Light would seem to be a hard thing to hold on to but that's just what so-called photonic crystals can do. Made of microscopic metal bars arranged into interlocking lattices, photonic crystals trap light of a particular frequency inside. And by introducing impurities at specific sites, scientists can bend the light along a prescribed pathway. Now scientists have found a new use for photonic crystals made of tungsten. Instead of bending light inside a crystal, researchers at Sandia National Laboratories have used a tungsten lattice to filter input energy so that it exits the crystal only in desired frequency bands. The heated lattices are said to emit significantly more energy than solid filaments can, paving the way for a possibly superior energy source.
Shawn Lin and his colleagues have been researching the tiny tungsten lattices for a number of years and were the first to take the crystals down to micron dimensions. The lattice they are currently working with is made up of tungsten rods that are half a micron wide and 1.5 microns apart. When the researchers placed the lattice in a vacuum heated to 1,250 degrees Celsius, they found that it converted radiation with an efficiency of 34 percent and emitted about 14 watts per square centimeter. Both of these results are significantly higher than the results predicted by the well-known Planck's law, which models the maximum emissions expected at specific wavelengths from ideal solid materials. Kazuaki Sakoda of Japan¿s Nanomaterials Laboratory at the National Institute for Materials Science notes that the "work clearly demonstrates that even Planck¿s law--the starting point of the era of quantum mechanics [used to predict these interactions]--can be modified."
The dramatically improved output values are seen only at the specific frequencies that the lattice's inner dimensions allow to escape. This property could help to improve the performance of some heat-driven engines. For example, photonic crystals could funnel excess heat from a power plant's generator and release it over a much smaller band of frequencies to drive engines--such as those in electric-powered cars that can absorb energy only within a small range--much more efficiently. The researchers report their latest findings in two papers that will appear in the journals Optics Letters and Applied Physics Letters.