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Refracted brilliance: How nature’s structures produce colour

By Shane Huntington.

Physicist Professor Ullrich Steiner explains how nature generates vibrant colors, as seen in many butterflies and beetles, through the structure of materials, and how these properties can be usefully reproduced.

SHANE HUNTINGTON
I'm Dr Shane Huntington, thanks for joining us. If we look at evolution on earth over the last half a billion years, it is hard to find an animal or plant species that has not adapted to the use of light in some way. Whether we are talking about high altitudes and the visual acuity of the eagle, or the production of light by the strangest of creatures from the oceans deep, some form of vision or camouflage is important to survival. Of course, many animal and plant species go a step further and make use of different colours to provide advantage. The flower, with its brilliant colours, attracts the bees crucial to pollination and the butterfly wing provides camouflage against predators. But colour is not limited to just choosing the right pigment. Any of us who have held a prism up to the sunlight know that the prism, which itself is free of colour, is nevertheless capable of producing colour. A DVD or Blu-ray disc held up to the light will yield a similar effect. Physics tells us that there are many ways to produce colour and if we look closely enough we will find all of those methods being utilised in nature. So how is colour produced in some of these more exotic structures? Are there advantages in creating such structures for our everyday objects? And what can we learn from the way nature produces these structures? To answer these questions we are speaking today with physicist, Ullrich Steiner. Ullrich is Professor of Physics of Materials at the University of Cambridge and is visiting Australia on the Selby Scientific Foundation Fellowship. Welcome to Up Close, Ullrich.
ULLRICH STEINER
Thank you for having me here.

SHANE HUNTINGTON
Now, when we think about colour, a piece of fruit or a peacock's feather or something of this type, we often think of this in terms of pigments, the materials that are in there, and those materials have particular colours, but you are interested in structural colour. What exactly is this?

ULLRICH STEINER
Well, what pigments do is pigments absorb a certain part of the optical spectrum, so they take away part - a certain colour range of white light. Structural colours don't do that, structural colours superpose certain parts of the optical spectrum in such a way that, say for example, a blue colour is reflected and the rest of the optical spectrum is transmitted. The reason why this produces more brilliant colour is the pigments, they don't only absorb say the red part of the spectrum and scatter and reflect the rest, but they take a little bit away from all the optical spectrum, so the colour tends to be a little bit more dull, less bright. The structural colours don't do that, so you'll account for each part of the optical spectrum either in reflection and transmission, and if that is utilised in an effective way, that causes these really brilliant colours that we seen in birds and butterflies and so on.

SHANE HUNTINGTON
Can you give us some examples of structural colours? You mentioned birds and butterflies, but where are some of the more amazing examples we see in nature where this is occurring, where we would perhaps otherwise not think it's different to pigment?
ULLRICH STEINER
Well, the most commonly cited examples are of course birds and butterflies, simply because we can see them most easily. Sometimes we see structural colour in a context where it doesn't have a function, for example mother-of-pearl nacre is coloured but it's on the inside of the shell and it's a purely accidental effect of the structure that is produced in the way nacre is assembled. There are also effects that are rare in plants, for example, until recently it was thought that plants use only pigments. We have found quite a number of examples where plants also use structural colours. So some of them are exotic, so there is a fruit, a type of a berry, African berry - it also exists in Australia - that uses structural colours. Essentially it looks like a berry but in fact it's completely dry and the advantage of having such a dry coloured object is that it doesn't wilt, it keeps its colour even when it falls off the plant, and that has advantages for the organism. Another example where we don't see it, many flowers use structural colour in the UV to communicate with pollinators; birds and bees. So for birds and bees, for example, tulips and hibiscus look a little bit like - well if you look at a CD it's iridescent, so they see this type of iridescence that we can't see simply because we cannot see in this part of the optical spectrum.

