Attosecond Lasers (2023 Nobel Prize in Physics) - Sixty Symbols

Yeah, I'm a bit rushed. I've only got a few billions of billions of attoseconds to talk to you. A safety officer, Peter Milligan, shouted across, oh, attosecond physics for the Nobel Prize this year. And what went through your head? Well, I remembered a lab that I visited at the NRC in Ottawa about 20 years ago, a person called Paul Corcombe. And at the time I was working on quantum dots at the NRC, and there was a buzz around Paul Corcombe's lab. And I knew that this was serious work going on. And so when I heard the Nobel Prize had gone to attosecond physics, I thought, Paul Corcombe, it's got to be Paul Corcombe. But it wasn't. So it went to Anne Huillier, Frank Krauss and Pierre Agostino. So it's for probably the shortest pulse of light that we can get. And in some ways it's like the ultimate shutter speed on a camera. So when we take pictures of things that are moving rapidly, you need to have a fast shutter speed. If you've ever gone out and tried to take a picture of a hummingbird, right, the hummingbird's wings beat at 80 per second or something. So if you were to set your shutter speed at, say, a second, so it would have fluttered 80 times by then, you'd just see a blurred image. So you've got to set your shutter speed maybe a thousandth of a second to make sure you can isolate this. And what these people have done is they've developed the technology and built the experiments which allow you to actually probe the movement of electrons in an atom. So maybe it's worthwhile seeing why that needs to be in what's called an attosecond. An attosecond is a billionth of a billionth of a second. So that's 10 to the minus 18 seconds. So there are 10 to the 18 attoseconds in a second. There are more attoseconds in a second than there are seconds in the age of the universe. That's, okay, I saw your head go, whoa. And the universe is 13.8 billion years old. So that is about 10 to the 17 seconds. But there are 10 to the 18 attoseconds in a second. So they're making a pulse of light that lasts that long. It lasts about, I think the current rate is about 200 attoseconds. No, sorry, it's down to a few dozen attoseconds, 20 attoseconds or so. Yeah, that's what a current pulse of light can be. But they're not shining that light onto an electron and seeing it like the wings of a hummingbird. Well, they are able to look at the, it's not an individual electron. I believe it's, they can't be looking at an object and saying the core of that object is the electron. Because we've got an uncertainty principle, right, which tells us the closer you get to pin down the position of an electron, the less you know about its velocity, for example. So what they're looking at is the wave, the wave function which you associate with the electron, where it's maximized. And they can look at that at different moments. And in 20 attoseconds the electron can have gone from one side of an atom to another side of an atom as it orbits, right. Electrons orbit atoms. And these can probe that. So they can actually sort of see where it's maximized here and then 20 attoseconds later see where it's over here. So when we think of an atom, we think of something that's about an angstrom, right, 10 to the minus 10 metres. And we're using light to probe it. And light is moving at a speed of 10 to the 8 metres a second. So every second a photon of light goes 100 million metres. So in 10 to the minus 18 seconds it goes one angstrom. You can go from that to that. So 10 to the minus 18 for light corresponds to an angstrom. So it's the typical time scale for an electron to go around. So that is the motivation for why attoseconds is a scale to go for. But people didn't think it was possible because we've had lasers for a long time. And these are laser pulses that are used. And lasers, it was thought, were the maximum time scale that you could get for a pulse was 10 to the minus 15 seconds. That's called a femtosecond. That corresponds to the frequency of light that you produce with lasers, you know, visible light. Or even if you go into the ultraviolet, you can't go much above a femtosecond. But that's still a thousand times bigger than an attosecond. So the electron would have gone whizzing around a thousand times. Like our wings on the hummingbird. It's like our wings on the hummingbird. So it was regarded up until the 1980s that this was, that was going to be the best you could achieve. And then a group of people, including our Nobel Prize winners, they started asking is that, can we do better? And they made use of the following, that it's possible to add waves of light together. And these waves can have different wavelengths and they can have different amplitudes. And you can add them together so that they will, individually they won't, they'll be too big or they'll be too long. But added together, they can actually form a profile of the right size and last the right length of time. So in principle, there was this process which is just adding waves together. We do it in mathematics all the time. It's called Fourier series. We add waves together to create new waves of different frequencies and