Tuneable Laser Technology
Professor Harry Coles, University of Cambridge
Dr. Philip Hands, University of Cambridge
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from the show ZAP! Lasers on trial...
Chris - COSMOS is a Cambridge University project that's developing a tuneable, portable laser for medical diagnostics and for scientific research. Here to show us how it works is Professor Harry Coles from the University of Cambridge and Dr. Philip Hands, also from the University of Cambridge. Let’s kick off with Harry. What actually is a laser first of all, Harry. Tell us about that.
Harry - LASER stands for Light Amplification by Simulated Emission of Radiation and it’s spelled LASER not the American way, with a Z. The easiest way to think of the laser and how it works is to imagine yesterday afternoon when it was raining. The rain comes down in a random pattern. But if that rain came down and got trapped on a washing line, for example, you'd have a whole lot of drops of water hanging on the line. Then if you flick that washing line, all of the water droplets drop down together. You get what's called a cascade process and because the line is at a certain height, you get the same energy dropping down in the water droplets.
Chris - But if I had a higher washing line...
Harry - You'd get more energy. So, changing the height of the washing line "tunes" the laser if you like to think of it that way. What you do in an actual cavity in a real laser, you put that light between mirrors and you bounce the light back and forth to give you a gain in a single direction.
Chris - Obviously, lasers don't have washing lines in them.
Harry - No.
Chris - What do they use to create the metaphorical washing line?
Harry - That is the molecule or the system itself where [you can excite the molecule]. So you excite electrons to a higher energy level. That's the analogue of the water droplet and then as the electrons de-excite, they give light out, they give photons out.
Chris - And they could only have discrete energy levels, so they have to give light with a certain colour.
Harry - That is correct, yes absolutely.
Chris - So how do you actually stimulate the emission of the radiation? What does the shaking of the washing line and actually makes more light where there was less to start with?
Harry - You can do it two ways. In our systems, we drive the laser with a pumped diode system to get light in to excite the system. You could do it electrically. For our application, we don't do it electrically. We want to use fairly low energies because of medical applications we’ll talk about later. We don't want high powered lasers.
Chris - Philip, so why is this special – what you've actually invented?
Philip - Well what we got here is a particularly small and self-fabricated laser and in my hand right here is the business end of our laser. It’s just a small glass cell – two pieces of glass separated only 10 microns apart. In this cell, we have a tiny amount of liquid crystal material and this liquid crystal material acts as both the gain medium, the washing line to use Harry’s analogy but also, as the mirrors of the system. In one small compact easily fabricated thing we have almost the entire laser system, all we now need to do is to excite it with a form of energy from the pump source.
Chris - So you have a laser which you put energy into your new laser cavity with, so that's the source of the energy in the first place, and that then produces new laser light of whatever frequency you want to. So the breakthrough here is not just the miniaturisation but the ability to create light of any colour you want?
Philip - Yes.
Chris - The previous laser cavity would be dictated by a certain height of washing line – those atoms in there. You can get around that problem and produce washing lines of any height.
Philip - That's right. There's a certain range of operation, but what we can do with this laser is we can take a fixed wavelength pump source and convert it into an any colour laser output specifically from the range of about 450 to 850 nanometres, that's all the way across the visible spectrum.
Chris - That is blue right through to red.
Philip - And into the near infrared a little bit as well, yeah.
Chris - So you've got it set up in front of us. Can you just talk me through what I am seeing here?
Philip - This unit in front of me right here is the laser itself. You can see it’s in two parts, the modular design. The largest part is actually the pump delivering energy into it. But the small box at the end here contains a liquid crystal cell which is like the one I've got in my hand right here. We shine light onto that crystal cell and it simply emits laser light. At the moment, you can see it’s focused onto the spot card here. It’s emitting red light because the twisted molecules that are inside our liquid crystal have a certain helix that reflects and therefore resonates red light. However, if I translate the cell across, I move a different part of the cell in front of the beam where the helix is more tightly wound and it now reflects and resonates green light.
Chris - So liquid crystals – the same things that might be in the TV in your living room these days, and calculators once upon a time - those are the display molecules. You're using certain configurations of those to cause the light to go through these contortions that produce the different colours.
Philip - That's right and inside the cell, there are three basic components. There's the dye itself which is the emissive part, the part that absorbs the radiation and emits the light. And then we have what we call a chiral nematic liquid crystal. It’s a twisted structure at a molecular level and that acts as a resonator of our laser. The amount of twist that we give it determines the resonant wavelength and therefore, the emission colour of our laser.
Chris - Are all of the crystal architectures premade on that little chip or do you actually, using electronics, change them dynamically?
Philip - Well the structure itself forms spontaneously. It’s important to say that first of all, all we need to do is mix the component ingredients together in a test tube and then fill the cell. That spontaneously forms the right structure. If you want to change the helix, you can do so in a number of ways. One is chemically: we can just change the ratio of our mixtures. You can tune it electrically to a certain degree as well. We can get about 100 nanometres or so of tuning electrically. What I'm doing here is a mechanical tuning. We’ve got a variation in the helix across the cell and as I press the button, I can tune the wavelength from red to green.
Chris - Shall we see it change?
Philip - Yes.
Chris - So you click a button on your laptop which talks to the device.
Philip - And we can go from red to orange...
Chris - It’s becoming yellow.
Philip - ...to yellow and if I keep going...
Chris - Green colour.
Philip - ...I come through to green. That's the range of this particular demo. We can actually go all the way through to blue and as say, into the infrared as well.
Chris - Philip, thank you. Harry, so why did you do this? What do you actually want to use this for? Why is this a big breakthrough?
Harry - Because every molecule has a different chemical structure and therefore has a different signal. So if you want to look at a whole range of different materials, it might be a biological system, then you will have absorption at different wavelengths, different colours, and you can look at systems naturally. In this case you don't have to put fluorescent dyes into your material. You can look at their natural absorption and indeed, their natural fluorescence.
Chris - So if you are a scientist using a microscope in a laboratory where previously you'd have to label cells with something that you would then flash light a certain colour onto to make it glow, now you're saying, you can just tune up your laser into the microscope, choosing a particular wavelength that will make things that are naturally there anyway glow.
Harry - Absolutely.
Chris - So you don't have to add anything.
Harry - Absolutely. All proteins that are in biological systems fluoresce beautifully in the blue violet. So you can pump that system and look at the output signals.
Chris - And because it’s so compact, you could – I presume, take that on the road, put it into a surgery, if you're looking for a certain substance in the blood, you could do that very easily. You wouldn’t have to have loads of equipment.
Harry - That's right. One of our aims is to develop these lasers for developing countries where we’d have portable devices, battery powered to use in the field – for malaria detection, things like that.