SHANE HUNTINGTON
When you mention iridescence I immediately think about some beetles and the way their wings have this amazing almost metallic appearance to them. Can you talk us through how that optical effect is created?
ULLRICH STEINER
A metallic surface, take for example, silver or polished aluminium, it is metallic but it is in a way colourless and what that basically means is it reflects all the colours. Now, structural colours are generated by a sequence of layers; one layer alternating with another layer of slightly different optical properties, different refractive index. If that layer stack is absolutely periodic, which means each layer has the same thickness one after the other, then we see a specific colour, say for example, green, because what happens is this stack is just tuned to reflect green colour. Now, what nature does is produce a stack of layers where the layer thickness changes gradually as you go through the stack, starting from a relatively think layer of periodicity, going down smaller and smaller. A stack like that would actually reflect the entire optical spectrum more or less like a mirror. So these beetles with a metallic appearance create a mirror in a rather different fashion by using this graded stack, where the thickness basically is continuously tuned from the red, which is a larger layer thickness, all the way through to the blue smaller layer thickness, reflecting all the colours of the optical spectrum and creating this metallic appearance.

SHANE HUNTINGTON
Now, when we talk about these stacks, we're not talking about something that can have a more or less random thickness. The optical wavelengths we're talking about here from blue to red range from 400-odd nanometres up to 700 nanometres. How precise do these stacks have to be? This is an incredible function of nature to be able to produce these, to get these effects.
ULLRICH STEINER
A few years ago we did an experiment in the lab where we just made single layers and were trying to find out how much colour change can we as humans still detect. It turns out if you change the layer thickness by somewhere between five and ten nanometres, it produces a colour difference that you can actually see, discern. Well, that's an amazingly small distance. So if you want to have a very well defined colour, you'll have to control your layer thickness by these few nanometres. If you use a stack you have to be even more precise, because small changes will actually slightly change the pitch of the colour and like probably most animals that can see colour, we are extremely sensitive to small colour changes.

SHANE HUNTINGTON
These materials that are in use, say for example, in the beetle's wing, are the layers all of the same material, or does there have to be some sort of difference in what's in them? Are the layers separated by materials? How would we copy what they're doing? What sort of materials would we use?
ULLRICH STEINER
The essential properties is that you have alternating layers of different refractive index. So the refractive index is how light gets refracted, how it gets essentially reflected and bent into the material as it propagates through it. So we need to have two different refractive indices in an alternating stack and we can do this, or nature can do this, in two different ways. One is simply using two alternating materials, the other thing is you can also use one material but alternate it between a stack that contains air, or mostly air, and one that contains mostly the material itself. That would also create then such an alternating stack, because air has of course also specific refractive index, which is defined as one.

SHANE HUNTINGTON
We often think about light just transmitting through materials or reflecting. When we hit one of these stacks, can you describe for us what's actually occurring, because I think depending on how you view light, as a particle or a wave, there's a different image in the mind of that particle passing through one stack, maybe the next and maybe being affected by the third. What is actually occurring in terms of the way the light hits these stacks and how it comes out? Like what's changing?
ULLRICH STEINER
It's probably easiest to visualise light as a wave in this particular case. The first simple experiment one has to do is looking at the interface between, as I said, these two optical materials that I'm talking about, say air and glass. So when light hits the surface of a piece of glass, then part of that light gets reflected and part of the light gets transmitted into the glass. It gets bent as it does and so we basically divide up the light beam into two parts; a reflected and a transmitted part. Now, if we make a layer of glass, say, surrounded by air, then of course we have two surfaces, one at the top and one at the bottom and we have this process occurring both at the top surface, where part of the light is reflected, and at the bottom surface, where part of the light is reflected. Since light consists of all the different colours, each colour is a wave of a given wavelength, then the light beam that reflects from the top surface and the light beam that reflects from the bottom surface, they superpose. So here we have this principle that if wave maximum comes in the way of maximum and wave minimum in the way of minimum, we get amplification of the light, so we see, for example, that the light will be amplified say for green light. If that doesn’t happen, if they are not what the physicists call in phase but they are out of phase, which means maximum meets minimum and vice versa, then that light cancels out. So we can have the situation where we have basically maximum meets maximum for green light, but maximum meets minimum for red light. So we see green light reflected and red light transmitted, for example. And we of course can see this in every day features. So an oil spill on a water bottle is coloured, it's a single film and we see these different colours simply because the film thickness varies a little bit from place to place over the puddle. Or a soap bubble which is coloured in all colours of the rainbow, that comes from the fact that the soap film has a certain thickness, just the right thickness to reflect these different colours.

