Ultraviolet VCSELs Are on the Way

Richard Gaughan

Ultraviolet laser sources are especially valuable because of their ability to excite fluorescence in molecules that do not react strongly in other spectral regions, including those molecules suitable for white-light illumination. The lasers also promise high-volume optical memory storage. Current UV lasers are bulky, however, reducing their attractiveness for these applications, but a new vertical-cavity surface-emitting laser (VCSEL) that radiates in the near-UV may change all of that.

Robert M. Biefeld, who supervised work on the laser at Sandia National Laboratories, explained that the device demanded extensive studies on the growth of AlGaInN as well as in situ diagnostics.

Lead scientist Jung Han adjusts the pump laser for a prototype vertical-cavity surface-emitting laser that emits in the ultraviolet. Photo by Randy Montoya; courtesy of Sandia National Laboratories.

Building on a sapphire substrate, the researchers from Sandia and Brown University in Providence, R.I., constructed a distributed Bragg reflector composed of 60-layer pairs of gallium nitride and aluminum gallium nitride. They laid an active region of 20 40-Å-thick indium-gallium-nitride quantum wells separated by 60-Å-thick GaN barriers over the Bragg reflector. They topped the structure with a 100-nm-thick AlGaN carrier confinement layer and a low-loss dielectric multilayer mirror of silicon oxide and hafnium oxide.

Previous attempts to build lasers from these materials had failed because lattice mismatch between the layers creates strain. To avoid cracking, the researchers introduced layers of aluminum nitride.

They pumped the VCSEL with a 355-nm tripled Nd:YAG, outside the reflectance band of the dielectric stack. They estimated that the structure usefully absorbed approximately 25 percent of the pump photons. Above 36 mW of pump power, the laser emitted at 383 nm with a bandwidth less than 0.1 nm. The circular beam divergence of about 7.4° agreed with predictions for the given pump-beam diameter.

"VCSELs have the same advantages for UV applications as they have for infrared applications, such as low threshold currents in injected lasers, single-lobed far-field output, high efficiency and compactness," Biefeld said. They have potential applications in lighting and optical displays, as well as in next-generation multimedia software. The group, which reported the work in the Oct. 12, 2000, issue of Electronics Letters, hopes to develop an electrically pumped version of the device.