Tungsten-filament bulbs, although the most widely used light source in the world, are inefficient, generating more heat than light. However, a new microscopic tungsten lattice developed at the Department of Energy’s Sandia National Laboratories has been shown to have the potential to redirect much of this wasted heat energy into visible light.
This could raise the efficiency of an incandescent electric light bulb from 5% to greater than 60%. In doing so it would greatly reduce the world’s excess consumption of electrical power by inefficient lighting, and the accompanying environmental impact caused by CO2 emissions.
Fabrication of the New Lattices
The first step toward this goal, achieved at Sandia by Shawn Lin and Jim Fleming, was reported in Nature in May. The tungsten lattice device was built using an extension of well-known microelectro-mechanical systems technologies that themselves have been derived from mature semiconductor technologies. As a result, fabrication of such devices could be cheap and easy.
The tungsten structures, usually made out of silicon, consist of tiny bars fabricated to sit astride each other at regular pre-set distances and angles (figure 1). Together, these form an artificial crystal. The spacing of the bars allows passage of only certain wavelengths of radiation they pass through but can also change direction as defects in the artificial crystal cause the light to follow the defect.
Figure 1. SEM images of Sandia’s 3-D tungsten photonic crystal. The images taken with (a) and without (b) oxide. The tungsten rod-width is 1.2µm and the rod-to-rod spacing is 4.2µm.
The Lattice’s Ability to Stop Other Frequencies
A further question considered by Lin and Fleming, with assistance from colleagues at Ames Laboratories in Iowa, was the tungsten lattice’s capability of ‘stopping’ other frequencies. If the crystals were built of tungsten, the metal could handle quite high temperatures and have a large and absolute photonic band gap in the visible range, where it is already known to emit light. But what would happen to the other, lower-wavelength radiation brought in by an electric current? Would the structure melt, or would the thermally excited tungsten atoms somehow prefer to reinforce emissions at higher wavelengths, such as in the visible frequency range?
Energy at the edge of the photonic band was observed to undergo an order-of-magnitude absorption increase energy was being preferentially absorbed into a selected frequency band. Meanwhile, periodic metal-air boundaries led to a large transmission enhancement. Experimental results show that a large photonic band gap for wavelengths from 8 to 20 microns proves ideally suited for suppressing broadband blackbody radiation in the infrared and has the potential to redirect thermal energy into the visible spectrum.
Results and Prospects
Lin and Fleming are delighted with the results, although the theory for the effect - re-partitioning energy between heat and visible light - remains unexplained. ‘It’s not theoretically predicted,’ says Fleming. ‘Possible explanations may involve variations in the speed of light as it propagates through such structures.’
Although the work was carried out with light in the mid-infrared range, no theoretical or practical difficulties are known to exist in downsizing the structure into the visible light range.