SHANE HUNTINGTON
Now, this is quite distinct from the idea of pigments, isn't it, because there you have, as you mentioned earlier, a scenario where a broad range of colours are essentially extracted, absorbed by the pigments. Whereas in this case, based on the thickness of these layers, you could presumably tune them to only reflect a very, very narrow range of colours, or even one colour. Is that right?
ULLRICH STEINER
Yes, that's right. So it's a very narrow band of colours and of course technologically we have been using these all along. Many optical filters work according to this principle. We are creating thin films to specifically select a very narrow range out of the optical spectrum, which essentially is just what you call one colour. Nature does this very effectively, so if you go through a collection of beetles, weevils or butterflies, you can see that they are very good in creating just one very specific, very brilliant colour. It's very difficult to do this with pigments; pigments are just not specific enough or narrow enough in the amount of light that they absorb or reflect.

SHANE HUNTINGTON
You're listening to Up Close. I'm Shane Huntington and today we're talking about how nature produces colour with physicist, Professor Ullrich Steiner. Ullrich, there are obviously a number of ways to do this. You have a scenario where we've been talking about essentially interference, so where you take a wave and you either add it or subtract part of it to another wave and you either enhance or decrease the intensity of that particular colour, or even completely remove the colour. But we also see, as in the introduction, I mentioned something like what happens with a prism where light is diffracted and you get colour, what sort of examples in nature do we find where that occurs?

ULLRICH STEINER
I mentioned one before which is an interesting one, so one of the effects you can have is again what is called a grating. It's basically just an array of thin lines and if the distance between the lines is again comparable to the wavelengths of a certain colour of the optical spectrum, then we get an interference effect which doesn't come from different layers, but comes from stripes essentially. The closest example that we know are, for example, the colours you see from CDs and DVDs. So what you have there is essentially you have a metallised track landscape, the tracks basically encode the information that the player extracts. It turns out that the distance between the tracks is comparable to the wavelengths. So when we hold it up in front of our eyes, we again see colour. Now, here the difference is you don't really produce one single colour, as you can do very easily with a stack as I described before, but you always have the entire rainbow present across the optical spectrum. So this is also something that one finds in organisms and the organisms we have concentrated on are actually flowers. It turns out that on the surface of flower petals one finds these striped patterns. The biologists call them striations and for quite a while it was not entirely clear what the function of these stripes are. And we've found out that when these stripes actually do create this kind of colour pattern, this interference pattern that we see from the CD, it's used to communicate with bees, for example, which is something that we demonstrated. So what we did is we actually reproduced a CD pattern and created an artificial flower from it, then offered that to the bees and the bees could actually distinguish between artificial flowers that had that effect, this iridescent grating effect, from the ones that don't.

SHANE HUNTINGTON
So you're talking about ultraviolet light there and often when people hear this, they think it's one type of colour, but actually it's an entire range. You're saying that you can actually modify that range in such a way that the bee is able to determine when a plant should be landed on essentially.
ULLRICH STEINER
Right, so essentially what it does is it enhances the reflection of a particular range of this UV light. How we model this is essentially when you rotate a CD in front of your eyes, the colour actually changes. So if you rotate it forwards and back it creates a kind of flickering effect. So the pigment colours, they are static. They look the same from every angle, so if you look out over meadow [of] flowers, you have all these different colours but they are all static, so it doesn't make it different from which angle you look at it. Now, imagine if just one flower has invented the trick of a changeable colour, a colour that flickers, so that would immediately draw the attention of the bee to it. So the flower is basically saying look, choose me out of the all the sea of flowers out there, and the bee apparently responds to that. So this is one of the strategies that has evolved to make some flowers more conspicuous to bees than others.

