concoct tiny lasers
Technology Research News
One way to cram more data onto compact
discs and through fiber-optic channels is to use shorter lightwaves. Ultraviolet
light is about half the wavelength of the red light generated by the lasers
in many commercial devices, and, unlike still shorter x-rays, ultraviolet
light can be controlled with lenses and mirrors.
The trick is to find suitable materials and techniques for making ultraviolet
lasers that are small enough to be built into semiconductor
Researchers at the University of California at Berkeley have taken a big
step in that direction by growing microscopic forests of vertical nanowires;
each is a laser 1,000 times narrower than a human hair.
The nanowires emit ultraviolet laser light when they are pumped by another
laser, according to Peidong Yang, an assistant professor of chemistry
When a material is pumped with energy, the electrons of its atoms move
to a higher energy state. When the electrons return to their low-energy
states they release the excess energy, often in the form of light.
Lasers work by bouncing this emitted light between two mirrors that are
positioned to send the light back and forth through the material. The
reflected light adds to the original energy source to produce yet more
emitted light, which in turn bounces between the mirrors. This feedback
increases the intensity of the light and results in a laser beam.
Semiconductor lasers, which are widely used in communications networks
and data storage devices, are a sandwich of three layers of semiconductor.
The middle layer is the amplification medium and the outer layers are
the mirrors. Electric current provides the initial energy.
Each nanowire is a laser because its ends are sharply defined, which allows
them to serve as mirrors that bounce light back and forth along the length
of the nanowire. "The [top] end is almost atomically sharp," said Yang.
The nanowires are grown by wafting a vapor of zinc oxide over a sapphire
chip that is coated with a thin film of gold. Most of the nanowires are
within 70 to 100 nanometers in diameter, according to the researchers.
The nanowires grow at a rate of about one micron per minute, and the researchers
have made nanowires 2 to 10 microns tall.
The crystal structure of the sapphire substrate causes the nanowires to
grow in a precise shape, said Yang. "In general, there are lots of methodologies
[for growing] nanowires. It's really a matter of growth control, with
careful control of the substrate," he said.
The gold film serves as a catalyst for nanowire growth, so putting it
on the sapphire substrate in a particular pattern causes the nanowires
to grow in that pattern, said Yang. "We can purposely put these nanowire
lasers at a particular place, depending on where we put the catalyst,"
The major hurdle to developing practical ultraviolet nanowire lasers is
figuring out how to produce the lasing action using electricity rather
than another laser. "We demonstrated... this optically-pumped laser. If
we can successfully put this device into electron injection configuration,
then it immediately can be commercialized," said Yang.
The researchers are also working on changing the chemical composition
of the nanowires to slightly altered the light wavelengths the lasers
emit. "We're working on tuning the composition of the systems so that
we can tune the wavelengths... from blue to deep UV," said Yang.
In addition to communications and data storage, nanowire lasers on labs
on a chip could be used for spectroscopy, said Yang. Spectroscopy is a
method of using light to determine the chemical composition of a substance.
The nanowire lasers could be used in practical applications in five to
ten years, said Yang.
Yang's research colleagues were Michael H. Huang, Haoquan Yan, Yiying
Wu and Hannes Kind of the University of California at Berkeley, and Samuel
Mao, Henning Feick, Eicke Weber and Richard Russo of Lawrence Berkeley
National Laboratory. They published the research in the June 8, 2001 issue
of the journal Science. The research was funded by the Camille and Henry
Dreyfus Foundation, 3M Corporation, the National Science Foundation (NSF),
the Department of Energy and the University of California at Berkeley.
Timeline: 5-10 years
Funding: Private, Corporate, Government, University
TRN Categories: Materials Science and Engineering; Optical
Computing, Optoelectronics and Photonics
Story Type: News
Related Elements: Technical paper, "Room-Temperature Ultraviolet
Nanowire Nanolasers," Science, June 8, 2001
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