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
"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
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
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.