UV/Blue Lasing Observed in Nanowires
Paula M. Powell
Building on earlier research that used near-field microscopy to capture lasing action of single ZnO nanowire arrays, scientists at the University of California, Berkeley, now have observed and characterized UV and blue laser action in single gallium nitride nanowires (see March 2002, page 39). In the process, they believe they have made a significant advance in the effort to create electron-injected nanowire lasers.
An electron micrograph of as-grown gallium nitride nanowires serves as the background for the combined near-field optical and topographic image shown in the foreground. The optical emission is strongly confined to propagation in the axial direction, leading to bright laser radiation observed exclusively at the nanowire ends. Single-mode and multimode ultraviolet lasing spectra are shown in green.
As in the earlier studies, the researchers employed both near- and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. Justin C. Johnson, a member of the group led by Richard J. Saykally that devised the imaging strategy, reported that the nanowires are highly monocrystalline in nature. They have lengths up to several hundred microns and diameters approaching 30 to 150 nm, although significantly smaller diameters are possible. He added that resulting images reveal radiation patterns correlating with axial Fabry-Perot modes observed in the laser spectrum. The images also indicate a red shift strongly dependent on pump power, which the scientists believe supports the idea that the electron-hole plasma mechanism is primarily responsible for the gain at room temperature.
According to Johnson, practical use of these nanoscale light emitters probably will dictate the need for electrical pumping, which isn't necessarily a bad thing because the material properties of GaN, combined with its affinity to doping, make it a good candidate for electron injection.
"The next stage of our research," he said, "is to determine quantitatively the capabilities and limits of these nanolasers. Specifically, pumping thresholds as a function of nanowire size are needed to determine the feasibility of using these structures in devices -- especially for the smallest of nanowires." In addition, Peidong Yang's group at the university is preparing GaN heterostructures that contain regions of different types of doping, which is a necessary step toward electron injection, he added.
Five to 10 years to application
Yang, head of the group that discovered the lasing in ZnO nanowires, noted that the nanowire technology is still in a fundamental research stage. He estimates it will be some five to 10 years before practical applications appear -- most likely in the areas of sensors or light sources. Both he and Johnson see several hurdles that must first be overcome: Yang in the area of how to integrate or assemble individual nanowire building blocks into parallel devices or functional systems and Johnson in how to synthesize and manipulate heterostructures for efficient pumping.
"Methods that have been used in the field of larger diode lasers are not always capable of doing the job for these free-standing submicron structures," Johnson said. "However, we believe that the technology we are developing will allow us the level of precision necessary to realize the goal of electron-injected nanolasers relatively soon. In the meantime, optical pumping continues to provide an excellent way to probe the physics that underlies the phenomenon of lasing in these novel structures."
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