Photon avalanches: a one pot method
Adam Murphy’s been looking into avalanches, but not ones made of snow…
Adam - When you think of an avalanche, you probably think of snow. One bit of snow moves, causing a cascade of snow beneath it. But lots of things can avalanche - nuclear reactions, the bubbles in champagne bottles, even particles of light, photons.
P. James - A photon avalanche is a process by which you have a material and that material is in a certain state, such that when it absorbs a single photon, it sets off this chain reaction that ultimately leads to a whole slew of photons coming out the other side.
Adam - That's P. James Schuck from Columbia University. Now photon avalanches aren't new. We've seen them for 40 years, but making this process happen has been difficult, only occurring in big bulk materials or at super low temperatures.
P. James - What we did here was create a material where we were able to realise this avalanching, that's not only it doesn't require being in a bulk piece of crystal, but actually happens in a little nano-scale packet of material and it can happen at room temperature. The particles themselves they're made of something called sodium yttrium fluoride, but it is a ceramic. So you think of them as basically a transparent ceramic. And then what we do is take lanthanide ions that have very interesting optical properties and we dope them into that material.
Adam - Sounds very complicated to put together. So is it really as hard as it looks?
P. James - No, it's actually a relatively straight-forward one pot synthesis as the chemists like to say. It really can be made at reasonable temperatures inside someone's lab. It's much simpler than what chemical companies go through I think to create gasoline, for example.
Adam - Another strange thing about these particles is that the photons that come out have a higher energy than the one that went in. Now, one of the biggest rules in physics is that you can't just make energy. It has to come from somewhere. So where is all this energy coming from? There's no such thing as a free lunch after all.
P. James - No free lunch. That's right. So the way that these actually work is the material works when bathed in infrared light. We're taking the material and we're surrounding it in infrared light. So these are sort of low energy photons but what happens is then when you're sitting in this bath of infrared, when your one photon does come into the material and gets absorbed, it puts the material up into this excited state that can then interact with this infrared bath very, very efficiently and that then leads to a chain reaction of events that leads to lots of photons coming out the other side.
Adam - And this isn't just nice physics to be put in a folder, labelled parlour trick. It's got some potentially huge applications. Like say you wanted to detect a molecule or a virus that only gives off single bits of photons or single bits of energy. Well, something like this might come in handy.
P. James - That means it can be a very energy efficient way to do things like make an ultra sensitive detector because what you want in a sensor is you want to detect a small change, and you want that small change then to lead to something big that's easily detectable. That's exactly what these do.
Adam - And there are some weirder applications. The cameras in night vision goggles detect infrared photons or heat, but the Silicon in the detector isn't usually very good at detecting infrared. Well?
P. James - If you were to then, for example, take our material, which happens to respond very nicely in the near infrared in fact, that has this very large response. Then you could imagine a scenario where you can simply take Silicon detectors, you coat it with avalanching nanoparticles, and now you've just made your camera an infrared camera, as well as a visible camera. Now that we know how these work, they give us a good clue as to how to make new materials and then these would even further expand the potential applications of it.