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The nitty gritty of the itty bitty

POSTED 31 JANUARY 2008

Reducing reflection
One of the fundamental properties of cars and other, less important, objects, is how they deal with light at the itty-bitty scale. Do they reflect light, like a chrome-plated grille on a classic Rolls? Do they absorb it like an oil-smudged engine block on an old pickup truck, or allow it to pass through, like the split windshield on a 1938 Buick?

For many applications in photonics, the emerging technology of using light for energy, lighting, communication and computing, it would be useful to control reflection, and thus we meet a new coating that is about as reflective as air, which is to say, it's not going to replace the rear-view mirror on your '38 Buick.

Collection of shapes, a white square, a white cylinder with black outline, and a gray shard shape.
Left to right: Reflection from silicon, silicon coated with a conventional anti-reflection coating, and silicon coated with the newly discovered anti-reflection coating. Photo: E. Fred Schubert, Rensselaer Polytechnic Institute.

Although the new coating does not reflect light, it's not a fancy black paint either, which would simply absorb the incoming light and convert it into heat. The new hybrid of non-reflection and non-absorption could be helpful in solar cells, brighter lights made from light-emitting diodes, and a bunch of other photonic applications, says E. Fred Schubert, a professor who directs the Future Chips Constellation at the Rensselaer Polytechnic Institute.

Schubert says he and his collaborators have made a coating that reflects less than 10 percent as much light as the previous champion.

Grainy, black and white side-view of thousands of tiny fibers, much like a carpet sample.
Layers of these nanofibers of silicon dioxide can eliminate reflections without absorbing the light. To ensnare visible light, each layer would be about 2 thousandths of a millimeter (2 microns) thick. Photo: E. Fred Schubert and co-workers, Rensselaer Polytechnic Institute.

The coating also differs from previous efforts because it can capture a broad range of wavelengths, not just one.

Seen from the side, the coating looks like Astroturf squashed to 45 degrees by a big ol' truck tire, with the leaves made of silicon dioxide (the stuff of glass or quartz), which does not absorb photons. "It's an inherent material property, it just lets the light go through without absorbing it," says Schubert. "It transmits the photons, but does not absorb them."

Getting hip to the index
A key characteristic of any material that transmits light is its refractive index, which tells us how fast light travels inside it. If you know the index for two materials that are adjacent to each other, you can predict how light will bend at their interface, and how much light will get reflected.

To design an eyeglass lens or a mirror, we need to know the refractive indexes of air and glass.

Large differences in the refractive index increase reflectivity, and conversely, creating gradual transitions are the key to reducing reflection, Schubert says. "If we continuously change the refractive index from the material's value to the value of air, we do not get any reflection."

And thus the new anti-reflection coating uses up to seven layers of material to gradually change from the refractive index of air to the index of the plate beneath the nanofibers.

Natural solution?
Oddly, although nature had supplied virtually no materials with a refractive index between window glass (1.45), and air (1.0003), the jellyfish has already figured out how to minimize reflection and make itself nearly invisible, Schubert adds. "Jellyfish have the same kind of anti-reflective property. They have layers of dense jelly, watery jelly, and then very water jelly" placed next to the surrounding water, which makes them very difficult to see.

Graceful, silent and see-through, a jellyfish glides through blue water
The jellyfish uses a gradual change in refractive index to disguise itself in the ocean by reducing reflection. Photo: David Burdick, NOAA

Unlike other anti-reflective coatings, Schubert says, his invention works with a broad range of wavelengths (colors), and incoming angles.

But the coating allows the light to pass right through, which matters in many photonics applications, Schubert says. "You want no reflection, but you also want no absorption. You just want to transfer the light."

One obvious use for a non-reflective coating -- if it's cheap enough -- is on solar cells, where it could increase the capture of energy-bearing photons. "If you have a solar cell, you want to avoid reflection from the silicon surface," says Schubert. "But you don't want the light to be absorbed, you want light to go into the silicon and be converted to electricity."

In photonics, he says, it's desirable to "tune the refractive index. You want to manage the photons, control them, and tune the flow of light, you don't want to interrupt it." And this can be done, he says by controlling the placement, thickness and composition of the various coating layers.

The beauty of iridescence.

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