Tungsten Photonic Crystal Could Provide More Power - New Technology

You can't get something for nothing, physicists say, but sometimes a radical innovation can come close.

Researchers at Sandia National Laboratories - exceeding the predictions of a 100-year-old law of physics - have shown that filaments fabricated of tungsten lattices emit remarkably more energy than solid tungsten filaments in certain bands of near-infrared wavelengths when heated.

This greater useful output offers the possibility of a superior energy source to supercharge hybrid electric cars, electric equipment on boats, and industrial waste-heat-driven electrical generators. The lattices' energy emissions put more energy into wavelengths used by photovoltaic cells that change light into electricity to run engines.

Because near-infrared is the wavelength region closest to visible light, the day may not be distant when tungsten lattice emissions realized at visible wavelengths provide a foundation for more efficient lighting - the first significant change in Edison's light bulb since he invented it.

"This is an important and elegant work," says Cal Tech professor Amnon Yariv of the research achievement. Yariv is a member of the National Academy of Engineering and a leading figure in quantum optics research.

The work has been granted two patents with another pending. Two papers describing the advance have been accepted by the journal Optics Letters. Another will be published by Applied Physics Letters.

Sub-micron-featured lattices - which resemble very tiny garden lattices carefully stacked one atop the other - can be mass-produced cheaply with today's computer-chip technologies.

The lattice itself can be visualized as a construction built of a child's Lincoln Logs. The tungsten "logs" of this experiment have diameters of 0.5 microns separated by distances of 1.5 microns.

The lattices are also known as photonic crystals because of the crystalline regularity of the spacing of their components. At first such crystals were of interest because they could bend specific frequencies of light without loss of energy. This was because the crystal's channels were constructed of exactly the right dimensions to form a 'home' for particular wavebands as they travelled. The innovation of the current method is to use the channels not to bend light but to permit input energy to exit only in the desired frequency bands.

The demonstration exceeds in output a well-known law formulated a century ago by Max Planck. The equation, called Planck's Law of Blackbody Cavity Radiation, predicts the maximum emissions expected at any wavelength from ideal solids.

The somewhat startling Sandia results exceeded these predictions by four to 10 times at near-infrared frequencies, says Lin.

In terms of electrical output, for the Sandia lattice heated in a vacuum to 1,250 degrees C - the typical operating temperature of a thermal photovoltaic generator - a conversion efficiency of 34 percent was calculated, three times the performance of an ideal blackbody radiator, predicted to be 11 percent.

Electrical power density was calculated to be approximately 14 watt/cm squared, rather than three watt/cm squared expected from an ideal blackbody radiator.

No deterioration of the tungsten lattice was observed, although long-term tests have yet to be run.

Lin says his group's work does not break Planck's law but only modifies it by demonstrating the creation of a new class of emitters.

A photonic lattice apparently subjects energies passing through its links and cavities to more complex photon-tungsten interactions than Planck dreamt of when he derived his.

Posted 8th July 2003

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