00:11
Hello and welcome to 英語でサイエンスしナイト. I'm Asami, your host.
And today we're missing Masako. She has a real job and I am currently fun-employed.
You know, fun plus unemployed. Fun-employed. Yeah. So I have all the time in the world and
she doesn't, so I'm doing this episode by myself. And the reason why I wanted to record this episode
is because, yes, it's that time of the month.
六月の科学系ポッドキャストの企画は町内細菌相談室の鈴木大輔さんがホストしてくださっています。
テーマは観測ということで。 Observation is the theme of the month.
And I gave some thoughts about, you know, what observation means to me. What can I talk about it?
Because, yeah, as a scientist, we do this every day, but we don't really, you know, deeply think
about these things. So this was a great topic and thank you, Daisuke-san, for suggesting this topic.
So I figured why not, why don't I share a thing or two about what I think of observation
specifically to my field since I'm solo recording today. So let's go. So in the past, I may have
mentioned in the past episodes that I am in a field of ultra-fast molecular dynamics.
And now I know it sounds kind of fancy and perhaps scary or intimidating, but let me break
it down so that you'll realize that this is just kind of a fancy way to describe a very simple
concept, although the experiment is very far from simple. But let's start from, you know,
in chemistry, which is sort of the larger umbrella field, right? Like ultra-fast molecular
dynamics belongs to the chemistry field. And so in chemistry, I think we can categorize
in a few ways what we're trying to observe, depending on the purpose of our observation,
like, you know, the why-why of observation. So why do we want to observe things?
So I came up with four categories. I don't think that encompasses everything, but,
you know, I came up with this in 30 seconds, so bear with me. So number one, in chemistry,
03:04
we typically want to characterize and identify the structure of the molecules or some kind of
signatures of the molecule, because, you know, you make a new molecule or you come up with a
new ways to make a molecule, and you want to make sure you want to know what you made at the end of
this process, right? So that's why we sort of do some kind of observation. Number two is a category
where you can identify, you want to identify something that is very low in quantity,
and you need some special observation tools for that. This might be the case of trace detection,
so you know that the molecule or something of an interest is in a mixture of this thing, but
you know that it exists in a very, very small quantity. So you're trying to look for the signal
of that molecule in a tiny, tiny signal in a very large sample size. So that's a challenge,
a different kind of challenge in observation. Number three, you're trying to capture something
that has very small probability of occurrence. So you're trying to capture an event that happens
very rarely, a reaction that usually doesn't happen, or some kind of chemical process that
doesn't happen unless it is specifically triggered in a certain way. So those are kind of rare events
category that I think about, and that's another difficult challenge, right? You have, you do some
kind of experiment, you know the probability of it happening by quantum mechanics, right? Not
because you suck at experiment, but by laws of nature, it happens so infrequently, and that's why
the observation is difficult. So that's category three. Then category four is you're trying to
capture something that has a very short lifetime. So it happens, but it happens so quickly,
so quickly in fact, that it's barely conceivable by human eyes, or sometimes your observation
tool's eyes. So those are sort of the four categories. And ultrafast molecular dynamics
kind of combines the last category with any of the one of the three that I mentioned earlier.
So we're in a business of trying to look at things that happens very quickly,
and that thing that we're trying to look for are very, very small, like atomic scales,
06:04
movement, or change in structure that is very, very subtle. In our field we have been able to
see things like ring opening reaction, you know cyclohexadiene opening up to hexatriene,
or isomerization reaction, and anything that has structural change associated
with a lifetime of less than one nanosecond can be sort of loosely defined as a part of
ultrafast molecular dynamics. But so it's really only recently, like what's fascinating about this
field is that it's really only recently that we have been able to observe ultrafast phenomena at
all. In order to observe the ultrafast phenomena, we needed to have the ultrafast
tools to be able to capture that moment and have a technique that lets you do that.
And we have had mainly two big hurdles to overcome in my field, ultrafast molecular dynamics. So
the two hurdles are temporal resolution challenge and the spatial resolution challenge.
So for the rest of this episode, I'll talk about the temporal resolution challenge.
Wow, that Japanese sucked. Bear with me once again. Anyways, so we wanted to see something
that happens very, very fast. So in order to do that, we needed something that can capture
the molecule at the fast speed, fast rate, one can say. And the technique that we rely on
is ultrafast laser technology. We could not do our research without this. So briefly,
I wanted to sort of go over the mini history of ultrafast laser technology development.
So I think it really started in the 1950s, 60s, when people started to be interested in
not just the beginning product, the starting product and the end product of the chemistry,
but people started to ask questions like, how does chemistry take place? And coincidentally,
we had ultrafast laser technology developing at the same time. So in the 1960s,
09:03
the development of mode-locking technique enabled production of stable and precise,
but also regularly spaced pulse trains. And this is how most pulse laser today is being seeded.
And this, yeah, without going too, too technical, I don't want to go too technical about it,
but it's basically a clock of the ultrafast lasers. So a clock needs to tick and talk
at a regular interval, and you don't want a clock that ticks sometimes at one second,
but some other times at three seconds. You want a clock that ticks and talks at every second.
