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One of the smallest lasers ever made -- far too small to be seen even with the aid of
the most powerful optical microscope -- has been successfully tested by a team of
researchers with Lawrence Berkeley National Laboratory (Berkeley Lab) and the University
of California at Berkeley. This device, which emits flashes of ultraviolet light, is
called a "nanowire nanolaser" and it measures just under 100 nanometers in
diameter or about one ten-millionth of an inch
The nanowire nanolasers are pure crystals of zinc oxide that grow vertically in aligned
arrays like the bristles on a brush. These crystal wire "bristles" range from
two to 10 microns in length, depending upon how long the growth process was allowed to
proceed. By comparison, the tiniest solid-state lasers in use today are fashioned from
thin films of either gallium arsenide or gallium nitride and generally run several microns
thick, or about one hundred thousandths of an inch. A typical human hair is about 100
microns thick.
"The ability to produce high-density arrays of light-emitting nanowires would open
up lots of possible applications that today's gallium arsenide devices can't do,"
says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and a
professor with UC Berkeley’s Chemistry Department who was the lead scientist on this
project.
Collaborating with Yang from Berkeley Lab were Henning Feick and Eicke Weber, also with
the Materials Sciences Division, and Samuel Mao and Rick Russo of the Energy and
Environmental Technologies Division. Joining them were Michael Huang, Haoquan
Yan, Yiying
Wu and Hannes Kind, with the UC Berkeley Chemistry Department. A paper reporting this work
appeared in the June 8 issue of the journal Science.
To produce these nanowires, the research team used a standard crystal-growing technique
called epitaxy in which one crystalline material is grown over the surface of another. In
this case, sapphire crystals, coated in patterns with a thin film of gold, were plunged
into a hot gas of zinc oxide. The gold film served as a catalyst and within ten minutes,
millions of zinc oxide nanowires formed over the patterns on the sapphires.
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PICTURED (LEFT TO RIGHT) ARE RESEARCHERS HAOQUAN
YAN, SAMUEL MAO, RICHARD RUSSO. AND
PEIDONG YANG
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Stated the authors in their Science paper, "This capability of patterned
nanowire growth allows us fabricate nanoscale light emitters on a substrate in a
controllable fashion."
To produce light, the researchers used a process called "optical pumping."
Working at room temperatures, the zinc oxide nanowires were flashed with light from a
visible light laser. This light excited the zinc oxide molecules, causing them to emit
photons. Cross-sections of the wires show them to be hexagonal and capped on the end. This
cap is perfectly flat as is the interface between the wire and the sapphire substrate.
These two perfectly flat surfaces act like mirrors and cause the photons emitted by the
excited zinc oxide molecules to bounce back and forth between them. This reflected light
causes the zinc oxide molecules to emit even more photons until the light becomes
sufficiently amplified to pass out of the nanowire’s end cap in a burst of
ultraviolet light.
Whether the process proves to be a scientific curiosity or a first step towards the
development of a significant new technology may hinge on the Berkeley Lab researchers
finding a way to extract light from their nanowires via electrical pumping. This would
entail the use of electrodes attached to both ends of the nanowires and stimulating photon
emissions inside the wires with electrons. Electrical pumping is required for the
nanowires to be integrated into an electronic circuit. If it can be done, it could open up
a broad range of potential applications including photonics, the use of light for
superfast data processing and transmission, and the so-called "lab on a chip," a
microchip equipped with nano-sized light sources and sensors to perform instant and
detailed analyses for chemistry, biology, and medical studies.
"With something this small, people will think of new applications for it,"
says Russo.
This research was supported by funds from the Chemical Sciences Division and the
Materials Science Division of the Office of Basic Energy Sciences in the U.S. Department
of Energy; UC Berkeley; the Camille and Henry Dreyfus Foundation; the 3M Corporation; and
the National Science Foundation.
Reproduced with permission.
Copyright Laurence Berkeley
National Laboratory
The original articles can be found at : http://enews.lbl.gov/Science-Articles/Archive/nanowire-laser.html
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