different amplitudes. The big question was how are you going to do it in a physical context? And this was a woman from the UK, I think she's called. She realised that there was a potential way to do it. And what she did was, she fired infrared light into a gas of, in fact it was neon. Now let's just go into the atom of the gas for a moment. We've got an atom, we've got an electron sitting there happily. It's a rare gas, it's very relaxed, not doing much. It's trapped by the electric field of the rest of the atom pulling it in. And then in comes this wave and it hits the atom. It excites the electron and a photon of light has an electric field and a magnetic field. And it's propagating along. The electric field of this light hits the electron. If it's got enough intensity to it, the electron tunnels out of the atom and now it's free. Okay, so the electron disappears off. And you think, well, okay, it's going to go and leave me with an ion behind. But what actually happens is the electric field of this photon is changing all the time. It's oscillating backwards and forwards, up and down. Its direction is changing. And an electric field is a force. So as it passed the atom initially, let's say it's pointing in that direction, it's forcing the electron in that direction. As it goes by, it switches direction. So the down bit of the wave comes back past the electron and it forces it back. And so from being in a regime where it pushes the electron off, it changes sign and pulls it back. And the electron goes up and comes back down towards the atom. And it's got this energy. It's gained this energy because the photon of light has passed it onto it. It can't get into the atom with all this energy. It's got to release that energy. So this kinetic energy of the electron gets released as an ultra-high frequency beam of light, ultraviolet beam of light, goes shooting off. The electron gets reabsorbed. This is happening all around this gas, right? This gas is full of atoms. Each of these atoms, I mean, I don't know if it's every one of them, is getting excited. The electrons are shooting off, different distances, coming back, emitting this light. So now, all of a sudden, I've got these pulses of light which are emerging, all with different frequencies. But a key ingredient is that each of these frequencies is related to the frequency of the original light coming in, because they've had a certain amount of energy transfer. So there's a very important relationship between them. And what she showed was, first of all, that the intensity of all these, these are called overtones, by the way, these extra high frequency light that's coming out, they're called overtones, that the intensity of all these overtones was very similar, so that they all were contributing the same amount, typically, to what was going to then happen. Okay, now we come to school physics. What happens when you have two waves that overlap one another? Do you remember? Crest plus crest, big crest. Trough plus crest, zero. You've now got all these waves superimposing on one another, and every now and again, you'd get crests building. So you get a big peak, all around it, then it drops down. Can you see what I'm drawing? I'm drawing my first pulse. And then it happens here, crests, drops down. Second pulse, third pulse. So out of this, she generated these pulses. Each of these pulses is about a few attoseconds. Now, as far as I know, initially she didn't have a way of determining the actual time scale of this. Maybe she did, I don't know. But she had shown how it was possible for this to happen, to form these short pulsed objects out of long pulsed initial waves, and she'd shown that the intensity of these overtones, which make up these pulses, was such that you could be guaranteed that you'd have these strong signals. It seems like a really roundabout, messy way to be doing it. And I think this is why they've got the prize. It is such a difficult thing to do. I think that's been the technology that they've had to introduce, is probably why they've got it. Because it's gone to three experimentalists. The Wolf Prize last year, which is another huge prize in physics, went to two of these people and a theorist. The theorist doesn't run it this time around. So two of the Nobel Prize winners, he shared the Wolf Prize, and that's generally thought to be a kind of precursor of the Nobel. Quite a few people have won the Wolf Prize one year and the Nobel the next year, as happened this year. But in this case, Paul Corcombe didn't share the Nobel. He was replaced by Pierre Agostino. Is anyone talking about why? Have you seen any rumours on the web, or is it just a mystery to you? Not really. I googled it. And in fact, Frank Krauss himself said it was a shame that more than three people can't share the Nobel Prize, because what Paul Corcombe had done was absolutely fundamental and underpinned the work that led to the Nobel. So I think probably the Nobel Committee focused on experimental work, whereas what Paul Corcombe did was a lot of really interesting work on the thing called the recollision electron model, where a laser can pull an electron away from an atom. That electron