In this 2011 Scientific American "Cocktail Party Physics" blog, Jennifer Ouellette explains the structure and properties of photonic crystals -- those found in nature in opals, dragonfly wings and abalone shells, and those synthesized in laboratories for use in optical devices, LEDs, and solar cells. Source: Scientific American, August 26, 2011
Shiny Things: An Ode to Photonic Crystals
By Jennifer Ouellette August 26, 2011
Recently, physicists at the University of Illinois reported success in making very high quality 3D photonic crystals for use in light-emitting diodes — a huge step in making their use in these kinds of optical devices (which also include solar cells of light bulbs) commercially viable. And it inspired me to dig up a 2007 post on photonic crystals, offered here with some minor changes and additions.
Photonic crystals are less exotic than they sound. You’ve seen them (or their effects) if you’ve ever seen the iridescent wings of a butterfly or dragon fly, or admired a lovely piece of opal. Jen-Luc Piquant is particularly fond of opal jewelry — especially when presented to her by an attractive suitor, accompanied by fine wine and lobster thermidor over candlelight and a single orchid.
Jen-Luc has some excellent company sharing her taste in gemstones. Queen Victoria adored all things opal, often giving them as wedding presents, and was thrilled when Australia (then part of the British empire) turned out to be an area rich in opals. In the Middle Ages, according to Wikipedia, “Blonde women wore opal necklaces to protect their hair from losing its color.” They didn’t have L’Oreal back then, I guess. They were also believed to be beneficial to eyesight. Pliny, the Roman historian, believed that opal combined “the beauty of all the other gems.”
Opals are Mother Nature’s photonic crystals — a topic I’ve covered before, both here and for the now-defunct (alas!) magazine, The Industrial Physicist. Way back in 1887, Lord Rayleigh first studied how light interacted with the simplest form of a photonic crystal: a one-dimensional periodic structure (useful today in things like reflective coatings).
It took an entire century to produce 2D and 3D versions, an honor that goes to Eli Yablonovitch and his collaborators, who accomplished the feat in 1991 when Yablonovitch was at Bell Communications Research in New Jersey. They mechanically drilled tiny holes into a block of material now known as “Yablonovite.” As expected, the material prevented microwaves from propagating in any direction (a photonic bandgap). And the field exploded, although it took more than a decade to make photonic crystals that blocked near-infrared and visible light.
Mind the Bandgap
Here’s why photonic crystals are so unique: The atoms or molecules in a photonic crystal are arranged in a precise lattice structure, similar to a 3D honeycomb, or an egg carton. Depending on the spacing between those building blocks, this creates a “photonic bandgap”: certain frequencies of light are blocked, while others are preferentially let through. There’s been a great deal of interest in using photonic crystals as a replacement for the pricey and sometimes toxic dyes used to color clothing or buildings, or even cosmetics. L’Oreal has a line of makeup featuring these iridescent effects, in which the colors only appear when the makeup is applied and exposed to light.
Therein lies one of the issues with manmade photonic crystal films to date: they have been highly dependent on viewing angle. Hold up a sheet of one of those films, and “You’ll only see milky white, unless you look at a light reflected in it, in which case certain colors from the light source will be preferentially reflected,” per Jeremy Baumberg, a physicist at the University of Southampton in the UK. Change the viewing angle, and the perceived color changes. (I wrote a short piece on his work back in July 2007 for New Scientist; the original paper appeared in Optics Express, an online open access journal of the Optical Society of America.)
He thinks that scientists have mistakenly assumed that the same effect was at work in both manmade and natural photonic crystals: the lattice structure causes the light to reflect off the surface in such a way as to produce the perception of color in the human eye, and which color one sees depends on the angle of reflection. But Baumberg says that the natural structures appear to selectively scatter rather than (or in addition to) reflecting the light, more like a diffraction grating.
Look at an opal under a powerful microscope, and you’ll see that it’s made up of spheres of silica stacked together like the oranges and other spherical fruit on display in your standard grocery store. Because those spheres of silica aren’t much bigger than the wavelengths of visible light, the stacking acts as a kind of grating.
The regularity of the sphere sizes and how precisely they are packed together determines the wavelengths of light that will be diffracted and hence the colors that one sees. Baumberg copied that structure when he set about making polymer opal films.
Even though it’s technically a mineral-based gem, opal is considered to be a “mineraloid” because it doesn’t have a truly crystalline structure as science narrowly defines it. Yet that very “imperfection” gives rise to very strong color effects — stronger than the iridescence produced by manmade films.
