30 July 2009

Nano lasers can relieve computer processing bottleneck

When lasers get smaller, that will quickly pave the way for the performance of computers to improve and also possibly speed up Internet service.

Lasers are everywhere in the world of electronics. They are essential components of CD and DVD players, not to mention the automatic check-out stations in supermarkets.

Small lasers used in technology enable communications across continents, and soon nanolasers will see use for communications between the parts inside a computer.

Engineers, in a joint effort between researchers at Arizona State University and Technical University of Eindhoven in the Netherlands, are making lasers smaller because it would enable the devices to more effectively integrate with small electronics components. The more lasers that can work with these components, the faster electronic devices could perform, said Professor Martin Hill, who leads the Eindhoven team, and ASU team leader Cun-Zheng Ning, a professor in the School of Electrical, Computer and Energy Engineering in ASU’s Ira A. Fulton Schools of Engineering.

The size of lasers in any one dimension has always been limited to one-half of the wavelength involved.

So, for lasers used in optical communications the required wavelength is 1,500 nanometers, which means experts said a 750-nanometer laser would be the smallest a laser could be for optical communications.

In an optically denser medium such as a semiconductor, this limit reduces by a factor of the index of refraction (expressed mathematically as ~3.0) of a semiconductor—in this case to about 250 nanometers.

The limit is the diffraction limit, a property associated with any wave, such as a beam of light. Under the current theory, you cannot make a laser smaller than this diffraction limit—or smaller than 250 nanometers for a semiconductor laser for communications devices.

The research teams at ASU and Eindhoven are showing there are ways around this limit, Ning said.

One way to get around this limit is the use of a combination of semiconductors and metals such as gold and silver.

“It turns out that the electrons excited in metals can help you confine a light in a laser to sizes smaller than that required by the diffraction limit,” Ning said. “Eventually, we were able to make a laser as thin as about one quarter of the wavelength or smaller, as opposed to one half.”

Ning and Hill have achieved something like that by using a “metal-semiconductor-metal sandwich structure,” in which the semiconductor is as thin as 80 nanometers and goes between 20-nanometer dielectric layers before putting metal layers on each side.

They have demonstrated such a semiconductor/dielectric layer, thinner than the diffraction limit, and squeezed between metal layers, can actually emit laser light, which is the smallest thickness of any ever produced. The structure, however, has worked only in a low-temperature operating environment. The next step is to achieve the same laser light emission at room temperature.

Researchers across the globe remain interested in integrating such metallic structures with semiconductors to produce smaller nanolasers because of the promise of applications for smaller lasers in a wide range of technologies.

“This is the first time that anyone has shown that this limit to the size of nanolasers can be broken,” Ning said. “Beating this limit is significant. It opens up diverse possibilities for improving integrated communications devices, single molecule detection, and medical imaging.”

Nanoscale lasers can also integrate with other biomedical diagnostic tools, making them work faster and more efficiently, he said.

These advances also represent a major step in nanophotonics, the study of the behavior of light on the nanometer scale and the ability to fabricate devices in nanoscale.

“Nanolasers can be used for many applications, but the most exciting possibilities are for communications on a central processing unit (CPU) of a computer chip,” Ning said. As computers get faster, the communication between different parts in a computer creates a processing bottleneck, he said.

Since a signal can transmit between computer components much faster by a light wave emitted by a laser than by metal wires, optical communication (communication using light) is “the ultimate solution for improving on semiconductor chip communications,” Ning said.

“But before this becomes a reality, lasers have to be made small enough to be integrated with small electronics components,” he said. “This is why the Department of Defense and chip manufacturers such as Intel are working on optical solutions for on-chip communications.”

For related information, go to www.isa.org/manufacturing_automation.