Beatin' those low-life blue-laser blues

It took scientists decades to construct a solid-state laser that would shine a beam of blue light. Now, they may finally have built one robust enough to be useful commercially. 

 Last year, a team of Japanese researchers announced that it had developed gallium nitride (GaN) semiconductor laser diodes that provide a continuous output of blue light at room temperature. Those diodes had a distinct limitation, however: They had an operational lifetime of only 27 hours. 

 Now, the same team, led by Shuji Nakamura of Nichia Chemical Industries in Tokushima, reports in the Dec. 1 Japanese Journal of Applied Physics that it has produced GaN diodes that have already lasted over 100 times longer in tests conducted at room temperature. Tests at higher temperatures indicate that the diodes have an estimated life of over 10,000 hours.

 The most likely first use for these blue laser diodes will be in optical data storage, where the amount of information stored on a given area of a disk's surface could be three or four times higher than that written by the infrared laser beams used today. The diodes could also find uses in high-resolution laser printers, full-color electronic displays, and undersea optical communications, says Nakamura.

 The researchers used two different fabrication techniques to extend the life of the diodes. First, the new components include 120 layers of GaN, each 25 nanometers thick, alternating with 2.5-nm-thick layers of GaN that also contain small amounts of aluminum. Previous versions of the diodes contained thicker layers of the aluminum gallium nitride material, which tended to crack under stresses induced by temperature changes during operation.

 Second, Nakamura's team formed the new laser diodes on top of a 20-millimeter-thick base of GaN specially designed to constrain the growth of crystal defects. 

 The researchers built the GaN base atop a series of silicon dioxide strips separated by narrow gaps. These strips distort slightly the structure of the GaN base above them, creating physical stresses that steer the spread of any crystal defects. Instead of ascending to the surface, where they would affect the performance of the layered diodes, most of the defects remain confined to the bottom 5 mm of the 20-mm GaN base.

 The few crystal defects that eventually grow to the upper surface of the GaN base do so at predictable locations. By selecting other spots on the surface on which to fabricate the diodes, Nakamura's team dramatically improved the laser's useful lifetime.

 The laser operates at a wavelength of 401 nanometers, about half the wavelength of red and infrared lasers. Its color lies at the extreme blue end of the visible-light spectrum.

"Producing a blue laser diode is a great step, but you need to get the cost down, the power output up, and the right color to use this in [an electronic] display," says R.L. Melcher of IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y.