Plastic Lasers Starting to Shine

Photo: Karl leo

Organic lasers could be tuned to emit a broad range of wavelengths, could be built on sheets of plastic, would be flexible enough to bend, and very inexpensive to make. But while organic LEDs are a big part of the smartphone display market and are making inroads in solid-state lighting and flexible solar cells, the laser remains elusive.

“The OLED display works so well, it would be really nice to have a laser as well,” says Karl Leo, who heads the Institut für Angewandte Photophysik of TU Dresden and the Solar Center at King Abdullah Unhiversity of Science and Technology, Saudi Arabia.  Leo, who spoke at the Fall Meeting of the Materials Research Society (MRS) in Boston last week, says his lab has come up with a possible path toward an electric-powered organic laser by adding some metal to the laser cavity.

Optically pumped organic lasers, which use light from another laser as a power source, already exist. At another MRS session, a German company, Visolas, described an optically pumped organic laser they’re close to commercializing as part of a mobile blood analysis system. But lasers are usually considered viable only when they can run on electricity, and that’s Leo’s goal. The trouble is that such electrical pumping requires a high density of excited charge carriers, on the order of kiloamperes per square centimeter. Such levels are not a problem in an inorganic material, such as gallium arsenide, but the carriers would create additional detrimental absorption and the heat generated could damage the organic materials.

Metal, too, is usually a bad thing to have in a laser cavity, because the metal absorbs photons to such an extent that it kills the lasing effect. Leo’s team built a vertically oriented laser cavity that consists of an organic “active layer” between two mirrors. The mirrors are reflective gratings made from alternating layers of titanium oxide and silicon dioxide. In between the bottom mirror and the active layer they placed stripes of silver, 40 nanometers thick and 1110 nm wide.

Placing the metal grating on top of the reflective grating caused the creation of so-called Tamm plasmon polaritons. Plasmon polaritons are oscillations of electron density that can exist at the interface between metal and other materials and amplify light, so the placement of the metal actually increased the lasing effect. “It’s possible to include a highly conductive metal contact into the cavity,” Leo told the meeting. “If you pump it hard enough, it can lase.”

He says, though, this is only an early step toward an organic laser. Reaching the threshold where the device begins to lase still requires very high currents that wouldn’t be practical in a real device. Therefore, he says, a useful organic laser could still be a decade in the future.

Still, he believes the pursuit is worthwhile. A cheap, broadly tunable laser would certainly be welcomed in optical communications, and it’s likely people will develop other applications, just as they did once inorganic lasers were created. “I’m sure if somebody makes an electric organic laser there will be a use for it,” says Leo.

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