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Plastic glows with bright laser light.

For 6 years, since they discovered it gives off light when electricity passes through it, scientists in Cambridge, England, have set their sights on a plastic called PPV. Now, the researchers have even higher hopes for the material. They have demonstrated that PPV can emit not just any light, but coherent laser light.

PPV is one of a class of polymers that researchers are developing as light-emitting diodes (LEDs) for use in electronic displays (SN: 10/16/93, p. 246). Getting PPV to produce laser light proves that "it's extremely good for light emission, which was questioned in the scientific community," says Richard H. Friend of the Cavendish Laboratory in Cambridge. That bodes well for polymer LEDs now in development. Friend and his colleagues Nir Tessler and Graham J. Denton report the work in the Aug. 22 Nature.

To create the laser, the researchers sandwiched the PPV between two mirrors: a thin silver film and a layered material called a distributed Bragg reflector, which reflects visible light with almost 100 percent efficiency. Like any laser, this device produces light by a process called stimulated emission.

Shining light through the reflector causes the PPV to emit photons, some of which are reflected back and forth between the mirrors, in turn triggering a cascade of more photons. The photons that are not reflected but pass through the silver film have a single wavelength and are in phase, forming the coherent laser beam.

The light emitted was yellow-green, but polymer laser light could, in principle, cover a range of visible wavelengths, Friend says. Most commercial lasers emit in the red or infrared range. The next big goal is to develop polymer lasers that can be excited by electricity rather than light.

Polymers can be easily deposited over large areas. How that quality can be exploited for lasers remains to be seen, but it's useful when making large, flat LED panels such as computer screens or billboards. With polymers, researchers "don't have to grow single crystals like for other semiconductors," says Alan J. Heeger, chief scientist at UNIAX Corp. in Santa Barbara, Calif.

At the International Conference on Synthetic Metals earlier this month in Snowbird, Utah, Heeger and other UNIAX scientists and a group at the University of Utah in Salt Lake City reported independent evidence of laser-emitting polymers.

"These materials look fantastic," Heeger says. "They will set the community looking in a new direction."
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Title Annotation:PPV can emit coherent laser light
Publication:Science News
Article Type:Brief Article
Date:Aug 24, 1996
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