. If you want to come back, you
need to submit an additional proposal. So,

yeah, I think most experimental groups try
to have several of these proposals running

at the same time, so that there is not a
two year delay between your data

acquisition. But yes. No possibility to
extend. It's booked already for the

complete next year. The schedule is fixed.
Herald: So I don't see any more people

queuing up. If you want to pose a
question, please do so now. In the

meantime, I would ask the signal angel if
there's another question from the

Internet.
Signal Angel: I have a question about the

dimensions of all those machines. The
undulator seems to be rather long and

contain a lot of magnets. Do you have an
idea how long it is and how many of those

electromagnets are in there?
Caroline: Yeah. Sorry, I didn't mention

it. It's about, I think one hundred and
seventy meters long in the case of the

European XFEL. I'm not sure about the
dimension of the individual magnets, but

it's probably also in the hundreds of
magnets, magnet pairs.

Herald: So is there more - excuse me,
there is a question on the microphone

number 3.
Microphone 3: Yeah. Hi. It's regarding the

harmonic light.
Herald: Please go closer to the

microphone.
Microphone 3: The harmonic light generator

that you were showing at the very
beginning, just before the one that won

the Nobel Prize. And can you also produce
light in the visible range? Or it has to

be in the visible range?
Caroline: The high harmonic generation? So

in the in the visible range, you cannot
create pulses that are so short that they

would be interesting for what I'm doing.
The pulse that comes in is already quite

short. So it's already femtoseconds long.
They just convert it into something that

is fractions of a femtosecond long. And
yeah. In the indivisible range that's kind

of a limit how short your pulse can be.
Microphone 3: So it is not a good

candidate for hyperspectral light source.
We need another kind of technique, I

guess.
Caroline: Well, I mean you are kind of

limited what short pulses you can generate
with which wavelength.

Microphone 3: Thank you.
Herald: So again, a question to the Signal

Angel. Are there more questions from the
internet?

Signal Angel: Yes I have another one about
the lifetime of the molecules in the beam?

How fast are they degrading or how fast
are they destructed?

Caroline: So probably the question is
about how fast they are destructed before

- so either before our pulses hit the
molecule the molecules should be stable

enough to survive in the gas phase from
the point where they are evaporated until

the point where the pump and the probe
pulse come together, because otherwise it

doesn't make sense to study this molecule
in the gas phase. When the probe pulse

hits and it flows apart, I guess pico
seconds until the whole molecules ...

Microphone 3: Like instantaneous.
Herald: So I don't see any more questions

on the microphones and we have a few
minutes left. So if there are more

questions from the Internet, we can take
maybe one or two more.

Signal Angel: Give me a second.
Herald: For people leaving already, please

look if you have taken trash and bottles
with you.

Signal Angel: So this one very, very
technical question. How do you compensate

the electronic signal that the electronic
signal reaction is probably slower than x

ray or light or spectrum changes at one
moment or at one particular moment, that

was interesting to analyze. I do not
understand the question, though.

Caroline: I think I understand the
question, but I don't have the answer

because again, I'm a - that's the problem
of speaking to a technical audience, you

get a lot of these very technical
question. Yeah. The data analysis is not

instantaneous. So the data is transported
somewhere safe in my imagination. And then

taken from there. So this data analysis
does not have to take place on the same

timescale as the data acquisition, which I
guess is also because of the problem that

was mentioned in the question.
Herald: I might interrupt here. Maybe it's

also about the signal transmission, like
the signal rising of the signal of the

electrical signal transmissions. Because
this would probably require bandwidths of

several megahertz, gigahertz, I don't
know, to transport these very fast

results.
Caroline: Yeah. I think that's also a

problem of constructing the right
detector. That has been solved apparently

because they can take these images. laughter
Herald: And on the other hand, we have 10

gigabit either nets. So we get faster and
faster electronics. More questions from

the Internet? Does not look like it, also
the time is running out. So let's thank

Caroline for her marvelous talk. Applause, final music

subtitles created by c3subtitles.de
in the year 2019. Join, and help us!