SHANE HUNTINGTON
It's interesting when we talk about the evolution of this, because it would appear that it's an accident just of size; certain size features on the flower that just happen to get in the right range where they've started to cause this effect, but then it's spread as an advantage through the system. Do we see other things like the sorts of crystalline structures in opals, for example? Obviously an opal doesn't have an advantage in having a crystalline structure for survival, but do we see that type of more complex structure in nature as well?
ULLRICH STEINER
Yes, so coming back, if one looks at weevils, beetles, butterflies, there are many very complex optical arrangements. So they're not just simple multi-layers, but many of them have three dimensional structures that create a very complex optical signal. And very often this optical signal has a specific function for the organism. So one of the organisms we looked at is a specific type of butterfly which looks green. Green is actually a good colour for the butterfly because of course it can hide against the green foliage, but it also has a disadvantage from the point of view of mate recognition. So female and male butterflies, they don't see each other very easily. So the butterfly has solved this problem by creating the green colour in a rather complex fashion. So if one looks very carefully at the wing scales of this butterfly, one actually sees that it has a pixelated type of colour, where each pixel has a yellow part and a blue part to it. If you look at this from afar, as everyone does, the blue and the yellow combines to green, so it appears to [be] green. Now, it turns out that one can create a filter that sees only the blue or the yellow colour. We are not entirely sure about this, but we know that some of the insects actually have eyes that are equipped with these filters, so it's likely that this particular butterfly has it as well. So you could create a situation where a butterfly sees another butterfly blue, but the predators would actually see the same butterfly as green. So it basically has both advantages using very clever tricks to modulate the light.

SHANE HUNTINGTON
It's extraordinary. Now, there will also be scenarios where it's not colour that's important, but in a sense the absence of it. So presumably these structures could equally be used to essentially absorb all wavelengths so that certain insects, animals and so forth are essentially invisible to predators.
ULLRICH STEINER
I'm not entirely sure if this invisibility effect is the right thing, but the absence of reflected colour is a very important principle. In fact we use it on our eyeglasses. We can put a coating on glass that reduced the light that gets reflected from the surface of the glass. In nature we see this on the eyes of again some moths and butterflies, which have such an unreflective surface and it presumably has two biological functions. The first thing is if you are trying to hide somewhere, a light reflecting surface is not a very good thing to have. Secondly, moths fly at very low light conditions, so they need to have eyes that couple in as much light as possible. I suppose it's a combination of these two advantages that have led to the evolution of these layers. Now, these anti-reflective layers, they are known to us technologically for more than 100 years, but we have a technological problem there. That is the layer has to be made of a material that has a very low refractive index and materials that have a sufficiently low refractive index to perfectly anti-reflection coat glass don't exist. Nature has found a way around this, so what nature does is it creates a surface structure where the structure - these are little bumps, if you want, they are so small that they cannot be individually seen by the light. So the light kind of averages over that material and so by modulating the structure, one can dial in any refractive index that you want. So what you can do is you can dial in the refractive index that is precisely the right one to prevent reflection of light. This is well known for nature and we can simulate that in artificial ways to create close to perfect anti-reflective coatings.

SHANE HUNTINGTON
We've been primarily talking about animals and plants and so forth that we find on the land. As you move into the oceans, of course, the situation changes quite dramatically, because you have to deal not with the refractive index of air, but with the refractive index of saltwater. What does that do in terms of the sorts of structures that we find in the ocean? People would be perhaps aware of some unusual creatures like the lobster, that have these very unusual channelled eye structures, but what sort of predator/prey relationships do we see in terms of this structural colour use in the oceans?
ULLRICH STEINER
This is not really an area of my expertise, I don't know much about structural colour underwater, but of course one effect is quite striking, that's just simply the scales of fish. So they are rather silvery and the purpose of that is to basically reflect the light that matches the background in which they live. So the reason why the fish have created these surfaces which are very often structurally coloured is so they can disappear in the background. This is a rather complex problem which I don't know much about, but it has to account for the angle of the sun that is overhead, because it's easy to create basically a coating, a scale, that lets the fish disappear under certain lighting conditions, but the same fish would then show up under different lighting conditions. Fish are really good in disappearing under almost any lighting conditions and as I said, I don't really know too much about the details, but there is a lot of variety of structural colour underwater and again combined with pigments can create all kinds of effects which you have in coral reefs and fish and what have you.

SHANE HUNTINGTON
You're listening to Up Close. I'm Shane Huntington and today we're talking about how nature produces colour with physicist, Professor Ullrich Steiner.Ullrich, given the intricacies of some of the structures that you've been describing, is this something that we've come across recently, structural colour? Is it something we've known about for a long time? There are some examples, I think, of pottery and so forth from hundreds of years ago, where it would appear that structural colour has been used, but perhaps without understanding.
ULLRICH STEINER
I think we know how layer interference works for a very long time, since Bragg. From an optical point of view we know how structural colour works very well and I think it has been described in nature over the years very well as well, but we still are surprised by finding new examples of how nature uses, how it modulates light in unforeseen fashions. I think it's not correct to say we know everything about structural colour in nature, because we continue discovering new effects.

