Scientists who recently developed new molecules that simultaneously absorb two photons of light today report that the molecules are sensitive enough to laser light that a myriad applications in materials science and photonics are possible.
Joseph W. Perry and Seth R. Marder of the chemistry department at The University of Arizona in Tucson and their colleagues, formerly at the California Institute of Technology, report the work in the cover story of the current issue of Nature (March 4).
The team described their design for efficient two-photon-absorbing organic molecules in Science in September 1998. They have developed a new class of light-absorbing compounds that, when pulsed with lasers, absorb photons two at a time and can trigger chemical or physical changes with light wavelength-long resolution in three dimensions. With tightly focused lasers, the processes can be initiated and controlled within light-wavelength size spaces: Because two-photon absorption varies with the square of light intensity, the processes occur only at focus, within the volume of a cube with sides the length of the laser's light wavelength. But slightly away from focus, two-photon absorption is negligible.
In today's journal article, Perry, Marder and their team report that highly efficient two-photon-absorbing polymers, or resins, can be produced and used for three-dimensional optical data storage and for three-dimensional manufacturing of microscopic parts for subminiature optical or mechnical devices.
The broad and far-reaching potential applications for the compounds, on which patents are pending, range also into medicine, for example in biomedical imaging, the researchers say. Their experiments show that lasers which are practical for industry supply enough power to excite the new two-photon absorbing molecules.
Perry, who uses laser spectroscopy to study the optical properties of molecules, and Marder, whose expertise is in making organic molecules tailored for specific, useful properties, have been working as a team since 1987 on the design of molecules with very sensitive nonlinear optical properties. (Nonlinear systems are those in which output is not linearly related to input.)
One nonlinear optical property widely regarded as a curiosity -- or as a downright nuisance by scientists trying to improve optical switching processes -- is the property of certain molecules to simultaneously absorb multiple photons of light. For optical switching, the goal is to get all the light going into a switching system to come out again. Light absorption is a problem.
By 1995, Perry and Marder -- then working at Caltech and the NASA Jet Propulsion Laboratory -- were seriously thinking about two-photon absorption not as a problem, but as a great opportunity.
"The basic idea that a molecule could simultaneously absorb two photons of light goes back almost 70 years," Marder said. "That concept was first published by a Maria Goeppert-Mayer, back in 1931. People have long realized there are many things one might in principle do with such molecules. The problem is, they didn't have molecules that were sensitive enough to snap up two photons efficiently."
In the summer of 1995, the researchers were shooting an orange laser beam into a black-cloth covered, optical cell filled with a clear solution of an electron-donor containing dye molecule for a type of nonlinear optical experiment. The effect they observed was striking.
"When I took the black cloth off to visually examine the cell," Perry said, "there was a very bright blue streak of light in the otherwise clear solution. The orange (lower-energy) laser beam was generating a blue (higher-energy) light -- fluorescence. We were seeing photons coming out of the system at higher energies than they were going in. That could only be explained if these molecules were simultaneously absorbing two photons, losing some energy and then emitting a photon still higher in energy than the ones being absorbed."
"We were very excited because the fluorescence was obviously generated by strong two-photon absorption. We realized that we had stumbled onto an example of what we would later show to be a whole class of molecules with an exceptional ability to absorb light two photons at a time."
Since, Marder and Perry have explored a few of many potential applications for two-photon absorption. In their Nature paper, they discuss how it might be used for high-density, three-dimensional optical data storage by rapid writing with a tightly focused laser in a thick storage medium.
"A lot of companies are working very intensively right now to try to develop media and to commercialize 3D optical data storage," Perry noted. But lacking sensitive two-photon absorbing molecules, researchers have had to use lasers so powerful that they are impractical for commercial memory systems.
The chemists also describe how two-photon absorption might be used to fabricate free-standing, three-dimensional microscopic structures or molds for structures from polymer, ceramic or semiconductor materials, for example, by moving a laser beam in any desired pattern, a "3D lithographic microfabrication" technique.
Two-photon-absorption adds the third dimension, depth, to such two-dimensional technologies as fluorescence microscopy and photo lithography.
"We have provided the knowledge and the molecules to do this well enough that people now can really start thinking about using two-photon absorption in ways they would not have considered feasible in the past," Marder said.
Marder and Perry, who joined the UA chemistry faculty three months ago, this week are setting up laboratories at the university's Science and Technology Park east of Tucson for continued research on two-photon absorption and other nonlinear optical properties.
More information about UA science and research is available at the News Services WWW site: http://science.opi.arizona.edu
Lori Stiles, UA News Services -- 520-626-4402 or 621-1877; firstname.lastname@example.org
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