Professor Roy Sambles, an optical physicist, describes how he stumbled across a scientific puzzle that was ripe for exploration

"Why should a moth’s wing, which is made of protein, look golden metallic?

It was this question, arising from the work of some South African researchers, which would simply not leave me alone.

In the early summer of 1996, as I browsed through some science journals over a sandwich lunch, I came across the studies of Brink and his colleagues who were investigating this extraordinary ‘gold spot’ in the wing of the moth Tricoplusia orichalcea.

It really was a puzzle. Nature does not readily incorporate metal, certainly not gold foil, into any living structure so the ‘gold spot’ was definitely not gold. So how was this metallic effect achieved? Was it diffraction? Was it interference? Was it perhaps some combination?

Brink had suggested an explanation based on a combination of effects and it struck me that there was much to be learnt from optical studies of structures like this. Could one make optically ‘metallic’ structures out of almost any old protein and, if so, how did one do this? Were there any other creatures with the same ability to produce metallic effects?

The answer to this last question is an obvious ‘yes’, for even the most unobservant amongst us must have noticed the vivid colours of kingfishers or peacocks or mackerel; colours with a ‘sheen’ to them which are not simply dyes but change with the angle of viewing.

Over the next few months, following discussions with biologists, it became clear that while many biologists had explored this area very few optical physicists had undertaken any systematic studies. It appeared to be an area ripe for exploration, wetting my appetite even more.

Coincidentally, in early 1997, one of my ex-doctoral students, Peter Vukusic, who was keen to return to the excellent Exeter environment, had enquired as to whether there were any new projects I had starting which he could work on as a postdoctoral research fellow. I therefore suggested this area of iridescence in nature, particularly in butterflies and moths. We began a cursory investigation. A rather hurried check of the literature revealed that a large number of butterflies, particularly from the tropics, give strong iridescent reflections, more often blue rather than golden, but all strikingly metallic.

Indeed, it is reported that some Morpho butterflies from South America are visible from about a kilometre. Because there appeared to be so much to explore I rapidly, with the help of Robin Wootton, a fellow academic but a biologist, rushed together a funding proposal to BBSRC for funding to allow Peter to explore this uncharted territory. The application was fortunately successful, the work commencing in earnest in January 1998, about eighteen months after the paper by Brink and colleagues had caught my attention.

During the three years of the project we have uncovered a wealth of new information concerning the way nature produces these exquisite optical effects. The work has received a substantial amount of media attention, appearing across the World on television and in newspapers.

There is still much to be unravelled. Our latest studies reveal that at least one butterfly species uses an open lattice of protein struts in their wing scales in a superb tetrahedral (diamond-like) arrangement producing strong selective colour reflection through the periodicity of the lattice regardless of which direction the light comes from.

These studies (Peter Vukusic has been awarded a prestigious five year Fellowship on the basis of the original work so far completed) continue to elicit support and interest from a wide community. This includes those concerned with anti-counterfeit marking of goods, credit card companies, bank-note manufacturers, safety (high brightness) clothing manufacturers, dye-free paint providers etc. In addition, the knowledge we are gaining from the visible region of the spectrum is being used to guide new work at other, non-visible, wavelengths."

Roy Sambles is Professor of Experimental Physics at Exeter University. As head of the Thin Film Photonics Group his research covers optics of thin layered systems, liquid crystal optics, grating optics, surface plasmons, molecular rectification, butterfly wings, photonic surfaces and microwave structures. He has refereed more than 340 research papers. Professor Sambles is a Chartered Physicist and Fellow of the Institute of Physics. He was awarded the Gray medal in 1998 for his work on the optics of liquid crystals.

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