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.