There is something stunning about the colours of a peacock feather. It’s not just a simple matter of the sort of coloured pigments an artist mixes up on a palette. The colours in the feathers almost glow in their iridescence, changing subtly with angle to catch the eye. To produce this effect, the feather contains a natural nanotechnology that has the potential to transform optics when this remarkable approach is adapted for use in human technology.
Both the iridescence of that peacock’s tail and the swirly, glittering appearance of the semi-precious stone opal are caused by forms of photonic lattices. These are physical structures at the nano level that act on light in a way that is reminiscent of electronics, like the semiconductors that act to switch and control electrons, giving unparalleled manipulation of photons.
The colours of the peacock feather bear no resemblance to those of a pigment. In blue paint, for example, the pigment is a material that tends to absorb most of the spectrum of white light but re-emits primarily blue, so we see anything painted with the pigment as being blue. In the peacock feathers it’s the internal structure of the feather (or to be precise the tiny ‘barbules’ on the feather) that produce the hue.
The colouration is primarily due to internal reflections off the repeated structure of the barbule, similar to the way the lattice arrangement of a crystal can produce enhanced reflection. What happens is that photons reflected from a deeper layer are in phase with those from an outer layer, reinforcing the particular colours of light (or energies of photons) that fit best with the lattice spacing. This is a photonic lattice. These effects depend on the angle at which the light reflects, giving the typical ‘shimmer’ of iridescence.
The practical applications of artificially created photonic crystals can do much more than produce a pretty effect and striking colours. Because a photonic lattice acts on light as semiconductors do on electrons, they are essential components if we are ever to build optical computers.
These theoretical machines would use photons to represent bits, rather than the electrical impulses we currently employ in a conventional computer. This could vastly increase the computing power. Because photons don’t interact with each other, many more can be crammed into a tiny space. What’s more, one of the biggest restrictions in current computer architecture is the complex spaghetti of links joining together different parts of the structure. With photons, those links can flow through each other in a basket of light – unlike wires and circuits, photons can pass through each other without interacting, allowing more complex and faster architectures. Equally, optical switching – and in the end, a computer is just a huge array of switches – could be much faster than the electronic equivalent.
There are significant technical problems to be overcome, but the potential is great. Photonic crystals are already used in special paint and ink systems which change colour depending on the angle at which the paint is viewed, reflection reducing coatings on lenses and high transmission photonic fibre optics.
Another example of nanotechnology having a quantum effect on light is plasmonics. Something remarkable happens, for example, if light is shone on a gold foil peppered with millions of nanoholes. It seems reasonable that only a tiny fraction of the light hitting the foil would pass through these negligible punctures, but in fact in a process known as ‘extraordinary optical transmission’ they act like funnels, channelling all the light that hits the foil through the sub-microscopic apertures. This bizarre phenomenon results from the interaction between the light and plasmons, waves in the two dimensional ocean of electrons in the metal.
The potential applications of plasmonics are remarkable. Not only the more obvious optical ones – near perfect lenses and supplementing the photonic lattices in superfast computers that use light rather than electrons to function – but also in the medical sphere to support diagnostics, by detecting particular molecules, and for drug delivery. Naomi Halas of Rice University in Texas envisions implanting tiny cylinders containing billions of plasmonic spheres, each carrying a minuscule dose of insulin. Infra red light, shone from outside the body, could trigger an exact release of the required dose. ‘Basically, people could wear a pancreas on their arm,’ said Halas.
Over the last seven weeks since the first post, we have explored a wide range of the ways that nanotechnology, given a push in the right direction by nature, is starting to be important in our lives. At the moment we are most likely to come across relatively simple applications like the nanoparticles in sun block or technology making fabrics and electronics water repellent.
As our abilities to construct nanostructures improve we will see increased use of the likes of carbon nanotubes and the nano-optics described in this piece. And eventually? It is entirely possible that we will see Richard Feynman’s 1950s speculation about nanomachines come to fruition, though they are likely to be more like the ‘wet’ machines of nature than a traditional mechanical device.
When nanotechnology appears in the news it is often in a negative light. We might hear that Prince Charles is worrying about the threat of grey goo, or the Soil Association won’t allow artificial nanoparticles in organic products. But the reality is very different. Nanotechnology is both fascinating and immensely valuable in its applications. I, for one, can’t wait to see what comes next.
This series has been sponsored by P2i, a British company that specializes in producing nanoscale water repellent coatings. P2i was founded in 2004 to bring technologies developed at the UK Government’s Defence Science & Technology Laboratory to the commercial market. Applications range from the Aridion coating, applied to mobile technology inside and out after manufacture using a plasma, to protection for filtration media preventing clogging and coatings for trainers that reduce water absorption.
Image from Wikipedia