Problem solving lasers

26 August 2014

Interview with

Cather Simpson, Photon Factory, University of Auckland

Lasers are being used in lots of ways, now including harnessing the sun's rays to produce energy and detecting the gender of sperms. Cather Simpson spoke to Chris Smith and Simon Morton about how she's doing this... 

Cather  -   So a laser is a very special light source.  So, if you'll look at the lightsArgon-ion and He-Ne laser beams in the room, you'll see that they look white or maybe slightly blue.  That's because they're putting out light of all different colours in all different directions.  Lasers essentially pick out one colour and put it out in a very tightly focused - we call it a coherent beam.  So, I have three different lasers here.  One is very blue, one is very green, and one is very red.

Simon -   And Cather, these are like little Maglites, are they?  They're about the size of a sort of peg.  Do they actually burn?  Could you burn a hole on my head with any of those?

Cather -   Let's try it.

Simon -   Really?  Isn't it dangerous to try and laser someone's eyes?

Cather -   You should never shine laser at anyone's eyes.  Most laser pointers, I have two that are slightly bigger than the average laser pointer that you'd get in a $2 shop.  These are ones that we bought for my research lab.  so, we use these in experiments and that's why they've got duct tape on them because we take them apart and put them back together.  Never shine them at your eyes.

Simon -   What are you trying to discover then at the photon lab?

Cather -   So, what we're trying to discover at the Photon Factory is how molecules absorb light which is a kind of energy and turn it into a more useful form of energy.  And so, if you look for example - this is just tonic water - because we're scientists, I'll show you.  See, really tonic water - the Schweppes label.

Chris -   Where's the gin?

Cather -   That was you guys who were supposed to bring the gin.  So, tonic water has a chemical in it called quinine.  It's the stuff that makes it taste bitter and it's the stuff that cures malaria.  If I take my UV laser, my very purple one and I shine it at tonic water and all I've got - I've got two 2-litre bottles here and I've put some aluminium foil on one side to get a few more reflections.  What you can see - those of you who can see - and I'll hold it up.

Simon -   Wow!  It's amazing.  There's a very distinct beam there, going through the tonic water and it looks - I mean, that's sort of Han Solo sort of stuff, isn't it?

Cather -   It looks like a lightsaber, doesn't it?  So, for those of you who want to make a lightsaber, if you fill a tube with tonic water, you're right on...

Chris -   It would be quite heavy, wouldn't it?

Cather -   Yeah.  So, if you see this, it's a different colour, isn't it?  It's kind of a light blue colour, sort of white, compared to that purple colour.  And so, what the quinine molecule is doing is it's absorbing the light.  It's getting all excited and it's turning it into a different colour of light.  If I use my red laser, all you see is red.  If I use my green laser, all you see is green.  All you're seeing there are reflections.  The molecules aren't absorbing and converting it to anything.

Chris -   So in other words, the molecules of quinine in the bottle are exclusively and selectively responding to the purple light, but not the green and the red.

Cather -   Absolutely.  And so, what we study is the same light that's coming off of here or that you're getting out of these blacklights.  It's hitting the back of your eye and the reason that you're seeing something is because in a very, very short time, millions of a billionth of a second, the molecule in the back of your eye is converting it to molecular motion.  The molecules that are in plants that do photosynthesis are converting that light energy to a little battery.  The molecules that are in my finger when I take a green, even an intense green laser pointer, you can't see it at all, right?  the molecules that are in my finger are the hemes that make your blood red are absorbing that light and extremely rapidly sending it out as heat.  And so, we study how molecules and a lot of those are related to one another in structure, how molecules decide what they're going to do.

Chris -   What are the applications of this?  What sorts of things can we use our understanding of how lasers interact with materials, the way you've been showing us to do?

Cather -   So, in our lab, we do everything from that kind of fundamental research, looking at how these molecules change light into other forms of energy, and the students who are studying that are looking at things like how art pigments fade and how we might prevent them from fading, or how we might make better solar energy harvesting complexes.  But we also take our lasers and use the fact that they're dumping energy into a system to make them very practical.  So for example, Intel would love to be able to use one of our very short-pulsed lasers to dice chips for their semi-conductor industry.  Right now, the narrow pulses don't have quite enough energy in them, so it's too slow to be economical.  But we have a big grant from the government to try to use our physics knowledge to make them more efficient.  And that's very exciting.

Chris -   I was talking to a guy the other day who has actually found that you can put a lot of energy into something with a laser.  But if you were to put all of the energy in all at once, the temperature would be obviously very high because of the huge amount of energy going in.  but if you spread this as a series of little pulses over time, you can put enormous amounts of energy in but it's not going to get to a very high temperature and burn the thing.  But then you get the changes you've been showing us with light coming back out, you get that information coming out so you can interrogate materials and find out for instance what paints were used to paint a picture or tell if it's real or if it's a phony picture, or something someone knocked up in their back garden.

Cather -   You can certainly do that.  So, you can get the spectrum if you use a laser like you're describing, a pulse laser.  There's a really interesting thing that happens when you start taking a laser like this one which is coming out all the time.  this is continuous wave and you start taking the beam and you chop it into little pulses.  You don't get rid of light.  You essentially kind of - it's like if you had Playdoh and you made it into little hills, right?  At some point, you start making those pulses very dangerous because there's so much energy inside that you ablate the material - you remove it.  So, if I use one of the lasers in my lab, that is about a billionth of a second long then with a single shot - so, this is a nanosecond pulse - I could detach the retina from the back of your eye.  That's a little gruesome.  I wouldn't do that, I promise.

Chris -   I wouldn't love to work in your laboratory.

Cather -   But I could also drill through a piece of stainless steel.

Chris -   And just very briefly - I mentioned this at the beginning - you're also sorting sperm out with lasers.  How are you doing that?

Cather -   So, we sort sperm with lasers using the fact that the same kind of energy that we're talking about damaging stuff, you're essentially hitting materials.  And so, you're generating forces.  So, you can make a solar sail for example, put a big aluminium foil.

Chris -   And not the sperm though.

Cather -   No, but yes, we do with sperm.  So, one of the things that we do is we put them in a little channel.  These are bovine sperm for the dairy industry and they're disk-shaped.  And in fact, just like turning the sort of Frisbees.  So, we put them in a channel and we use the fact that these things generate pressure and that puts them all and oriented the same way, and they go down this channel and we say, "Ah!  There's a male sperm and a female sperm."

Chris -   How do you tell them apart?

Cather -   So, the male sperm has a Y chromosome which is a little bit smaller and so, when you use fluorescence, that means the males aren't quite as bright as the females.

Chris -   Just more aggressive and miserable and stroppy.

Cather -   No, it has nothing to do with aggression.  It has to do with brilliance.

Chris -   They refuse to ask directions.  The male sperm don't say which direction do I have to go.

Cather -   No, it's not about asking directions.  And then we use another laser pulse to take the ones that we've identified as female in this case and go boom, and we just move them over.

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