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Although tungsten-filament bulbs are the world’s most extensively used light source, they are inefficient and generate more heat than light. However, a new microscopic tungsten lattice created at the Department of Energy’s Sandia National Laboratories demonstrated the ability to redirect most of this wasted heat energy into visible light.
This lattice could increase the efficiency of an incandescent electric light bulb from 5% to over 60%. This, in turn, would significantly decrease the world’s surplus consumption of electrical power due to inefficient lighting, and the associated environmental impact caused by CO2 emissions.
Fabrication of the New Lattices
The initial step toward this goal, accomplished at Sandia National Laboratories by Shawn Lin and Jim Fleming, has been described in the May issue of the journal Nature. The tungsten lattice device was constructed using an extension of popular microelectromechanical systems technologies, that have been derived from established semiconductor technologies. Consequently, such devices can be produced easily and cost-effectively.
The tungsten structure, typically composed of silicon, comprises minute bars fabricated to sit astride each other at regular predetermined distances and angles. Together, these create an artificial crystal. The spacing of the bars allows only specific wavelengths of radiation to pass through, but can also alter direction, as flaws in the artificial crystal cause the light to follow the flaw.
The Lattice’s Ability to Stop Other Frequencies
The next question contemplated by Lin and Fleming, with help from colleagues at Ames Laboratories in Iowa, was the ability of the tungsten lattice to “stop” other frequencies. If the crystals were made using tungsten, the metal could endure relatively high temperatures and have a huge and absolute photonic band gap in the visible range, where it is already known to discharge light.
But what would happen to the other, lower-wavelength radiation caused by electric current? Would the structure melt, or would the thermally stimulated tungsten atoms somehow choose to reinforce emissions at higher wavelengths, for example, in the visible frequency range?
It was observed that energy at the edge of the photonic band was absorbed more by an order of magnitude, and this energy was being preferentially absorbed into a selected frequency band.
In the meantime, periodic metal-air boundaries resulted in a large transmission enhancement. Experimental results revealed that a large photonic band gap for wavelengths from 8 to 20 µm proves perfect for overpowering broadband blackbody radiation in the infrared, and is capable of redirecting thermal energy into the visible spectrum.
Results and Prospects
Lin and Fleming are pleased with the outcomes, although the theory for the effect—re-partitioning energy between visible light and heat—is yet to be understood.
It’s not theoretically predicted. Possible explanations may involve variations in the speed of light as it propagates through such structures.
Jim Fleming, Sandia National Laboratories, Department of Energy
Although the experiment was performed with light in the mid-infrared range, no theoretical or practical problems are known to persist in downsizing the structure into the visible light range.