The advantages of a vertical laser configuration have been amply demonstrated as longer wavelength VCSELs enter the commercial mainstream. By operating at much shorter wavelengths, blue versions of the device could increase data densities on networks and in other specific applications such as optical disks.

A joint research project among three universities — the University of Tokyo, Germany's University of W¼rzburg and Italy's University of Lecce — recently achieved blue laser emission at the 399-nm wavelength in a vertical cavity structure fabricated in the gallium nitride system. The lab demonstration, reported recently in the journal Science, used optical stimulation, rather than an electronic contact, to achieve short-wavelength laser emission.

While optical pumping represents a shortcut to getting photons into the cavity to verify laser operation, in principle the same effect could be achieved with electronic input, forming a true laser diode.

Critical factor

The critical factor for higher wavelength operation in a vertical laser cavity is the proportion of aluminum in gallium-aluminum-nitride layers. More aluminum is required to achieve good reflectivity at higher wavelengths, but the aluminum creates problems by widening the lattice mismatch with other compounds such as gallium-nitride. In addition, aluminum increases the thermal coefficient of expansion of the layers in which it occurs, leading to temperature-generated defects.

The experimental device was structured as an active region built from layers of aluminum-gallium-nitride that alternated with different proportions of the constituents, surrounded by two multiple layer Bragg reflectors. The reflectors had very high reflectivity — 98 percent for the bottom reflector and 99.5 percent for the top reflector.

Starting with a sapphire substrate, the researchers first deposited a gallium nitride buffer layer and then proceeded to build up the various reflecting and active multiple-quantum-well layers. Finally, an array of 18-micron-diameter islands, spaced 22 microns apart, were etched out of the multilayer system. The individual dots were illuminated with a helium-cadmium laser emitting at a 325-nm wavelength. The optical response of the island was recorded through the back of the substrate.

The researchers were looking for spectral narrowing in the output from the system. Ideal laser light consists entirely of photons of a single wavelength, so its spectrum would be a single vertical line.

Since real-world systems always depart from the ideal model, the spectral shape of laser light comes in the form of a very narrow peak; therefore, research demonstrations of laser activity look for a very narrow output of wavelengths. In this experiment, the situation was complicated by the ability of other optical phenomena to produce narrow-peak wavelength emission. Therefore, careful measurements were required to distinguish that behavior from true laser action. They found a spread of output wavelengths less than 0.1 nm, which was the resolution limit of their measuring system.