It seems that us Nerds are becoming a rather nostalgic bunch. Change is very much in the air. Adding to those changes, I have recently left the never-ending pursuit of knowledge that is academic science for the never-ending pile of submitted manuscripts that is academic publishing. It’s a bit too early for me to ramble on about the new world I’ve come to inhabit – I’m still getting used to the presence of windows – so I’ll leave my thoughts on publishing to a later post. Instead, I hope you’ll humour me as I try to sum up my time as an active scientist. George may have spared you such torment, but he worked on denoising of powder diffraction traces and not laser physics, so I think I have the upper hand. Plus, for all those hours I spent first screaming at and then pleading with the laser to do what it’s supposed to without shutting down every half an hour, I do kind of miss it.
Physics: it's pretty much as glamorous as it looks.
Besides, I won’t get another chance to be asked what I do and give the answer: “shoot lasers at little indistinguishable pieces of black stuff”. So I’m going to seize this opportunity to try to explain actually what it was that my colleagues and I were trying to do with all those hours in the dark. Other than drinking tea and listening to really loud rap music.
Broadly speaking, my field is ultrafast materials science. This nicely melds two worlds: that of ultrafast laser science and optics with the study of complex matter. Let me clear up those two areas a little more. Ultrafast laser science is concerned with developing laser sources and techniques on really fast (by which we mean ‘short’) timescales – typically we talk about picoseconds (a millionth of a millionth of a second) or femtoseconds (a millionth of a billionth of a second, or 0.000 000 000 000 001 seconds). Complex matter isn’t matter with issues – although it kind of has those too – but is the broad class of materials that can’t be explained by the relatively simple models laid down in the early part of the 20th century that describe most of the substances we are familiar with, like metals and insulators. In those everyday materials we consider the electrons whizzing around the constituent atoms as being totally independent of each other – each one just sees the nucleus, screened by all the other electrons, and it goes about its business as though the others aren’t there. This approximation is remarkably powerful, and more less enables you to calculate all the properties of metals, insulators, and those all important semiconductors, to incredible accuracy. The issue is that the model breaks down eventually because, of course, the electrons do see each other and sometimes they see a lot more of each other than at other times. This gives you a whole load of unexpected behaviour, like superconductivity or the new and rather mysterious topological insulators.
The indistinguishable bits of black material that are at the heart of it all. These are the exciting compound 1T-TaS2.
So naturally, people are really interested in complex matter, first because it presents a number of difficult challenges and puzzles to solve, and secondly because we might be able to do lots of really exciting things with it. Also, people are really interested in lasers because they’re cool and they come in a range of colours and you can get them to make little balls of lightning in the air. So it makes sense to combine the two, right?
There are plenty of techniques out there to study materials and they all do an amazing job. But there’s one property of lasers that’s hard to beat: time resolution. This comes from their pulses, which are pretty short (as discussed above) and means they can capture very quick things happening. There’s a useful analogy we can make here with movies. As we all know, a movie works by taking a series of photos and then playing them back; each photo captures a split second of the action, so that when we put them all together we can watch the baddie blowing up the building, or the hero getting the girl, or the horse in motion. As the exposure time for each frame reduces, the speed of the activity we can witness increases. If we use millisecond exposures we can film things that happen on millisecond timescales – this is what we’re used to in movies and television. Now, with our laser system we can consider the light pulses as acting like the film: each one bounces off or passes through our material, capturing an ‘image’ of it that becomes encoded in the properties of the light, which we can record on a detector. Because the pulses only have a duration of femtoseconds, we can ‘film’ femtosecond-duration events. This is ideal, as processes like atomic vibration or the whirling of electrons all happen on this sort of scale. Thus if we’re clever, we can film all the goings-on inside these materials and try to work out what’s making them tick.
But you can’t just shoot the laser and watch what’s coming back. Your little piece of black matter isn’t doing anything special from one moment to the next – otherwise nothing would be stable. All you would detect is a constant, boring background signal. What you need to do is kick-start something: shout ‘action’ and get everyone running around. And the way we start things happening is with a second laser pulse, which pumps the material into a state of activity by transferring energy to it, getting everything all shaken up and disordered. Pretty soon, all the electrons and other inhabitants of the system begin to evolve, oscillating back and forth or cycling round and round, gradually settling back down before the next pump pulse comes along again. Then we take our filming laser (usually referred to as the probe), and watch what happens. By changing the arrival time between the pump and the probe, we can capture a frame of a particular point in this evolution; by scanning this relative delay, we can build up a picture of what’s going on. This is the ubiquitous pump-probe trace of ultrafast materials science:
They all look sort of similar: first the signal goes up or down, then it begins to return to where it came from, and if you’re lucky it might have some wiggles on it too. The trick then is to sort of squint at it until something useful pops out. For the time-resolved scientist, the important information is contained in the timescales: how quickly does it go up, how quickly does it come back, how fast does it oscillate? This is important in complex matter because you normally can’t tell what’s responsible for the phenomenon you’re interested in: the different elements contributing to the properties (the electron’s charge, its magnetism, its particular orbit around the atom, to name a few) all compete with one another and no one part dominates over the others. However, in the time domain – the realm of the pump-probe trace – you can separate out all the different components based on how fast they respond. Then the idea is to work out who is coupled up with whom, who’s lurking in the corner, and who’s responsible for the party.
This is more or less exactly what's going on when you do these experiments.
That’s the crux of the matter, at least. There are a whole ton of variations you can throw in there, like changing the temperature or applying magnetic or electric fields on top, or altering the wavelength of the laser light. That last one was mostly what I did during my time. This approach lets you compare processes having different energy, so you can look for fingerprints of different motion or reveal interactions that you didn’t expect.
There are a vast array of tools available to look for all of these things, and unfortunately a lot of them are located in exotic and interesting foreign locations. And some of them are also in Harwell. But what’s nice is that the fundamental approach is always the same: take your material, kick it, and see what happens. You just alter the angle you look at it from each time to learn something else. A great deal of information can be gleaned in this way, opening up a number of routes to figuring out what exactly is going inside all these fascinating materials. Plus, lasers are cool, so that’s a big bonus. You can always pretend you’re going to hold the world’s governments to ransom or something like that.
So the next time you see a pasty physicist squinting under the harsh glare of a hallway strip-light, hopefully now you’ll have some understanding of what it is he’s up to in that dark, noisy little room of his. Assuming he’s managed to get the laser running for longer than ten minutes, that is.
by nicky
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