Source: after NAS/NASA |
Fluorescent tubes could soon face a new rival for the top spot in indoor lighting. Scientists at Sandia National Laboratories and Brown University have devised the very first vertical-cavity surface-emitting laser (VCSEL) capable of producing ultraviolet light. Somewhat analogous to the red light LEDs that have supplanted ordinary bulbs in traffic lights, the UV VCSEL promises to last five to ten times longer than fluorescent tubes, and be more aesthetically pleasing. (People could conceivably arrange these tiny chips however they want on ceilings, walls or furniture.) Other applications might include identifying weapons-grade fissionable materials and E. coli bacteria, both of which fluoresce in the visible spectrum. "It was a dream, yet distant," says Sandia lead scientist Jung Han. "Now we have achieved it."
The invention, described in the October 12 issue of Electronics Letters, is currently in the laboratory stage, meaning it is now powered by larger lasers instead of an electrical source. But Han expects that will change within two years. The VCSEL itself is made from nanoscopically thin layers of semiconductor materials that emit photons when electricity passes through them. As the laser's name implies, its light comes from the surface of the semiconductor wafer, unlike traditional edge-emitting lasers (above). One important advance Han's group made was adding indium to the mix of semiconductors used. Although gallium nitride and aluminum nitride both emitted light in the UV range, the efficiency was only near 1 percent. Indium raised that figure to about 20 percent, although the wavelength of the light stretched out into the near-ultraviolet range. "We pay the price for the necessary evil," Han notes.
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The VCSEL is not the only semiconductor choice for producing white light, but it does have several advantages. Although commercially available blue LEDs give off white light, it is considered cold for lighting. And although red, blue and green LEDS can combine to make white light, they may not be cost-effective. Also, the light from the VCSELS is more monochromatic and directional.
