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Scientists at UC Berkeley and LBNL create world's smallest laser
07 June 2001

By Robert Sanders, Media Relations

Berkeley - A University of California, Berkeley, chemist has grown the world's smallest laser - a nanowire nanolaser one thousand times thinner than a human hair.

Among the potential applications are chemical analysis on microchips, high-density information storage and photonics - transmitting information via laser light. The laser, one of the first real devices to arise from the field of nanotechnology, emits ultraviolet light, but can be tuned from blue to deep ultraviolet.

"The ability to produce high-density arrays of nanowires opens up lots of possible applications that today's gallium arsenide devices can't do," said creator Peidong Yang, assistant professor of chemistry at UC Berkeley and a member of the Materials Science Division at the Lawrence Berkeley National Laboratory. "This process works, it is ultracheap, and it's the first real application of nanowires."

Yang and his colleagues in the Department of Chemistry at UC Berkeley and at LBNL report their development in the June 8 issue of Science.

Gallium arsenide and gallium nitride lasers are today's leading solid state lasers, cheap enough to be used in laser pointers. Made of multilayer thin films, they are several micrometers in size, on the order of one 10,000th of an inch. The nanolaser is about 100 times smaller.

Yang and his team grew the lasers, which are pure crystals of zinc oxide, using a standard technique called epitaxy, employed broadly today in the semiconductor industry. In epitaxy, a device is immersed in a hot vapor that is deposited in a very thin layer, sometimes only a few molecules thick.

The scientists painted a gold catalyst onto a piece of sapphire and placed it in a hot gas of zinc oxide (ZnO) - a compound often used in solid state lasers, but perhaps best known as an ingredient in sunscreens. The gold, when heated, formed regularly spaced nanocrystals that stimulated the growth of extremely pure zinc oxide wires only 20 to 150 nanometers in diameter. One nanometer is about the size of ten hydrogen atoms laid end to end.

The solid wires, which are hexagonal in cross section, grew to about 10 microns in length before the growth process was stopped, typically after two to 10 minutes. A human hair is about 100 microns in diameter.

"This technique is very compatible with current industry methods," Yang said.

Under an electron microscope, the arrays of nanowire nanolasers look like bristles of a brush, each bristle an individual laser. Bunched together like this, the nanolasers are bright enough to be used in different applications.

The key to getting these solid state lasers to emit coherent UV light is a perfectly flat tip that acts as a mirror in the way that, from underwater, the water surface acts like a mirror. The end attached to the semiconductor also is a mirror, so that light emitted by excited zinc oxide bounces back and forth between them, causing more molecules to emit and amplifying the light. The amplified photons produced by this stimulated emission - "laser" stands for light amplification by stimulated emission of radiation - eventually pass through the mirrored free end, producing a flash of UV light.

Though Yang now must use another optical laser to excite the zinc oxide molecules so that they will emit UV light - a process called optical pumping - he hopes eventually to "pump" the zinc oxide with electrons. Electron pumping is necessary for a laser to be integrated into an electronic circuit.

Once configured to work with electron pumping, the nanolaser could be put to any number of uses, Yang said. "Lab-on-a-chip" devices could contain small laser analysis kits - nanodetectors - capable of such things as Raman spectroscopy, a laser technique that can be used to identify chemicals.

A short-wavelength ultraviolet laser also could increase the amount of data that can be stored on a high-density compact disk, just as the advent of blue-light gallium nitride lasers boosted data density.

And in the field of photonics and optical computing, cheap bright lasers are essential.

Yang said that at this preliminary stage of development, the nanolaser is comparable to or better than the gallium nitride blue laser in terms of ease of manufacture, brightness and much smaller dimensions.

"It basically has high enough intensity to think about making a practical device," he said. Plus it operates at room temperature.

The research was supported by the Camille and Henry Dreyfus Foundation, the 3M Corporation, the National Science Foundation, the U.S. Department of Energy and UC Berkeley. Yang's colleagues are postdoctoral students Michael H. Huang and Hannes Kind, graduate students Haoquan Yan and Yiying Wu, all of UC Berkeley's Department of Chemistry; and PhD scientists Samuel Mao, Henning Feick, Eicke Weber and Richard Russo of LBNL.

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