Michael K. Robinson
DURHAM, N.C. -- Cree Research Inc. has demonstrated a pulsed blue semiconductor diode laser that operates at room temperature -- a first step in a race for a commercial device that would greatly increase optical storage capacity.
The company's 403-nm laser is based on a silicon carbide (SiC) substrate and a gallium nitride (GaN) lasing medium. Nichia Chemical Industries of Anan, Japan, demonstrated a GaN diode laser in 1995, but Cree officials believe their device is the first GaN laser built on a SiC substrate.
According to Alan Robertson, the company's chief financial officer, the SiC substrate represents a significant advance over Nichia's sapphire-substrate GaN blue laser: It offers a natural cleavage plane that channels the laser energy. The sapphire crystal has no such plane, so researchers must manufacture one using reactive ion etching. Silicon carbide dissipates heat 10 times faster than sapphire. The crystal lattice structure of SiC aligns more easily with GaN, averaging only about 3 percent dislocation defects, compared with about 15 percent for the sapphire substrate.
"In our opinion, this is going to be the right materials system for the future," said Robertson.
The blue laser, a long-sought prize of semiconductor photonics, promises improved efficiency in optical storage. Preliminary research indicates that using a blue laser could quadruple the storage capacity of an audio disc. Because a blue laser has a shorter wavelength, it can be focused on a smaller spot than the conventional GaAs lasers used in consumer compact disc players and CD-ROM drives.It also has potential for undersea communications because seawater absorbs less blue light than longer wavelengths.
Such commercial applications of blue diode lasers will require life spans of 10,000 h, and the Cree device is far from that goal. "We have observed a duty cycle from 0.01 percent to 1 percent. Between that range, at 0.1 percent, we have observed lifetimes on the order of a few minutes," Robertson said.
The company has not divulged the device's output power.
The development is the product of cooperative funding among Cree Research, Philips Laboratories-Briarcliff and the Defense Advanced Research Projects Agency.