University of California-Berkeley (Berkeley, CA) chemist Peidong Yang and colleagues have grown room-temperature nanowire nanolasers that, under optical excitation, demonstrate surface-emitting lasing action at 385 nm with an emission linewidth of less than 0.3 nm. The zinc oxide (ZnO) nanowire nanolasers are believed to be the smallest in the world, as well as one of the first real devices to arise from nanotechnology. At this preliminary stage of development, the scientists report that the ZnO nanolaser is comparable to or better than the gallium nitride (GaN) blue laser in terms of ease of manufacture, brightness, and much smaller dimensions.
"The ability to produce high-density arrays of nanowires opens up lots of possible applications that are not possible with today's gallium arsenide (GaAs) devices," Yang says.
To grow the lasers, ZnO nanowires were synthesized with a vapor-phase transport process using catalyzed epitaxial crystal growth. Sapphire substrates received a 1- to 3.5-nm-thick coating of gold (Au), and were then placed in an alumina boat. The materials and substrates were heated to between 880°C and 905°C in an argon gas flow to generate Zn vapor, which was transported to the substrates. The nanowires, which are hexagonal in cross section, grew 2 to 10 µm during the 2- to 10-min growth process. Yang and colleagues found that the nanowires formed natural laser cavities with diameters ranging from 20 to 150 nm, with the majority (95%) falling between 70 and 100 nm.
To study the possible stimulated emission from the nanowires, the scientists examined the power-dependent emission. Samples were optically pumped with the fourth harmonic output of a Nd:YAG laser (266 nm, 3-ns pulse width) at room temperature, with the beam focused at an incidence angle 10° to the symmetric axis of the nanowire. Light emission was collected in the direction normal to the end surface plane (along the symmetric axis) of the nanowires. During the evolution of the emission spectra, lasing action was observed with increasing pump power. When excitation exceeded the threshold of the ZnO nanowires (approximately 40 kW/cm2), sharp peaks emerged in the emission spectra. Linewidths of these peaks were less than 0.3 nm, which is more than 50 times smaller than the linewidth of the spontaneous emission peak below the threshold. The narrow linewidths and rapid increase of emission intensity led the scientists to conclude that stimulated emission does take place in these nanowires.
The lasing action in the nanowire arrays without any fabricated mirrors prompted the researchers to consider them as natural resonance cavities. One end of the nanowire is the epitaxial interface between the sapphire and ZnO, while the other end is the sharp (0001) plane of the ZnO nanocrystals. Both can serve as good laser-cavity mirrors, according to Yang, given that the refractive indexes for sapphire, ZnO, and air are 1.8, 2.45, and 1, respectively.
While the chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources, the scientists believe these short-wavelength nanolasers may find applications in optical computing, information storage, and nano-analysis.
REFERENCE
- Michael H. Huang et al., Science, 292, 5523 (June 8, 2001).