then moves back very quickly towards the atom and can emit photons in the extreme ultraviolet bit of the spectrum, as you need for these really short pulses. But it was theory that he did initially. Then, of course, he did experiments as well. How were they used as a tool? Right. So I think that part of the prize is where Louis-Lieu's work came in. And then you've got the other two who came in. Their work was actually to manipulate this. So in 2001, Agostini realised that there was a way in which he could use, once again, use this laser beam, this infrared laser beam. He split it in one of the channels. He did this thing with the atoms where it would go through neon atoms and then form these pulses. They call them a pulse train. So you can imagine the size of the pulse. Then dropping down. So that's one carriage and then another pulse. So you have this pulse train. And then what he did was he split this initial beam. In it came the original infrared radiation photons. It splits. The beam carries on and then this bit goes through the neon, creates my overtones. And then they recombine them. And then the mathematics for that recombining allows them to determine the size of the pulse that has been generated. So for the first time, I think, they were able to say that the length timescale of that attosecond pulse, and it was 250. Each of these trains had a 250 attosecond pulse length. And the third person that has got the prize, Krauss, I think he's called, he developed a technique to isolate out single pulses. So he could create single pulses. He found the length of his single pulse was 650 attoseconds. He did something really interesting. It harks back to the Nobel Prize of Albert Einstein. What did Albert Einstein win the Nobel Prize for? Not for relativity. He won it for the photoelectric effect. A calculation he did in 1905, not 2000, but 1905. A photon of light comes into a material, hits the material. If it's got high enough energy, it excites an electron which gets emitted. And you can predict the speed with which it comes out as being the difference between the energy of the light coming in and the binding energy keeping the electron there. And then that experiment was confirmed, and he got the Nobel Prize in 1921. There was no way of knowing whether this is an instantaneous effect or is there a time delay, but Krauss did this experiment with these pulses. He followed the photon coming in, so it hits the atom. 100 eV pulse, I think, coming in. It excited the electron, and the electrons came from two different orbitals. You know atoms have orbitals, right? Different energy levels. One came out from a 2s orbital, the higher one, and one from the 2p, which is lower down. The 2p one came out, he said, 21 attoseconds later than the 2s one. Okay? So he first of all measured a time difference, but this difference could be calculated using quantum chemistry, and basically atomic physics. The theoretical values were different from his values by a fraction of two. And L'Huillier, she came onto the scene and redid this experiment in her own way, and she got the same result as the theoreticians. So there was something called, what did they call it, shaking in Kruse's experiment that she'd managed to avoid, apparently. And so their result matched the theoretical results of quantum mechanics. So you've got these experiments, and what they seem to be doing, I think the Nobel Prize people have been quite quick in giving it, in that it's still early days in the development of the technology, in that the kind of physics that it's been applied to is kind of really academic, if you like. It's trying to understand charge distributions around an atom, and charge swapping, when one atom will swap charges with another and ions will form. It's sort of homing in on using this very fine scale of time to be able to probe the motion of electrons. That's the key thing they're doing here. They can probe the motion of electrons. You're telling me these pulses of light can be used to geolocate the position of the electron? Yeah, I mean it depends what we mean there by electron and geolocate. It cannot pin down a point and say the electron is there. It can't? It can't, because if it could, if you could do that, then what would happen from Heisenberg's uncertainty principle is then you'd have no idea about the speed of that electron. Well that's OK, if I don't want to know that, if I just want to know where it is and I don't care what speed it's going at. Yeah, but I don't think that's the case here. I think they do want to know, but it's a really good and important question. I think what they're looking at is the waveform you associate. Do you remember in quantum mechanics you have particle and wave duality? You can describe something either as a particle or you can describe it as a wave. And I think these descriptions are wave-like descriptions, so what you're looking for is the peak amplitude of the wave because that tells you where the electron is most likely to be. It feels like there's this sort of growing body of world-class scientists trying to hide the Nobel Prize because of the rules. Yeah, no, it does seem to be... I mean, science