Material turns infrared to green

By Kimberly Patch, 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 contrast imaging.

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, he added.

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 to Prasad.

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
Funding:   Government
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.


February 20, 2002

Page One

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Real birds change virtual evolution

Material turns infrared to green

Nanotube chips draw near

Chip provides more bang


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