A STACK of ultra-thin tungsten wire could one day form the basis of a super-efficient light bulb, converting twice as much energy to light as fluorescent tubes and 10 times as much as ordinary light bulbs.
The discovery, by Shawn Lin and colleagues at Sandia National Laboratories in Albuquerque, New Mexico, also challenges Planck鈥檚 law of black body radiation, a century-old law of physics that predicts how much light in each different wavelength a substance will emit at any given temperature.
The key to the discovery lies in the delicate structure of the iridescent material. It is made from orderly stacks of tungsten rods, each of which is a mere 500 nanometres in diameter 鈥� within the range of the wavelengths of visible light.
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When the researchers heated the lattice by passing an electric current through it, they found that it emitted up to 10 times as much near-infrared light as Planck鈥檚 law predicted (Applied Physics Letters, DOI: 10.1063/1.1592614). 鈥淲e don鈥檛 quite know how it works yet,鈥� concedes Jim Fleming, a member of Lin鈥檚 team. But they suspect that the longer infrared wavelengths normally emitted from tungsten light filaments are for some reason trapped in the lattice, and only escape as shorter wavelength, near-infrared beams.
The researchers reckon that if they can make the tungsten rods even finer, the wavelength of the emitted light will become shorter, bringing it into the visible range. Lin estimates that with rods 100 to 200 nanometres wide, up to 60 per cent of the energy flowing into the lattice should be converted into visible light.
These tungsten lattices could then be used to make filaments for super-efficient light bulbs that are as bright as today鈥檚, but use a tenth of the power to do so, and give off a twentieth of the heat. The team has been granted two patents, with a third pending.
Amnon Yariv, professor of applied physics at the California Institute of Technology in Pasedena, is impressed by the work. But he insists that the discovery does not violate Planck鈥檚 law, which he calls 鈥渙ne of the towers of modern physics鈥�. 鈥淩ather it recognises that in very sub-micron structures the law does not apply,鈥� he says.
