Imitating a butterfly's wing

At the Nanophotonics centre at Cambridge University a team of researchers are using this technology to developing new materials including a polymer which changes colour as it stretches. Kate Lamble spoke to the head of the centre, Jeremy Baumberg.

Jeremy - So, nanophotonics is a way of trying to control light in a new way. Nano are sort of small things and nanoscale is something on the scale of maybe tens of atoms, hundreds of atoms. It's a billionth of a metre, but it's also a way of trying to build things on that length scale so that we actually change the way that light interacts with matter.

Most of the clothes that we're wearing, they're coloured, but the reason they're coloured is that they've got molecules in them and when we shine light on them, some of the electrons in those molecules absorb particular colours of light. We see the rest of the colours of light coming out. So, the idea of nanophotonics is, can we design materials where they have this really intricate architecture on a really small scale, which is smaller than the wavelength of light to change the colour of the material.

Kate - So in nature, what animals or what plants do use colour in a different way from how we use it?

Jeremy - So, the classic example is a butterfly. So, in nature, it's really difficult to make a good blue colour and you can actually see that from blue M&Ms or Smartie's where they've actually tried taking out some of the artificial dyes and put in natural ones, and they're really poorly coloured. So butterflies solve this problem not by using a dye, but often by actually having really intricate wing scale structures. What they do is they scatter blue light very strongly, but absorb the other light. So, only blue light comes off and you have this wonderful iridescence.

You can also see it in plants. That's actually the cellulose in the plant walls which is spiralling in a certain way and that generates colour in the same way. We're just about to start working on squid and clams, and they also have very strong iridescent colours which they generate by layers of proteins in their cells.

Kate - We were talking earlier about the colour of our clothes. I'm wearing this really bright pink jumper today, but that's all wool. I suppose when we're looking at the structure and imitating the structure of nature, we have to look at different materials rather than the ones that we're used to.

Jeremy - So sometimes, we actually do use the materials that's in nature. As I was talking about, cellulose is just wood pulp. But often, we're trying to use materials which we control and produce very simply so often polymer materials. So, I have in front of me a piece of a material that we've been playing with for the last 5 years or so and that's actually made of spheres of a polymer which is polystyrene. So basically plastic bags. What happens here is if you take a sheet of this material, it's transparent. But if you chop it up finer and finer, it starts to get a colour and what makes it really work is we make tiny spheres of this, less than the wavelength of light. We've learned how to make them so they stack up in regular arrays. When they stack up, they make wonderful iridescent colours. So, this piece here is a wonderful bright green. The nice thing is, because it's made out of an elastic material, when I stretch it, it changes colour because I'm changing the separation of all the different components.

Kate - This material is amazing. I mean, holding it just in your hand, it looks a bit like the back of a beetle, that iridescent shine that you get in a back of a beetle. But when you stretch it, it goes from a green to a definite blue. Why does that happen?

Jeremy - So, what happens here is that we have balls, all sitting in regular sheets. So, imagine you're taking marbles and putting them in a tray. We have a whole set of the balls like that and then another layer on top, and another on top of that. The light bounces against these stacks of spheres and when it bounces against each layer, there's a small amount of the light that's actually scattered in different directions. When you add up all the waves from the light reflecting at different interfaces, only for some colours do they add up constructively, do they add up to make a strong amount of light coming off.

It's a bit like the phenomenon of waves. You can see waves bouncing around near the shore of a sea and when the wave patterns come together, you get these patterns of crests and troughs. That's happening with light here. What we have to do is we have to get the spacing of these layers exactly right. So what we can do is we can change the colour of the material just by changing the size of the spheres. We can make red ones or blue ones or green ones, just by starting with different sizes of the spheres.

Kate - And how did you go about making these spheres and working with them on such a tiny level?

Jeremy - This is in some ways a really simple material, but until now, it's been incredibly difficult to make and it's actually an opal. So, this stacking of spheres is what you find happens in the ground in Australia. It takes 5 million years, high pressure, and special water to do it, and we don't have the time for that. So, what we do is we actually grow these particles. You start by seeding them and then you throw in some chemicals which are called monomers. What you do is you grow a chemical reaction so they actually come stick on the seeds. And you throw in more monomers and they grow more and more, and more. Each seed grows at the same rate and you can stop at any point, and then you've got a whole load of spheres all the same size. So, when you do that and we make a sphere which is maybe a quarter of the wavelength of light across, a couple of hundred nanometres. and then we graft onto to edge of it, we add on a sticky layer - it's like chewing gum.

So, what we end up with is all of our spheres are coated with chewing gum and then we dry everything out. Then what we can do is if we heat that up a bit and we sheer it which is, we actually put it between our hands if you like and move our hands in opposite directions then these spheres have to move over each other. It turns out, when they have this chewing gum coating, as they move over each other, they start to like to form a really regular lattice. So, it's a self-assembly process and that's the secret of what we're trying to do, instead of putting every sphere where we want it. We want all the spheres to go to the right place.

Kate - How long has this sort of work been possible? How long has it been that we've been able to get down to the scale of nanometres and work on this level?

Jeremy - So, it's been evolving over the last 20 years or so and what we've been learning steadily over that time are different little tools and techniques. In that time, what's happened is with these building blocks, we've learned how to grow much better. But actually, how to glue them together, we're still only slowly learning.

Kate - If it's only been around for 20 years and yet, we've got so far in that considerably short period of time, in another 20 years, where do you think it will be and sort of, do you think these type of nanophotonic materials will be in our everyday lives and we'll be taking them for granted?

Jeremy - Completely so. I think the first place they'll come in is in the sort of areas of sensing and health. So, we're starting to learn ways that we can make materials that might respond to thousands and thousands of components that are in our immune systems. So, you get some sort of optical readout. We'll definitely see that within 10 years. I also think we'll start to see materials on buildings where you can change colours. There's a big demand for doing that. Certainly textiles, people are very interested in smart textiles. So, all of this, in the next 20 years, I think will become very, very mainstream.