Nanolasers Heat Up

Researchers have cleared a major hurdle to the practical use of nanoscale lasers, opening the way to fundamentally new capabilities in biosensing, computing, and optical communications. A team at the University of California, Berkeley, has demonstrated the first semiconductor plasmon nanolaser, or “spaser,” that can operate at room temperature.

While traditional lasers work by amplifying light, spasers amplify particles called surface plasmons, which can do things that the photons in ordinary light waves can’t. For instance, photons can’t be confined to areas with dimensions much smaller than half their wavelength, or about 250 nanometers, limiting the extent to which optical devices can be miniaturized. Plasmons, however, can be confined in much smaller spaces and then converted into conventional light waves—making them useful for ultra-high-resolution imaging or miniaturized optical circuits that might, for example, operate 100 times faster than today’s fastest electronic circuits.

Working with Berkeley mechanical-engineering professor Xiang Zhang, postdocs Ren-Min Ma and Rupert Oulton designed and demonstrated the new semiconductor spaser. It uses metals and semiconductors, long recognized to be attractive materials because of their ubiquity and resilience. But previous spasers made of them lost too much energy to sustain lasing unless cooled to extremely low temperatures, below -250 °C.

“For a time there was a lot of criticism that plasmon lasers would only work at low temps,” says Martin Hill, a professor of electrical engineering at the Technical University of Eindhoven, in the Netherlands, who researches nanolasers. “This [is] an interesting demonstration and a step towards making useful devices and encouraging more people to look at plasmon-mode nanolasers.”

The team’s device contains a 45-nanometer-thick, 1-micrometer square of cadmium sulfide, a semiconductor used in some solar cells and photoresistors for microchip manufacturing. The square rests on a a 5-nanometer slice of magnesium fluoride, atop a sheet of silver. When light from a commercial laser hits the metal, plasmons are generated on its surface. But the cadmium sulfide square confines the plasmons to the gap, reflecting them back each time they hit an edge. Less than 5 percent of the radiation escapes the structure, allowing sustained surface-plasmon lasing, or “spasing,” at room temperature. The research was published online in Nature Materials on December 19.

This isn’t the first spaser to work at room temperature. In fact, the very first spaser used dye-based materials that work at room temperature. But these materials can only be activated with pulses of light—called optical pumping—which limits applications. The Berkeley team used optical pumping to demonstrate its laser because “it’s easier,” says Oulton, but the major advantage of semiconductor lasers is that they can be pumped electrically—the team’s ultimate goal. “We need to be able to plug real-world devices into a wall socket. This is without question,” Oulton says.

While Hill is excited by the Zhang group’s demonstration, he notes that “electrically pumped devices are a much more technically difficult thing. For example, for photonic crystal lasers, it took many years from the first optically pumped laser until an electrically pumped device was made.”

The Nature Materials paper describes only sustained lasing within the cadmium sulfide cavity, which Ma says is useful for applications like single-molecule detection, important in high-sensitivity biological and medical testing. The researchers are working on demonstrating a biosensor based on the laser, and Ma says a practical device might be possible within a few years. They have also developed ways to couple the light output of the spaser so that it can be used in plasmonic circuits for optical computing or communications. Building simple plasmonic circuits is another project Zhang’s group is pursuing.

Other possible applications for the spaser include using it to focus light beams in photolithography, making possible the manufacture of microchips with features smaller than 20 nanometers, about the limits of optical lasers. It could also be useful for packing more data onto storage media such as DVDs and hard disks. Ma notes that both applications would require the addition of a plasmonic lens to further focus the light; this is something else that Zhang’s lab has worked on.

The group is enthusiastic about the potential to eventually commercialize this design, since it uses inorganic semiconductors already common in computing and communications. Ma says it should be “very easy” to integrate devices based on the design into current fabrication processes. Oulton and Hill both also mention that the materials are extremely robust and have long lifetimes inside devices.

Optimistically, say Ma and Oulton, proof of principle—electrically injected plasmonic lasers that run at room temperature—should be possible within a couple of years, and commercial devices could follow quickly.