Photonic Crystals (Beautiful Iridescent Colors of Nature and What They Betoken)

Today I wanted to write more about giant clams and their astonishing ability to “farm” algae within their body (and then live off of the sweet sugars which the algae produce).  I still want to write about that, but it proving to be a complicated subject: giant clams mastered living on solar energy a long time ago, and we are still trying to figure out the full nature of their symbiotic systems.

Today, instead we are going to look at the phenomenon which gives the mantles of giant clams their amazingly beautiful iridescent color. It is the same effect which provides the shimmering color of hummingbird feathers and blue morpho wings, or the glistening iridescence of cuttlefish.  All of these effects are quite different from pigmentation as generally conceived:  if you grind up a lapis lazuli in a pestle, the dust will be brilliant blue (you have made ultramarine!) but if you similarly grind up a peacock feather, the dust will be gray, alas! This is because the glistening reflective aqua-blue of the feather is caused by how microscopic lattices within the various surfaces react with light (or I suppose, I should really go ahead and call these lattices “nanostructure” since they exist at a scale much smaller than micrometers). These lattices are known as “photonic crystals” and they appear in various natural iridescent materials—opals, feathers, and scales.  Scientists have long studied these materials because of their amazing optic properties, however it is only since the 1990s that we have begun to truly understand and engineer similar structures on our own.

Physicists from the 19^th century onward have understood that these iridescent color-effects are caused by diffraction within the materials themselves, however actually engineering the materials (beyond merely reproducing similar effects with chemistry) was elusive because of the scales involved.  To shamelessly quote Wikipedia “The periodicity of the photonic crystal structure must be around half the wavelength of the electromagnetic waves to be diffracted. This is ~350 nm (blue) to ~650 nm (red) for photonic crystals that operate in the visible part of the spectrum.”  For comparison, a human hair is about 100,000 nanometers thick.

The actual physics of photonic crystals are beyond my ability to elucidate (here is a link to a somewhat comprehensible lay explanation for you physicists out there), however, this article is more to let me explain at a sub-rudimentary level and to show a bunch of pictures of the lovely instances of photonic crystals in the natural world. Enjoy these pictures which I stole!

But, in the mean time don’t forget about the photonic crystals! When we get back to talking about the symbiosis of the giant clams, we will also return to photonic crystals!  I have talked about how ecology is complicated.  Even a symbiotic organism made up of two constituent organisms makes use of nanostructures we are only beginning to comprehend (“we” meaning molecular engineers and materials physicists not necessarily we meaning all of us). imagine how complex it becomes when there are more than one sort of organism interacting in complex ways in the real world!

 

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