Plastic Lasers Starting to Shine

Polymer-based lasers, the OLED’s more powerful cousin, inch closer with the use of a plasmonics trick

2 min read
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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3 Ways 3D Chip Tech Is Upending Computing

AMD, Graphcore, and Intel show why the industry’s leading edge is going vertical

8 min read
A stack of 3 images.  One of a chip, another is a group of chips and a single grey chip.
Intel; Graphcore; AMD

A crop of high-performance processors is showing that the new direction for continuing Moore’s Law is all about up. Each generation of processor needs to perform better than the last, and, at its most basic, that means integrating more logic onto the silicon. But there are two problems: One is that our ability to shrink transistors and the logic and memory blocks they make up is slowing down. The other is that chips have reached their size limits. Photolithography tools can pattern only an area of about 850 square millimeters, which is about the size of a top-of-the-line Nvidia GPU.

For a few years now, developers of systems-on-chips have begun to break up their ever-larger designs into smaller chiplets and link them together inside the same package to effectively increase the silicon area, among other advantages. In CPUs, these links have mostly been so-called 2.5D, where the chiplets are set beside each other and connected using short, dense interconnects. Momentum for this type of integration will likely only grow now that most of the major manufacturers have agreed on a 2.5D chiplet-to-chiplet communications standard.

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