By John McHale
PALO ALTO, Calif. - American scientists are bolstering blue laser diode technology with gallium nitride semiconductor material, a move offering the potential to enhance covert communications for the military, as well as improve data storage and biological sensing.
Commercial applications for this improved blue laser technology range from high-resolution color printers, projection television, laser surgery, and high-capacity optical storage devices.
Researchers making the breakthrough are on three teams from the Xerox Palo Alto Research Center in Palo Alto, Calif.; Cree Research Inc. in Durham, N.C.; and the University of California at Santa Barbara (UCSB).
The three teams have been working for the past two years to develop the laser device with partial funding from the Defense Advanced Research Projects Agency (DARPA).
"This demonstration is a critical step to realizing a key military component," says Anis Husain assistant director of electronics technology at DARPA and program director for blue laser research. "Short wavelength blue lasers are critical for high data storage military systems and chemical biological reagent sensing, as well as covert communications. The base technology could also be extended for solar blind missile detection systems."
Although DARPA officials say they are pleased with the accomplishments, they believe "more work remains to achieve a viable blue laser component for military systems," Husain notes.
Blue lasers, which are of shorter optical wavelengths than the more commonly used red laser, are important because they hold forth the potential to increase the capacity of optical storage media such as compact disks by as much as 10 times.
"It is basically storing more information in a smaller space," says David Bour, principal scientist at the Electronic Materials Laboratory in the Xerox Palo Alto Research Center.
Despite their initial success, the scientists have a long way to go before matching the accomplishments of a Japanese scientist at Nichia Chemical Industries in Anan, Japan, Bour says.
Shuji Nakamura, a solid state physics researcher for Nichia, developed the first blue light diode, which is expected to reach 10,000 continuous life hours this year - well ahead of his U.S. competitors.
The Xerox diodes operate at wavelengths from 422 to 432 nanometers under pulsed current-injection conditions from a 10 quantum well heterostructure grown on a sapphire substrate through metal organic chemical vapor definition.
Scientists from the Xerox team are shooting to create laser printers with two to four times the resolution of today`s 600-dots-per-inch machines, Bour says.
Scientists at Cree built their device with silicon carbide (SiC) wafers combined with a gallium nitride thin-film process. "The next step is to improve device performance and process repeatability," says John Edmond, Cree co-founder and the senior scientist in charge of the program.
"This is the first known nitride laser using a silicon carbide substrate and the laser emission demonstrated was textbook. The heat going into a laser is a principal failure mechanism, and as an ideal heat sink, silicon carbide overcomes this far better than sapphire," says Jan Schetzina, professor of physics at North Carolina State University in Raleigh, N.C., and a partner in the DARPA effort.
"This success really emphasizes the advantage of growing nitride laser structures on SiC," says Kathy Doverspike, a senior scientist at Cree. "SiC is a vastly superior heat sink to sapphire or other substrates on which blue laser structures have been grown"
The UCSB researchers built blue laser diodes that operated pulsed for as long as six hours at room temperature. "Except for the seminal work of Nichia Chemicals Ltd. in Japan, ours is clearly one of the best results in the world," says Larry Coldren, professor of electrical and computer engineering at UCSB.
"No one else, except Nichia, has reported threshold current densities as low as ours - 12.7 kiloamps per square centimeter," adds Steven DenBaars, a UCSB researcher.
U.S. researchers are making breakthroughs on the kinds of blue laser diode technology pictured above that offer to increase the optical storage capacity on compact disks by as much as 10 times.