And the precision of that one second is basically determined by mode-locked laser.
And you can imagine that the timescale of interest you're trying to go for is
tens of femtoseconds. And by the way, femtoseconds is
one second is equal to 10 to the negative 15 femtoseconds. So wait, that's the other way around.
One femtosecond is equal to 10 to the negative 15 seconds. So that is
very, very, very, very fast. It's all you need to know. Anyway, so we wanted a really precise clock.
In 1960s, we managed to get that, but that's not the only thing we needed, right? A major breakthrough
in ultrafast laser technology came in the 1980s with the development of this technique called
chirped pulse amplification. So basically in this technique, it lets you generate a very, very short,
ultra short pulses by first stretching the pulse in time domain,
amplify that stretched out pulse in the series of amplification. There's many different ways
to amplify a pulse. You could use regen amplifier or single pass amplifier. There are many ways,
but we don't need to get into that. But anyway, we stretch, we amplify, and then we compress at
the very end of the process, which lets you have a high intensity, high energy, ultra short pulses.
And this technology is simple in conception, very, very non-trivial in the physics of it all.
And this technology was pioneered by Gerard Moreau and Donna Strickland, who,
by the way, Donna Strickland probably deserves a whole last episode on her own because
12:00
she not only was only a grad student when she worked on this project with Moreau,
you know, her Nobel prize winning project was in her grad school days, which is bonkers to think
about. This Nobel prize was recognized in 1985, but she was only the third woman to be awarded
the Nobel prize in physics. Any guesses on who were the first and the second?
Um, well, first one is Marie Curie in 1903. Second one is Maria Goppermeier in 1963. So,
60 years after the first one, second one came through. And it thankfully didn't take that long,
but still took a long time, 1985, to get to the third person. That just, you know, in today's
standard, pretty insane, but it just goes to show how far we've come, you know, women in science,
very proud of it. But that's another episode on their own. So, I digress.
Anyway, so, all right, where are we? We figured out how to make intense, high intensity,
super short pulses. All right, now what, right? And I'll tell you now what. So, during 1980s and
1990s, so, you know, we're coming pretty close to the, you know, today's time. But we started to,
we discovered this thing called titanium-doped sapphire. And that became sort of like a
workhorse of ultrafast lasers. So, this titanium-doped sapphire, we typically in the field
call Ti-sapphire laser. So, what is this titanium-doped sapphire doing in the laser, right?
So, it's, without, again, getting too technical, this is a part of the laser that's called gain medium.
And that's where the lasing of the laser takes place. By the way, did you know that the word
laser is an acronym? No.
Anyway, again, so, well, it's basically just a very, very important part of the laser.
And depending on what gain mediums you use, you get access to different energy or different
wavelength, probably more importantly. And Ti-sapphire just lets you have a whole options
15:03
of tunability. And thanks to the pump sources that became, you know, way more efficient and
reliable like diode laser at this time around, you know, Ti-sapphire laser became a household,
or I should say, lab hold staple instrument. And in my lab in grad school, I also had a
Ti-sapphire laser. The Ti-sapphire itself, it's this beautiful pink crystal. And it's kind of
fun to see. Yeah, you can see that the doping makes the sapphire pink. So that's interesting.
Anyway, and lastly, right, so we're back where we're up to 1990s. You know, in like the 90s to
2000s and onwards, the advancement of nonlinear optics and pulse shaping techniques in the late
20th century also enhanced the ultrafast laser specs. So I again, don't want to go too, too
technical. But basically, what that means is that now we can have very fast pulse in different
wavelengths, have a huge wavelength accessibility now thanks to techniques like parametric
amplifications. So, you know, lasers today, you can have access from, you know, deep UV,
like 195 nanometers to IR, like 1500 nanometers easily from just one Ti-sapphire laser. So that's
really useful and helpful for chemistry experiments, because you want to, you know,
molecules have some absorption cross section, and you want to make sure if you're hitting it
with a laser that the laser's photon is properly absorbed by the molecule. So typically, you want
to target the wavelength, and that's very helpful for this experiment. And yeah, and what did I say?
So different wavelengths, and we can also have precise sort of manipulation of temporal and
energy profile of the laser pulse as well. And this, again, was also very important for
ultrafast molecular dynamics. And more recently, we have entered the era of fiber-based ultrafast
lasers. So none of the other complicated stuff beforehand, you just seed the light through a
fiber. So it's a hikari fiber de yaru reiza no koto nandakedo. This is, again, sort of like still
18:01
a developing field, although the portability and stability and amazing tunability, you know,
from deep, deep UV to longer wavelengths have just became, you know, extremely helpful. And
I think for the future, this is sort of the direction we're going for,
rather than going through the whole, you know, parametric amplifications and other many,
many steps that requires for the pulse to be generated.
Okay. Yeah. So that is a lot. I think this is the most technical episode yet.
But so that's the story of the temporal challenge, why we wanted to go for
ultra-short pulses in order to look at ultrafast molecular dynamics.
That's it for the show today. Thanks for listening and find us at EigoDeScience on
Twitter. That is E-I-G-O-D-E-S-C-I-E-N-C-E. See you next time.
