on butterfly’s wings

One of the best-known poems by Gerard Manley Hopkins — a Victorian-era minister whose writings frequently centered on the glories he observed around him in nature — opens with a tribute to the phenomenon of iridescence: the wings of kingfishers and dragonflies, in Hopkins' poem, but it can also be found in the wings of cicadas, and butterflies, in certain species of beetle, and in the brightly colored feathers of male peacocks. A firm believer in divine hierarchy, Hopkins found a metaphor for man's relation to God in this peculiar attribute of nature: "Each mortal thing does one thing and the same/… Crying 'What I do is me, for that I came.'"

I don't share Hopkins' religious ecstasy, but I have always appreciated his skillful use of language and meter, and his unabashed appreciation for the natural world. Nature fits form to function, and everything has its place in the delicately balanced ecosystem. You don't need to believe in God to marvel at that, or at the many examples of iridescence in the world around us. Equally marvelous is the unusual cause of those bright flashes of hue. The color we see doesn't come from actual pigment molecules, but from the precise lattice-like structure of the wings (or shells, or feathers), which forces light waves passing through to interfere with itself, so it can propagate only in certain directions and at certain frequencies. And the brilliant colors that result change depending on one's point of view. In essence, they act like naturally occurring diffraction gratings.

Physicists call these structures photonic crystals, an example of "photonic band gap materials", meaning they block out certain frequencies of light and let through others. (If you prefer an explication of the science from America's beleaguered pop princess, Jen-Luc Piquant suggests you check out Britney Spears' Guide to Semiconductor Physics. Now that Britney has lost the baby weight and the loser husband, Jen-Luc fervently hopes she will return to her cutting-edge physics research.) This makes them "tunable", particularly the manmade varieties, because of those highly ordered arrays of periodic "holes". Anything tunable is by definition controllable, and therefore useful for practical applications. Photonic crystals are used most often as waveguides for light in telecommunications/fiber optics systems, or other places where scientists want to be able to control either the frequency or the direction of light.

Over the last six months, there's been several interesting new developments in the effort to exploit the features of naturally-occurring photonic crystals in innovative ways. (I've been collecting newsy items on the topic for several months now, in hopes of finally finding time to write a blog post about it. That time has come.) Most recently, in November, Chinese researchers (Jin Zhang and Zhongfan Liu) at Peking University announced that they have figured out a way to use the wings of cicadas as stamps to pattern polymer films at nanometer size scales — a feat that is quite challenging using conventional microfabrication technology. The wings are rigid enough so that when they are pushed down onto a smooth polymer film, that film is imprinted with a negative version of the array pattern.

The wings are also chemically stable, plus they have a waxy coating which results in very low levels of surface tension. This is important, because the wings don't end up getting stuck to the polymer film after imprinting. They can be removed while leaving the stamped pattern intact. That pattern is then transferred to silicon via a more traditional etching process, thereby forming "nanowells" on a silicon chip. Such chips "show promising anti-reflective properties," according to Zhang and Liu, and could be useful for optical imaging, or in the use of Raman spectroscopy for detecting molecules. Liu phrased it best: "There is a lot that nature can teach us about nanotechnology."

Butterfly wings get their color from naturally occurring photonic crystal structures in scales made of chitin, a polysaccharide that shows up in all kinds of insects. Those scales are arranged like tiles on a roof, except they measure a mere tens of micrometers across.  Last September, New Scientist reported that a group of researchers have measured the structure and optical characteristics of the photonic crystals in butterfly wings for the very first time. They did it by studying electron microscope images of the scales. It turns out that each side of the wing contains different photonic structures: a metallic blue produced by single crystals, and a dull-ish green that results from a more random arrangement of crystals. Precisely ordering the lattice structure is critical to achieving the most brilliant colorful effects — and to controlling the propagation of light at the desired frequencies. Which is why telecommunications applications rely on manmade photonic crystals rather than nature's more random arrangements.

Still, butterfly wing crystals can produce green, yellow and blue colors, depending on their overall effect, and the researchers managed to generate red reflections as well. That's significant because such a palette could be used in flat panel displays, simply by mounting an array of crystals only tiny MEMS arms to change their orientation. So any given "pixel" could produce red, green or blue. A September 1, 2006 paper in Optics Letters by a team of Swiss scientists described a similar approach using diffraction gratings and piezoelectric polymers (which contract whenever an electric voltage is applied) to faithfully reproduce a fuller range of colors than can currently be achieved in conventional displays, whether they be standard TVs, LCDs, or plasma screens. (For instance, they can't reproduce the blues observable in the sky or in the sea.) Manuel Aschwanden of the Swiss Federal Institute of Technology in Zurich headed the project, and described the grating as having one side molded into something that looks for all the world like microscopic pleated window shades.

More frivolously, copying the structure of butterfly wings is giving rise to spiffy new kinds of make-up, giving a whole new meaning to the term "butterfly effect." For instance, L'Oreal offers eye shadow, lipstick and nail polish featuring these iridescent effects, bringing nature's beauty to the cosmetics counter. This is achieved by stacking nanoscale layers of materials like mica, silica or liquid crystals, of varying thickness to give each material a specific refractive index. For instance, a stack 80 nanometers high produces blue, while one 120 nanometers high produces red. In the package, though, the stuff just looks white; the colors appear when the makeup is applied and exposed to light. There are the usual concerns about using nanoparticles in cosmetics, when little is known to date about potential health risks, but that hasn't dampened the enthusiasm for such novelties. Yet.

Researchers at the University of Toronto have developed a new elastic type of photonic crystal that changes color with the application of pressure. It also mimics the structure of butterfly wings and opal (the gemstone is another common example of a naturally occurring photonic crystal): it resembles a 3D honeycomb. They hope to develop the material further in hopes of using it to, for example, capture full-color fingerprints. The obvious advantage is the enhanced contrast and sensitivity to detail, making it easier to analyze prints for identification purposes. But any impression picked up by the elastic photonic crystal is visible immediately in bright hues, with no need to first convert that raw data into electrical signals for computer analysis. The Toronto material could also be used as pressure sensors in consumer electronics or airbag deployment — or just for children's toys.

Imagine a toddler being able to squeeze a toy and watch the color change right in front of his/her eyes! Imagine the wonder the child would feel, especially when s/he was old enough to realize that it wasn't magic, but a one that arises from Nature itself, that man has seen fit to copy and put to good use. I think even Hopkins would be suitably impressed at what the scientific study of a simple butterfly's wing has wrought. So it seems fitting to close by quoting another Hopkins' poems, "God's Grandeur." It presents a vivid image of the Holy Ghost brooding over our imperfect world "with warm breast and with ah! bright wings."