SHANE HUNTINGTON
When we bring it in now from nature into our everyday environments, where do you see the use of structural colour being applied to our many and myriad devices that we have in our everyday lives?
ULLRICH STEINER
Well in technology, as I said, one of the ways you have them is on say, for example, in the anti-reflective case and eyeglasses and camera lenses, we use filters that are based on these effects. We don't see it actually in too many cases to create colour directly, presumably because it's relatively expensive to make. So we can, of course, technologically very easily create this stack structure that we've been talking about, but it is a bit more expensive to make than just to create pigment and mix it into whatever object you want to have. So we find it more in a technological setting rather than in an everyday kind of setting.

SHANE HUNTINGTON
I suppose people would be familiar with the type of holographic structure they'll find as a security measure on their credit cards. Presumably this is an example of structural colour.
ULLRICH STEINER
Yes, it's a grating interference and it's modulated in such a way that it creates a holograph. Holographs are actually interesting objects, they are intrinsically just these line patterns that we were talking about before, but they are sculpted in such a way that they can recreate an object that looks almost three dimensional in front of you.

SHANE HUNTINGTON
Now, we obviously have access to a lot of different materials that you could argue nature has not had access to. Does this give us certain advantages, especially some of the particular metals and so forth we can refine to make very unusual types of structures that give us different effects?
ULLRICH STEINER
One of the things we can do is we can amplify the effects that we find in nature, simply because we can use materials that have properties that nature cannot easily do. For example, nature is limited by the refractive index of the material it has access to, so it actually has to use, for example, many, many layers to create a certain effect. Whereas we can do something similar with fewer layers, simply because we have materials that are much more strongly refractive. Also we have access to metals which have interesting optical effects that nature has a much more limited access to.

SHANE HUNTINGTON
Tell us about that. What do metals offer us? I think most people would think you start putting metal in and things just become opaque like metal, but on this scale something else is happening.
ULLRICH STEINER
Well if you use nanometals, small spheres, then what one has is the light interacts with the electrons inside the metal and creates a resonance. So what you do is you'll actually have rather than all of the light reflected back, which is what a normal plain metal surface would do, only part of the optical light is reflected back if you have a small nanosphere say of gold or so. Well, the Romans used that, they used that to stain glass, or very often the stained glass actually contains more metal particles. One can tune the colour by changing the size of the metal particles, so this is one of the tricks that we can use which I don't know of an example in nature where nature uses this particular effect.

SHANE HUNTINGTON
I understand that if you were to take a layer of gold and you keep thinning it out and thinning it out, at some point the pigmentation effect actually no longer works and it changes colour, is that right?
ULLRICH STEINER
Yes, you can actually create gold in almost any colour and we showed this in the lab. So if you take gold and you structure it on a very small length scale, on a length scale of about 10 nanometres or so, gold suddenly starts becoming transparent and it starts becoming coloured, depending on what kind of structure you use. So this material people call metamaterial. In a sense it plays with the light in a very funny fashion, one can even create optical effects that cannot be created in any other way.

SHANE HUNTINGTON
Many people would be aware of some of the applications of some of these technologies already, of course. The stealth bomber from the United States is well known and it is essentially one of these scenarios where a particular wavelength, being radar, is not able to be reflected off this device. What sort of things do you expect to see in the coming decade or two with regards to more use of this technology in everyday lives of our listeners?
ULLRICH STEINER
I think what we can do is we can actually control light with more precision than we are able to now. If we, for example, master these metamaterials, we can create lenses that are more powerful, that focus light much better using smaller objects. So that will help us on the one hand, of course, to miniaturise existing devices such as cameras and so on. It will also possibly help us to build a new type of device, such as an optical computer. Will this all happen? Well, it's an interesting question. From the scientific point of view we are interested in the basics of how this all works. Well, experience shows us once we figure out how things work we usually employ them in new types of devices and very often they are quite different from what you have imagined at the outset.

SHANE HUNTINGTON
Ullrich, thank you very much for being our guest on Up Close today.
ULLRICH STEINER
Thank you very much for having me.

SHANE HUNTINGTON
Ullrich Steiner is Professor of Physics of Materials, University of Cambridge.

University of Melbourne