is teamwork now. Everything is done in teams and typically those teams have got a lot more than three people in there. So having to condense a team into three people that actually win the award from the whole global team of people actually that develop the field is pretty tough. And in this case it seems to be very tough to me. What do you think? Yeah, and to make sure that controversy doesn't just dominate the award every year and the science takes centre stage. Is the Nobel Prize still important? Absolutely. Yeah, I mean, it really shines a spotlight on the field. It shines a spotlight on what science can do, not just the fundamental physics which often attracts young people into science, but also what can be done with that science. Thank you for having me in your lab. What do you do here? There's all this impressive equipment, kit, machinery. What kind of physicist are you? What do you do? Yeah, we like working with lasers because they're really cool and allow us to do a lot of investigation of matter with particular application in medicine. So what we want to do is to use light to probe matter and to understand it. And it provides a tool to record the light that comes back that carries out the information we need to understand what's happening in our sample. Professor, why does the light have to be a short pulse? Why couldn't you just put a constant source of light on the thing you're studying? Well, it all depends on what the process you try to understand. So, for example, in this experiment here, we are trying to discriminate between the light that interacts and the light that can induce other processes, which take, for example, the absorption of electrons and the remission of light. But that takes time. At that time, it's very, very short. But, however, in order to discriminate between the two, we have to use very short laser pulses. The Nobel Prize this week in physics seems to have been related to the sort of work you do. Was it closely related? Tell me about that. Well, it's a phenomenon that we want to observe here. We use what's called picosecond laser pulses. Now, the Nobel Prize is about because that is what is relevant for this particular experiment. For the Nobel Prize work, the pulses of light that were used there, they were at a second. To put it into context, this means it's like comparing in one second compared to two weeks it will take in our experiment. Do you need much different machinery to achieve an attosecond compared to your picosecond? Well, you can imagine this is very novel work and it is even actually measuring the pulses. Of course, it's very complex. It's very much the fundamental physics. Here, we are applying for measurements on real biological samples to try to understand biological events that happen. If you flashed a light in my eye for a picosecond, was it too quick for me to know it happened? I think that would be too quick. Obviously, because of the effects depending on the power, it may damage your eye pretty certainly. Whether your brain will recognise it, I don't know. When a Nobel Prize is given in your field, what does it mean for you? It's just amazing. It's exciting. The way in which I start the first lecture of my modules in advanced super-resolution imaging, I start with the Nobel Prizes in this field. I think we are lucky. Coming back from the 1964 with the Nobel Prize awarded for the laser, at the time we were saying it's a technology looking for applications. Now, come 60 years and lasers are everywhere and Nobel Prizes are coming over very, very often. One of the applications has medical applications. What does an attosecond laser have applications for? I think at the moment what we have to think is the extraordinary science that we can do with it. We have to think that the electron takes a few hundred attoseconds to go around the nucleus of hydrogen. What we can do is actually look at how atoms vibrate, chemical reactions that happen at those sort of timescales. This is just amazing. I guess this is so exciting that coming later I think applications will be much later than that. Professor, you told me the 1964 Nobel Prize for lasers forms part of your lectures here at Nottingham. Is the 2023 Nobel Prize going to be part of your lectures too? Oh, absolutely. It's going to be there definitely on the slide and potentially the fourth year modules we are teaching. Absolutely. We always like to expose the students and to excite. So this is just great for my area of physics. Can you show me where is your laser here? I can't. Where's the heart of your experiment here? Well, one problem working with lasers the beams have to be fully enclosed just for the safety. So you can see the laser will be in this part here and it's fully connected and covered by tubes to be delivered under microscope. So that will be a tiny laser. Just that box? Yep. Do you buy your lasers off the shelf or do you have to make them? We buy off the shelves because we as I mentioned my work is very much in understanding biophysical processes and potentially developing medical devices using lasers. So the first thing what you want to do is to have reliable lasers that you can actually use.