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Organic Lasers Go Electric

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Richard Gaughan

Physicists at Lucent Technologies Inc.'s Bell Labs have developed the first electrically driven organic semiconductor laser. Constructed of crystalline tetracene, a molecule that comprises four benzene molecules, the device emits light at 575.7 nm. The achievement promises to enable the development of inexpensive laser sources across a wide spectral range for large-screen displays, compact disc players and other applications.

Until now, it was thought that high threshold current densities would make electrical pumping impractical, so previous organic lasers were optically pumped. Jan Hendrik Schön and his colleagues selected the high-quality crystalline tetracene rather than an amorphous medium. This material displays low absorption at the lasing wavelength and a threshold current lower than what has previously been achieved in an organic lasing medium.

The researchers thermally evaporated gold electrodes onto both faces of a freshly cleaved tetracene crystal 25 µm long and several hundred microns wide. They sputtered amorphous aluminum oxide onto the crystal and deposited zinc oxide gate electrodes. In operation, the field-effect transistors that the deposited electrodes created served as sources for both holes and electrons.

Researchers at Bell Labs have demonstrated an electrically driven semiconductor laser constructed of crystalline tetracene. Electrically driven organic lasers could find a place in a variety of consumer applications.

"The use of field-effect electrodes," Schön explained, "allows for a precise control of the injected hole and electron current, leading to efficient and balanced charge injection necessary for electrically driven lasing." A fixed voltage across the drain and source contacts drives the holes and electrons toward each other, where they radiatively recombine.

The device has an average optical power of more than 150 µW and displays continuous-wave operation at temperatures as high as 200 K. Schön said that the team has achieved lasing at room temperature, where the lasing threshold is about 800 A/cm2, and that enhanced room-temperature operation will follow. "Improving the cavity will lower the threshold," he said. "We also plan to change the six-terminal device structure to four- or three- or maybe even two-terminal structures."

Now that the researchers have demonstrated electrically driven operation, other laser parameters may be controlled. For example, Schön said that it should be possible to tune the emission wavelength throughout the entire visible spectrum by designing or selecting other molecules with charge transport properties similar to tetracene.

The group also will focus on the manufacturing process, including production of the lasers as thin-film devices on flexible plastic substrates.

Photonics Spectra
Nov 2000
ConsumerindustrialResearch & TechnologyTech Pulse

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