Artemis - the Superfast XUV Laser
Helen - Artemis is one of the devices at the STFC Central Laser Facility in Oxfordshire. Artemis produces light known as XUV which is a wavelength between UV and x-rays. It fires very short burst of energy lasting 30 femtoseconds. That's 3 x10-14 or 30 thousanths of a millionth of a millionth of a second long and that's what scientists like Dr. Emma Springate can use to probe the fundamental properties of matter...
Emma - We're in Artemis. It's the CLF's facility for ultrafast laser and XUV science. So we have a big laser system that produces really, really short pulses of light, 30 femtoseconds or so and we use that to generate pulses of XUV which is radiation in the region between the UV and the x-ray region. So, at 10 to 100 nm or so.
Ben - In order to come in, we've had to don aprons and shoe coverings. We've just walked across a sticky floor, and we've got goggles on. Obviously, there are a lot of safety issues but I'm guessing the aprons and shoes are not for my safety but to stop me from contaminating the laser?
Emma - They are, yes. They're trying to keep down the amount of dust in the lab. If you get dust on some of the optics, the lasers are intense enough that they can actually burn the dust onto the optic and make a hole in the laser beam. We also use a lot of ultra high vacuum equipment as well. We generate the pulses of XUV in a vacuum and then all of our experiments are done under very high vacuum as well. It helps keep the lab cleaner.
Ben - So the lasers in this room are at x-ray strength, but we're still wearing goggles. Surely, x-rays are going to get straight through goggles and affect the rest of our bodies just as much as our eyes. So are the goggles really necessary?
Emma - The lasers themselves generate light at 800 nm which is just on the red end of the visible spectrum. There's also a lot of green light around from the pump lasers that put the energy into the titanium doped sapphire crystals that actually emit the 800 nm light. So those are the main things that the goggles protect us against. The x-rays that we produce are in the vacuum ultraviolet XUV region of the spectrum. So basically, they're absorbed in air. So we generate them in vacuum and through most of their journey they're inside a vacuum chamber with stainless steel walls. The x-rays are the least of your worries in this room.
Ben - So, the x-rays that you're using for science essentially an end-product and these goggles protect us from the other lasers that you use to create those.
Emma - That's exactly right, yes.
Ben - It's very noisy around here as well. Is that just the pumps for the vacuums or are there other bits of kit that are making that noise?
Emma - The noise you mostly hear is the power supplies for the pump lasers. The chugging noise that you can hear, kind of like an old man wheezing, that's the cryocompressor. The titanium doped sapphire crystals in the laser that we use have to be cooled down to cryogenic temperatures with helium pumped around them. Most of the vacuum pumps, we keep underground to try and keep the noise level down a lot.
Ben - There's a lot of kit in here that I couldn't even begin to understand. What are each of these machines?
Emma - At the back of the room over there, we have the laser system, it's an ultrafast laser system. Then we have two tables full of optics to generate much short pulses of light by focusing the light into a hollow fibre filled with gas and also to generate light across the wavelength range from the UV to the mid-infrared. The idea is that we can use any combination of these pulses as a pump to initiate a change in a molecule of solid for instance, or to generate x-rays, and then another laser pulse to probe, at a later time, what's going on in the experiment. Then we have a whole series of vacuum equipment. So a vacuum chamber where the x-rays are generated. The XUV can only propagate in vacuum so we have a series of vacuum chambers to monochromatise the light, filter it, and refocus it down, and the experimental interaction stations at the end of the beam lines.
Ben - So, the interaction stations are where scientists can go and actually get to do some science with all this amazing kit.
Emma - Yes, indeed they are. So we have two. We have one for materials science, for people looking at experiments in condensed matter. We have a couple of groups who are interested in highly correlated electron systems which are the kind of materials that high temperature superconductors are made of, and people interested in ultrafast de-magnetisation. So, using lasers to switch on and off the magnetisation in materials and see how fast that changes. That's the kind of things they do in the material science. We also have a group in at the moment doing an experiment on our station for gas phase experiments looking at small clusters of helium.
Klaus - My name is Klaus von Haeften from the University of Leicester from the Department of Physics and Astronomy. I'm investigating a very basic effect in condensed matter physics and gases and liquids. So we want to find out how molecules that rotate, how they couple to their environment and how the environment slows their rotation down.
Ben - Is this a pure physics problem or are there some applied examples that we could think of?
Klaus - There's no direct application but the problem is relevant not only for physics, but also for chemistry and biology because molecules play a central role in everyday problems. We have chemical reactions for example and chemical reactions depend on how molecules are aligned to each other and as the molecules rotate, they get misaligned, and dragged in different ways, so we need to understand this.
Ben - How are you using lasers to actually study this?
Klaus - Well it's difficult to see how molecules rotate for two reasons. One of the reasons is that they're very small and secondly they rotate very fast. So, we need a sort of microscope and the laser light acts as this sort of microscope. The lasers here are particularly fast and they allow us to track the rotation of the molecules.
Ben - So, you fire a laser at a molecule very, very quickly. I'm assuming in very, very short bursts. How do you then pick it up and what can that tell you?
Klaus - Well the trick is to use two laser pulses and to have one as a reference and to fire the second laser pulse with a delay time between the two and we control this delay time and see how much the position of the molecule has changed with this information on the rotation that they want to get.
Ben - And what's going to be the next step? Once you've collected that information, what do you do with it?
Klaus - We have to think and analyse the data and we need to develop some finer theories for that. This is a true cutting-edge experiment because we have preliminary data that tells us that this effect exists but has never been observed before or seen before. So, it's very difficult to answer this question because it's something never done before, so how can you predict what happens in the future? We will look at the data and we have theoreticians in the team who are smart enough to develop ideas.