If you look at nature there are some stunning colour effects. Rainbows are amazing things. Stunningly bright, vivid butterflies from South America. Dragonflies. How do they achieve those gorgeous greens? We just need to know!
If you take a butterfly and you look at its wing scales, it has got very tiny wing scales about 15 microns by 100. We found that within the wing scale, which is more or less fingernail material, that the butterfly had created tiny sculpted structures, nanostructures, which gave interference, which gave diffraction. Those effects combined can give you a variety of different vivid colour effects.
From the electron microscope images we discovered an intricate structure in the wing scale of the morpho butterfly. And what we decided to do was replicate that on a much larger scale. Here, we have a structure corresponding to the scale of the wave length of microwaves with detailed ridges, and these Christmas tree structures internally. Then when we shine microwaves at this it responds like light does to the butterfly wing scale.
By unravelling the butterflies we discovered a whole raft of metamaterial type structures. If you structure matter on a fine enough scale, it doesn’t respond in a simple way like a bucket of water or a piece of aluminium. It has new properties, new optical properties, new properties for different wave lengths.
There was a fascinating paper by some South African scientists who were looking at a particular moth. It’s got a gold, metallic, gold spot on its wing and it turns out that it uses a zig-zag grating, which is just basically a grating and you zigzag it. Zigzag gratings haven’t really been studied before and they’re fascinating. They have weird polarisation properties, odd diffraction properties.
We then make zigzag gratings. We then metallise them. And they have very interesting optical diffractive properties.
If flat silver is placed into the scatterometer, all we would see is reflected green light. If we pattern the silver surface with a grating then what we see on the screen is some missing portions of light. The light has gone into exciting surface plasmons.
Scaling up from the visible to microwaves, we make samples of this kind. These scatter microwaves as the butterfly wings scatter light but if we now metallise these they will have very different properties. Properties that you would not achieve in a butterfly wing scale. They will deflect microwaves in particular directions. They will stop certain frequencies of microwaves. You could even make lighthouses, if you like, of microwaves so you can steer them around.
This is useful in a variety of applications. One of these is RFID tagging. RFID tags are going to be placed on many objects to make records of where they are: stock checking, movement of goods, drugs, blood samples etc. Conventional RFID tagging, the success rate of monitoring a large number of RFID tags can be as low as 70-75%. By using these structured metal surfaces we’ve raised that success rate to well above 99.9% which is really a massive improvement.
Our most recent work is to combine the patterned metal surfaces, and the surface waves they have, with some of these new ideas in what are called transformational optics, theoretical developments in recent years where you take an ordinary optical design and use a new mathematical approach to design new types of structures.
One of the things we have been playing with most recently is an object called a Luneberg lens on a surface. If you shine a surface wave, which is a plain wave at the lens, it focuses it to the point on the circumference of the lens.
I go around from place to place talking about physics and I find that from three year olds to 93 year olds, they are still fascinated. How does it all work? That’s it, isn’t it? How does it all work?
About the film
Filmed on location at Department of Physics, University of Exeter, Exeter, United Kingdom.
Director: Martyn Bull
Producer: Thomas Delfs
Camera: Mark Whatmore
Editors: Liam Angell, Mike Willbourne
Cast: Professor Roy Sambles