turns infrared to green
Technology Research News
Since the invention of the laser in 1960,
its synchronized wavelengths have been put to use uses in fields as disparate
and widespread as fiber optics, surgery, and precise conference room communications.
The key to generating this useful type of light is finding a way to synchronize
the wavelengths. Lasers are named for the process that allows them to
do this -- Light Amplification by Stimulated Emission of Radiation. Common
light sources like light bulb filaments emit photons of many different
wavelengths in all directions. In a laser, however, atoms emit photons
that are all the same wavelength because they are stimulated, or pumped
with photons of the same energy level.
If a material has enough electrons at a high-energy level, the stimulated
emissions can stimulate still more emissions, leading to a narrow beam
of intense, coherent laser light.
A team of researchers from the State University of New York at Buffalo
has made a material that absorbs three photons at once, and contains enough
high-energy electrons so that the material can lase. The three-photon
process also produces wavelengths that are higher frequency than those
that are absorbed, an effect known as upconversion. The process could
eventually lead to lasers with shorter wavelengths, which could enable
faster optical communications and higher-density storage applications.
The researchers carried out the three-photon process through "a combination
of developing a material capable of efficiently absorbing three photons
at the same time, and having a high-power infrared laser source that produces
stimulated emission at the proper wavelength," said Paras Prasad, executive
director of SUNY Buffalo's Institute for Lasers, Photonics and Biophotonics.
"For a short period of time the material can have more molecules at higher
[energy levels], which allows [for] for stimulated emission."
Using a "new organic chromophore developed in our research group, and
powerful infrared [lasers], we have recently accomplished the first demonstration
of direct three-photon excited frequency upconversion lasing," said Guang
He, a senior research scientist at the University of Buffalo.
To demonstrate the process, the researchers used a 1-centimeter-long quartz
container filled with a type of chromophore, or dye, suspended in a solution.
They focused a high-power infrared laser beam into the center of the sample
to stimulate the material and start the lasing action. The laser pulsed
1,000 times per second and the pulses lasted about 150 femtoseconds, according
to Prasad. A femtosecond is a trillionth of a second.
The chromophore molecules absorbed infrared light, but emitted higher
energy green-yellow light, according to Prasad. This is because the molecules
were releasing the combined energy from the three absorbed photons in
the form of one higher energy, and therefore shorter-wavelength, photon.
This is possible because the material is efficient at absorbing three
photons at once, said Prasad.
The infrared wavelength the researchers used to stimulate, or pump, the
lasing action was 1.3 microns, which is one of two wavelengths used commonly
in optical communications. The emitted green-yellow lightwaves measured
0.53 microns from crest to crest.
The process could eventually prove useful for short-pulse optical fiber
communications; it could be used to shift the size of a wavelength and
to reshape and stabilize light pulses, according to Prasad. The 1.3-micron
infrared radiation penetrates more deeply into biological tissues, but
causes less damage than visible light. Because of this, materials that
provide efficient three-photon absorption could prove useful way to trigger
therapeutic reactions inside living tissue, Prasad said.
The three-photon process has some inherent advantages over conventional
lasers, according to He. "Two-photon absorption shows a square dependency
on the input light intensity, whereas three-photon absorption exhibits
a cubic dependence. Due to this difference, we could get much higher data
storage density and three-dimensional imaging resolution" with three-photon
absorption devices, he said. The cubic dependence provides a stronger
spatial confinement for the beam, which in turn will allow for higher
The research is promising, said Dmitri Strekalov, a research scientist
at NASA. It may eventually result in a "fundamentally new tool for photochemistry
[with applications in] biology and medicine... or may allow for improvements
in optical lithography [that bring] higher resolution and lower cost,"
he said. This could also lead to a new type of short-wavelength laser,
What is needed next is "a better selection of materials with various properties...
and optimization of their response," said Strekalov.
What's required to bring the three-photon process to its potential, is
"developing more highly efficient three-photon absorption and exploring
various three-photon absorption... techniques in fields such as ultrashort
wavelength frequency upconversion lasing, short-pulse optical communications,
high-density three-dimensional data storage and biophotonics, said He.
The work could find practical application in 10 years or so, according
Prasad and He's research colleagues were Przemyslaw P. Markowitz and Tzu-Chau
Lin. They published the research in the February 14, 2002 issue of Nature.
The research was funded by by the Air Force Office of Scientific Research
and the Air Force Research Laboratory at Dayton.
Timeline: 10 years or more
TRN Categories: Biology; Physics; Materials Science and
Engineering; Quantum Computing
Story Type: News
Related Elements: Technical paper, "Observation of Stimulated
Emission by Direct Three-Photon Excitation," Nature, February 14, 2002.
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