How do lasers work?

They're the stuff of sci-fi, but how do lasers actually work?
12 June 2018



How do lasers work?


Chris Smith asked Deirdre's question about how lasers work to materials physicist Jess Wade, who shined a light on the topic...

Jess - Oh, this is a great question Deirdre. Everyone loves lasers right, along with slime. They’re the second best thing in physics. Lasers work because we have atoms and inside an atom are some protons and neutrons, which is what Fran studies. We’re more interested in the electrons in material science, and what we need to do is in a laser is we need to excite one of these little electrons up to kind of excited state we call it, but it’s really unstable in this excited state. It’s like you give a child loads of slime to play with and they suddenly go ferociously energetic.

Chris - Loads of sweets!

Jess - And then really quickly it needs to stabilise, so it emits some light to get back down to its ground state. And you can do a really clever thing where you trap loads of atoms together so that when that one little electron jumps down and emits its light it excites another electron that’s nearby it and makes that emit its light also. And you have this kind of cascade affect where it keeps happening and you can do that by trapping these atoms between mirrors or something clever. What’s really exciting about laser light and why physicists like it so much is that little jump that the electron makes. That energy that it gives out is really specific to the atoms that you’re exciting. So laser light is only one colour, it’s only one energy and that lets us do all kinds of clever physics because we can then use that laser light to excite different atoms which absorb that particular energy, and it’s also super focused. You get these laser beams. You have a light bulb that gives out this really diffuse light but laser beams come at super focused, and we can keep them focused for a very very long time.

Chris - And, of course, by using different flavours of atoms you can get different colours of light that have different applications. Because if you want to do say medical imaging or you want to burn bits of tissue, you want a laser that will interact very well with tissue but not other things?

Jess - Exactly. And it’s very similar to fluorescence. So fluorescence is this thing where we excite an atom and very quickly they emit light and it happens over a very short timescale, and when it happens it’s gone. You can tune that fluorescence, you can tune that colour to whatever atoms that you are exciting.

Chris - Eleanor?

Eleanor - So talking of applications, something I’ve always wondered about lasers, taking this to a really highbrow level, I love Star Wars and one of the weapons they use is this kind of laser shooters. Is that ever going to be possible having kind of weapons that are lasers?

Chris - Do you have nefarious intent or something?

Eleanor - Of course not. I don’t know why you would suggest that.

Chris - When working with the ants becomes too much.

Eleanor - Well, you know.

Chris - It’s gone from burning them with a magnifying glass to a laser blaster!

Eleanor - I would never do that.

Chris - A laser blaster. Jess?

Jess - If you wanted a laser blaster you can make a laser blaster, right. If you get a very powerful laser you could fire it at something. You can pop balloons with certain lasers. You could certainly blind yourself with a laser. The majority of time spent at the beginning of your PhD in materials is trying to complete laser safety courses so you don’t blind any of your friends with a laser. So I don’t think you’re far off the Star Wars thing if you want one.


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