Figuring that Nature might be onto something, Baumberg used arrays of plastic spheres stacked in three dimensions, rather than the layers used by most researchers doing similar work in the past. L’Oreal’s makeup line, for instance, is made by stacking nanoscale layers of materials like mica or silica, of varying thickness; the thickness imparts the specific color.
That Is So Dope(d)
The colors produced by Baumberg’s films are much stronger, and far less dependent on viewing angle, than earlier versions produced by other researchers. That’s because Baumberg added a twist: he “dopes” his crystals, by embedding tiny carbon nanoparticles wedged tightly between the spheres, a structure he likens to “cannonballs stacked in rubber.”
The end result: light doesn’t just reflect at the interfaces between the plastic spheres and the surrounding materials, it also scatters weakly off the embedded nanoparticles. A similar effect makes the sky look blue, except the effect is enhanced by embedding the particles inside the lattice structure of a photonic crystal. You need both effects, it seems, to get the optical properties you need for these polymer opal films.
In fact, Baumberg points out that many mined opals have been “cooked” — either deep in the earth for millions of years, or by the miners themselves in manmade laboratories — and that process produces organic nanoparticles around the silica spheres that make up the opal. It’s especially pronounced in the orange-y Australian fire opals.
(The amazing photograph at left, by the way, is just one of many close-up shots of common objects made by Florida State University’s Molecular Expressions Website: “Exploring the World of Optics ad Microscopy.” I encourage readers to do their own exploring. The opal page is here, but it’s a terrific site in general, and one could spend hours — I certainly have — perusing its varied pages.)
Many opals just look whitish without this cooling process. “In trying to glean the secrets from the miners who extract these gems, it seems clear that the ‘recipes’ are exactly of enhancing the scattering that underpins the effect we’ve shown,” Baumberg told me.
Much like natural opals, you can change the resulting color simply by changing the size of the spheres, so it’s another example of a “tunable” material. For green-orange films, Baumberg and his colleagues use 100nm radius spheres. Ergo, it’s ideal for practical applications. But you can also tailor the embedded nanoparticles to suit a particular application.
For instance, you can create nanoparticles that react with certain dangerous chemical agents or toxins. This would cause the film to stretch, thereby changing the spacing between the spheres in the lattice structure — and thus, the color changes, indicating the presence of the target substance. That’s why the military is interested in Baumberg’s work, among others: the films would be ideal for making sensing patches that could be woven directly into soldiers’ uniforms, and could alert them when dangerous toxins are present in the field.
The same effect makes the polymer opal films an excellent candidate for food packaging applications — the area of interest for Unilever. Imagine being able to look in the fridge and tell immediately whether some perishable food item has gone off: the packaging would change color. The nanoparticles would be tailored to react to whatever happens when food spoils, the film stretches, and the spacing between spheres changes.
Use the films to, say, make currency, and you’ve got a handy anti-counterfeit measure: someone could tell whether a bill was genuine simply by twisting or stretching it to see if it changed color. EADS has something less exotic in mind: they want to use the films to coat the tails of its Airbus craft. Because colors = pretty! For his part, Baumberg’s interested in seeing if he can use fibers woven from his new material to make a flexible display.
Disorder in the Court
There are other physicists experimenting with “doping” photonic crystals, just to see what happens. Last November, there was a paper in Applied Physics Letters about recent work by a team of Italian and German physicists who have developed a new, flexible fabrication technique for “rewritable” photonic crystal devices. Unlike Baumberg’s work, which is focused on more consumer-oriented applications, these researchers are interested in using the devices in optical computing, where photons process information in much the same way that electrons do in our present computers. They take a 2D lattice of tiny pores arranged in a beehive pattern, and insert defects by injecting different materials directly into the pores.
Baumberg has experimented in the past with 2D films for optical chips, and for LEDs, made of very dense glass and semiconductors — it’s how he got into the field, when Merck asked him about using photonic crystals for optical devices. Such films aren’t useful, he says, unless they have a very high refractive index contrast, and are also much more expensive to manufacture when compared to polymer opals, because optical chips simply “can’t tolerate disorder in the lattice.” The polymer opal applications actually require a certain disorder; the spheres don’t need to be perfectly stacked, “in fact, it’s better if they are not,” says Baumberg.
This tolerance for a bit of messiness is the biggest reason that Baumberg’s German collaborators at DKI in Darmstadt have
been able to mass-produce the films in rolls as long as 100 meters — the first time scientists have been able to do so and still get materials with such strong color properties, that can be so easily tailored to such a wide range of commercial applications.
In nature, opals tend to form over millions of years, the result of sandy sediments along the shorelines of seas or oceans depositing silica in some type of fluid solution into cracks in rocks, layers of clay, and even a few fossils. When the silica solidified, some of it turned into opal. While not copying this process exactly, there are some common elements with the approach taken by Baumberg and his German collaborators to make opals.
