Thinnest semiconductor laser holds promise for on-chip interconnects

Semiconductor lasers with thicknesses down to 80 nm have been built by researchers at Arizona State University (ASU; Tempe, AZ) and the Technical University of Eindhoven (Eindhoven, The Netherlands).¹ The work opens up possibilities for using nanoscale lasers in optical integrated circuits to significantly improve the performance of computers and speed up Internet access.

Authors of the report include professor Martin Hill, who leads the Eindhoven team, and ASU team leader Cun-Zheng Ning, a professor at the School of Electrical, Computer and Energy Engineering in ASU's Ira A. Fulton Schools of Engineering.

Engineers have been trying to make lasers smaller in part because it would enable the devices to be more effectively integrated with silicon electronics, but the size of lasers in any one dimension (for example, thickness) has been thought to be limited by diffraction to one-half of the wavelength involved. For instance, for lasers used in optical communications the required wavelength is about 1500 nm, so a 750 nm laser was thought to be the smallest possible for optical communications. In an optically denser medium such as a semiconductor, this limit is reduced by a factor of the index of refraction of a semiconductor--in this case to about 250 nm.

The research teams at ASU and Eindhoven are showing there are ways around this supposed limit, Ning says. One way is by using 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 says. "Eventually, we were able to make a laser as thin as about one-quarter of the wavelength or smaller, as opposed to one-half."

Semiconductor, dielectric, and metal sandwich
Ning and Hill use a metal-semiconductor-metal sandwich structure, in which the semiconductor is as thin as 80 nm and is sandwiched between 20 nm dielectric layers before putting metal layers on each side. They have demonstrated a laser with such a structure--a laser with the smallest thickness of any ever produced. The structure, however, has worked only in a cryogenic environment. The next step is to achieve the laser emission at room temperature.

"This is the first time that anyone has shown that this limit to the size of nanolasers can be broken," Ning says. "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 be integrated with other biomedical diagnostic tools, making them work faster and more efficiently, he says.

Nanolasers can be used for many applications, but the most exciting possibilities are for optical communications on a central processing unit (CPU) of a computer chip, Ning says. "But before this becomes a reality, lasers have to be made small enough to be integrated with small electronics components," he adds. "This is why the Department of Defense and chip manufacturers such as Intel are working on optical solutions for on-chip communications."

Research in this field in the United States is being funded by the Defense Advanced Research Projects Agency (DARPA), the central research and development organization for the U.S. Department of Defense. The agency is supporting a collaborative team partnering researchers at ASU, the University of California at Berkeley and the University of Illinois, Urbana-Champaign.

ASU's collaboration with Hill's team at Eindhoven happened by coincidence, Ning says. "We discovered we were working on the same problems and trying to achieve similar goals using similar ideas," he notes. "So the partnership developed."

REFERENCE

1. Martin T. Hill et al., Optics Express, Vol. 17, issue 13, p. 11107, June 18, 2009.