A short-wavelength InGaN-based multiple-quantum-well laser emitting at 417 nm was reported at Photonics West (San Jose, CA) in January (see Laser Focus World, March 1996, p. 41). Nichia Chemical Industries (Tokushima, Japan) senior researcher, Shuji Nakamura, said that the stimulated emission occurred at room temperature using pulsed current injection. Previously, GaN-based lasers have had to be optically pumped to lase. And while current-injection pumping of ZnSe-based II-VI materials has achieved emission at 490 nm, lifetime has been only a few hours at room temperature.
The wide-bandgap III-V nitride material lasers were grown by Nichia on a sapphire substrate. Because it is difficult to cleave the GaN crystal grown on the sapphire, reactive-ion etching was used to form the mirror cavity facets. High-reflection (60% to 70%) facet coatings were used to reduce threshold current.
Diode-laser characteristics were measured for pulsed current with a period of 2 ms and a pulse width of 2 µs. Threshold current was 1.7 A, corresponding to a current density of 4 kA/cm2. Differential quantum efficiency of 13% per facet and pulsed output power of 215 mW per facet were obtained at a current of 2.3 A. The dominant 417-nm emission above the threshold current had a FWHM of 1.6 nm.
Nakamura noted that although no lifetime data have been gathered yet, the diode laser has operated pulsed for up to 24 hours. The goal is to achieve CW operation, he said. "Considering the long lifetime of III-V nitride-based high-power blue LEDs, the realization of reliable diode lasers fabricated from [the same] materials seems easier [to obtain than with] diode lasers fabricated from II-VI materials."