The manufacturing process doesn’t involve any advanced lithography or etching, just a couple of simple polymers. They use a kind of directed self-assembly, first growing polystyrene spheres in a flask — in batches of an impressive 350 kilograms, no less. Once the spheres reach the right size (about 200 nm in diameter), they stop the growth process and irradiate the spheres with UV light to harden them. The spheres are then coated with a sticky outer later of another soft polymer (poly-ethyl-acetate, or PEA, for the curious).
Per Baumberg, the spheres are then “squished” at 150 degrees Celsius. This makes the outer shell runny — the shell melts, basically, into a fluid, so there are naturally generated shear flows that take place. “All the spheres run over each other and want to line up in a beautiful lattice,” he wrote in answer to a hastily emailed query. “Nature appears to favor the spheres lining up together when they flow over each other, but we are [still] trying to understand exactly why.”
So now we have lovely natural-seeming photonic crystals, with an imperfect photonic bandgap effect because at this point, all we have are visible Bragg reflections, although the material is both flexible and malleable, changing color as it bends and stretches. Introducing much smaller (less than 50 nm) carbon nanoparticles into the interstices of the lattice structure is the final touch, bringing in the all-important diffraction grating/scattering effect. You maintain the lattice structure, and yet also enhance the desired color effects, resulting in a material that provides strong colors and less dependence on viewing angle.
I suspect Baumberg is quite intrigued by the recent work by Paul Braun at the University of Illinois, who has succeeded in building 3D photonic crystals for LEDs and similar optical devices. This is desirable because in 3D, it’s possible to control how light propagates in all dimensions (2D crystals only control how light propagates in two dimensions.) The key is his manufacturing method, a point neatly summarized by Technology Review:
Making these structures is tricky. Photonic crystal structures vary, but they’re often made by drilling nanoscale holes, rods, and other features into a material. Patterning a flat slab of material with the necessary nanoscale structures to make a two-dimensional photonic crystal is a relatively simple process. It’s far more difficult to get that kind of patterning into a thick chunk of material to make a three-dimensional structure without degrading the material.
And the kinds of photonic crystals that are most useful—those that can actively convert between electrical signals and optical ones, in addition to precisely manipulating the flow of light—are the hardest to make because material flaws are introduced during the process. This light-to-electricity and back conversion is critical in LEDs, solar cells, and optical data interconnects for computing.
I also have a soft spot for nacre (a.k.a., mother-of-pearl), particularly of the type found inside abalone shells; it’s also produced by pearl oysters and freshwater pearl mussels. The substance is secreted by epithelial cells that make up the mantle tissue of those aforementioned organisms, and it’s secreted constantly, deposited onto the inner surface of the shell — as a means of smoothing the interior, and as a defense against any invading parasitic organisms or foreign objects. When invaders do get in, the mollusk responds by encasing them in successive, concentric layers of nacre, ultimately forming a pearl.
Like opals and other natural photonic crystals, nacre exhibits iridescence, although it doesn’t seem to arise from a lattice structure; it’s more like bricks in parallel stacked neatly together. Nacre is mostly made of nano-thin layers of calcium carbonate crystal called aragonite, and just as with opals, the iridescence is due to the interference of light as it reflects from the those crystalline layers.
A July 29, 2007 paper in Physical Review Letters shed some light on the microscopic architecture of nacre and how it might grow, according to a 2007 article in Physical Review Focus. This in turn, could explain why nacre is so resistant to cracks — 3000 times more resilient than its organic and crystalline components.
Researchers at the University of Wisconsin, Madison, used a finely tuned, polarized x-ray beam to investigate the structure. The crystals are shaped like irregular disks and arranged into alternating tablet and scaffold layers. The Focus article describes it as being “a bit like single layers of coins touching at their edges, alternating with sheets of paper.”
As with Baumberg’s polymer opals, nature’s imprecision has a purpose: the Madison group’s theory is that because nacre forms layer by layer, those layers aren’t perfectly aligned, although they do tend to form ragged columns that resemble “snaking stacks of quarters.” Small holes pierce the layers, and these could be scattered quite randomly rather than being vertically aligned. The resulting structure is more resistant to cracks because the cracks can’t propagate as easily — they keep running into these holes, which serve as crystal boundaries. Considering that nacre forms from the biological processes of living creatures, perhaps it’s not surprising that its structure isn’t entirely determined by the usual intrinsic properties of crystal growth.
See? Opals and other shiny things aren’t just about aesthetic beauty.
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