Organic Lasers Promise New Lease on Light
Robert F. Service 

 Tiny solid state lasers are big business: Almost $500 million worth of the devices are sold each year for uses ranging from compact disc players to telecommunications equipment. But they have big drawbacks: Many of today's lasers are made from ceramic chips--similar to those at the heart of computer processors--that require expensive clean-room facilities to manufacture, and their color palette is somewhat limited. Researchers have long pinned their hopes on organic materials, which are typically easier and cheaper to process. But to date they have managed to coax organic solids to lase only when blasted with a beam from another laser--hardly a commercial advantage. Now, however, organics have finally begun to shine on their own.

 On page 599, a team at Lucent Technologies' Bell Laboratories in Murray Hill, New Jersey, reports that they've devised the first electrically powered solid state organic laser, a step that could open the floodgates for novel lasers that are cheaper and that shine in colors inorganics can't match. The feat is "big news," says Yang Yang, an optics researcher at the University of California, Los Angeles. "This is really a milestone. People have tried to make organic electrically pumped lasers for a long time." 

Beaming. Transistor "gate" electrodes on top and bottom cause electrical charges to flow between the two additional pairs of electrodes (top). A voltage applied between transistors then causes these charges to enter the middle layer, where they produce photons (center) that generate a laser beam (bottom). 

To work as lasers, solid materials must function something like an interstate freeway: They must allow lots of traffic--photons in this case--to speed along without hitting potholes, and they must have plenty of on-ramps for electrical charges to get into the device. In conventional ceramic lasers, the on-ramps are metal electrodes placed above and below a semiconductor crystal. When a voltage is applied between the electrodes, electrons flow into one side of the crystal and out the other. The electron vacancies left behind, called "holes," act like positive charges that can move through the material as an electron on a nearby molecule jumps into the hole, leaving a vacancy where it originated. When electrons coming from one side of the device meet holes coming from the other, they annihilate one another, creating photons in the process. The photons bounce back and forth between mirrors on opposite sides of the crystal, prompting the crystal to release additional photons of the same wavelength. This creates a surge of light, some of which escapes in a beam through a predesigned leak in one of the mirrors.

 Ceramics such as gallium arsenide chips make great lasers because, like a good freeway, they are clean and fast and have easy- access on-ramps. Solid organics, on the other hand, have been more like old country roads: Defects in the materials act like hazardous potholes, trapping photons and causing them to dissipate their energy as heat. And even high-quality organic materials have had big trouble with their on-ramps: Conventional metal electrodes are just too slow at injecting electrons and holes into organics.

 The Bell Labs group--physicists J. Hendrik Schön, Ananth Dodabalapur, and Bertram Batlogg, along with materials scientist Christian Kloc--tackled those two problems in turn. Initially, Kloc used a specialized gas furnace to grow high-purity crystals of tetracene, a molecule that consists of four linked rings of carbon atoms. That gave the researchers the multilane, high-speed freeway they needed for the photons. For their on-ramps, the team did away with the standard electrodes and turned to a pair of transistors, known as field-effect transistors, or FETs. FETs work by applying a voltage to one electrode, called a "gate," that triggers a flood of electrical charges to flow through a channel between two additional electrodes. Depending on the makeup of the FET, the flowing charges can be either electrons or holes.

 The researchers placed the FETs above and below the tetracene crystal. The bottom FET was designed to flood its channel with electrons, while the top FET sent holes. The team then applied a voltage between the two FETs, which drew the flood of positive and negative charges into the tetracene, where they produced a burst of photons that triggered the lasing process. The scheme worked to perfection, generating a yellowish-green laser pulse. This novel use of FETs "is an important concept, because it allows them to control the charge injection, which is the key to getting this to work as a laser," Yang says.

 Despite the organic laser's success, it may be a while before organics take over that $500 million market. Growing high-purity organic crystals requires manufacturing processes nearly as exacting as those used to grow conventional ceramic chips. And researchers must also learn how to mass-produce lasers with transistors positioned above and below. Still, Batlogg notes that researchers should easily be able to change the tetracene to other organics to produce a whole range of different colors of laser light. That should give lasermakers something to beam about.

Volume 289, Number 5479, Issue of 28 Jul 2000, pp. 519-521. 
Copyright © 2000 by The American Association for the Advancement of Science.