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Photonic crystals light up tungsten

17 Jun 2002

Research published in this week's Nature reveals how to make 3D, all-metallic photonic crystals with infrared bandgaps.

US scientists have made a three-dimensional, tungsten photonic crystal with a large infrared bandgap from 8 to 20 µm. If this crystal were made into a filament, the efficiency of incandescent bulbs could increase tenfold. (Nature 417 52)

The all-tungsten crystal has many surprising and unexplained optical properties. For example, when heated to above 1500 °C, it converts infrared thermal radiation to visible light.

The team consists of researchers from the Department of Energy's Sandia National Laboratory and Iowa State University. Sandia researcher Jim Fleming said: "The large bandgap forces the infrared thermal emission into a selective emission band in the visible."

Fleming and co-workers say this could potentially raise the efficiency of a standard incandescent electric bulbs from 5% to greater than 60% as energy is not wasted in heat generation.

"An absorption peak near the band edge acts as a channel for light emission," says Fleming. "The conversion of heat to visible light is unexplained and not predicted by theory." One explanation involves variations in the speed of light as it propagates through the structure.

Because metals are dispersive and absorb in the infrared and visible spectrum, studies of metallic photonic crystals have so far have concentrated on microwave and millimeter wavelengths.

"The tungsten 3D photonic crystal was made by selectively removing silicon from already fabricated polysilicon/SiO2 structures," explained Fleming. "The resulting mould is then back-filled with chemical-vapour-deposited tungsten."

He continued to say that this method could be extended to create almost any 3D metallic photonic crystals with infrared bandgaps. Such crystals have never been made.

To test the crystal's properties, the team used a Fourier-transform infrared measurement system. High reflectance and low transmittance for wavelengths greater than 6 µm confirmed the existence of the bandgap. A strong absorption peak at 6 µm highlighted the photonic band edge whilst light at 5 µm was transmitted through the crystal.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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