[BLANK_AUDIO] So, that's the detection of regular, visible wavelength light. Now lets talk about the infrared. So, first thing, why we want to measure infrared light. Lots of very interesting things in the universe emit more infrared light than visible wavelength light. So, cold things, things shrouded behind dust, and in particular, high redshift objects at the edges of the universe emit lots of infrared. So we really do want to detect infrared light. But here's the problem. So visible wavelength light, so, the wavelength of light you can see with your eyes, is around 500 nanometers, or 0.5 microns. If we take near infrared light, the sort of thing we want to measure, at say a wave length of two microns - if we ask, well okay, how much energy does each photon have, for that near infrared light? The answer is about 0.6 electron volts. Now you may have remembered, that for silicon, that band gap between the the bound states and the conducting states, that gap, is 1.2 electron volts. Now, that means for regular visible light, each photon could easily bump you up into the conduction band of silicon. But the infrared light can't do that trick. The photons don't have enough energy. So w need to do something else. So basically, the answer is to turn to different substances. There are other semi-conductors. So silicon is the most popular in the electronics industry. So, it makes very cheap devices because everybody uses it, and it's easy to make printed circuits and so on. But there are other semi-conductors we can use as detectors. So for instance, the popular choice in the near infrared, a few microns, Is an alloy of Mercury, Cadmium and Tellurium, or usually known as Mer-cad-tel. And in fact, if you adjust the Cadmium content, you can change the the size of the band gap and tune it. So that's pretty cool. But it's got a, a smaller band gap, so the infrared light can do it. When we move into mid infrared, 10, 20 microns, that's the sort of wavelength warm bodies, and most of the things in this room are radiating at, that has even lower energy per photon. So to detect that, well there are various choices, but the popular one for example, is silicon arsenic. Then we move into the far infrared at 100 microns there we use germanium and gallium. So there are choices, there are ways we can do this, but I won't go into details, but in order to actually make working devices, with these other materials, it's much harder work, the cost has not been driven down by the big electronics industry the way it has for silicon. So those devices are more complicated and a lot more expensive. So this is an infrared chip right here made of Mercadtel, and this is a working model, a tester for an infrared camera that's on a telescope called VISTA that's in Chile, run by the European Southern Observatory. But the camera was designed and built here in the UK in Edinburgh and Oxfordshire. And, that is, it looks pretty similar to a CCD, I know, it just looks like a flat, smooth, surface. It's made up of lots and lots of pixels. But it's not made of silicon. It's made of Mercadtel. So, it works very well, but, it's very expensive. So, in infrared astronomy, it's very important to be cool. That is, to keep everything cold. There are two reasons for this. The first one, is that everything around us is radiating in the infrared, at the sort of temperature that we are here on Earth - your body, the telescope - everything is radiating in the infrared. That's just the peak wavelength of radiation for warm bodies. So we want to try minimize that to keep down the glare. That's a background for detecting faint things. The second reason is maybe not quite so obvious. Let's think again about how the detector works. Remember that we've got a set of bound states and then a small gap, and then the conduction band, and what we want is for light to strike the surface, knock electrons up into the conduction band, and then we trap them. However, if the material itself is warm the atoms will be jiggling about. And then some atoms will bump into other atoms and those collisions between atoms, can also bump up electrons into the conduction band. And we get a signal that's not due to light, but it's just due to this so called thermal noise. So we want to cool the material down, to minimize that problem. And this is especially bad for infrared detectors, Which have a very small band gap. So, what we do is to put our detectors such as this infrared array here, I need to put it inside a kind of super fridge basically, in a bath of liquid nitrogen, or even liquid helium, and keep it as cold as possible.
