A new technique allows plasmon lasers to operate at room temperature, freeing the technology from vacuum chambers cooled to cryogenic temperatures and overcoming a major barrier to their practical use. The achievement, by researchers at the University of California, Berkeley, is a "major step towards applications" for plasmon lasers, said the research team’s principal investigator, Xiang Zhang, UC Berkeley professor of mechanical engineering and faculty scientist at Lawrence Berkeley National Laboratory. Schematic of a plasmon laser showing a cadmium sulfide (CdS) square atop a silver (Ag) substrate separated by a 5-nm gap of magnesium fluoride (MgF2). The cadmium sulfide square measures 45 nm thick and 1 µm long. The most intense electric fields of the device reside in the magnesium fluoride gap. (Images: Renmin Ma and Rupert Oulton) "Plasmon lasers can make possible single-molecule biodetectors, photonic circuits and high-speed optical communication systems, but for that to become reality, we needed to find a way to operate them at room temperature," said Zhang, who also directs at UC Berkeley the Center for Scalable and Integrated Nanomanufacturing, established through the National Science Foundation’s (NSF) Nano-scale Science and Engineering Centers program. In recent years, scientists have turned to plasmon lasers, which work by coupling electromagnetic waves with the electrons that oscillate at the surface of metals to squeeze light into nanoscale spaces far past the natural diffraction limit of half a wavelength. Last year, Zhang's team reported a plasmon laser that generates visible light in a space only 5 nm wide, or about the size of a single protein molecule. But efforts to exploit such advancements for commercial devices had hit a wall of ice. "To operate properly, plasmon lasers need to be sealed in a vacuum chamber cooled to cryogenic temperatures as low as 10 kelvins, or minus 441 degrees Fahrenheit, so they have not been usable for practical applications," said Renmin Ma, a postdoctoral researcher in Zhang's lab. In previous designs, most of the light produced by the laser leaked out, which required researchers to increase amplification of the remaining light energy to sustain the laser operation. To accomplish this amplification, or gain increase, researchers put the materials into a deep freeze. To plug the light leak, the scientists took inspiration from a whispering gallery, typically an enclosed oval-shaped room located beneath a dome in which sound waves from one side are reflected back to the other. This reflection allows people on opposite sides of the gallery to talk to each other as if they were standing side by side. (Some notable examples of whispering galleries include the US Capitol's Statuary Hall, New York's Grand Central Terminal and the rotunda at San Francisco's city hall.) Instead of bouncing back sound waves, the researchers used a total internal reflection technique to bounce surface plasmons back inside a nanosquare device. The configuration was made out of a cadmium sulfide square measuring 45 nm thick and 1 µm long placed on top of a silver surface and separated by a 5-nm gap of magnesium fluoride. Electron microscope image of the plasmon laser. The scientists were able to enhance by 18-fold the emission rate of light, and to confine the light to a space of about 20 nm, or one-twentieth the size of its wavelength. By controlling the loss of radiation, it was no longer necessary to encase the device in a vacuum cooled with liquid helium. The laser functioned at room temperature. "The greatly enhanced light-matter interaction rates mean that very weak signals might be observable," said Ma. "Lasers with a mode size of a single protein are a key milestone toward applications in ultracompact light source in communications and biomedical diagnostics. The present square plasmon cavities not only can serve as compact light sources but also can be the key components of other functional building blocks in integrated circuits, such as add-drop filters, direction couplers and modulators." The US Air Force Office of Scientific Research and the NSF helped support this work, which was described last month in an advance online publication of the journal Nature Materials. Ma and Rupert Oulton, a former postdoctoral researcher in Zhang's lab and now a lecturer at Imperial College London, are co-lead authors of the paper. Other co-authors are Volker Sorger, a UC Berkeley PhD student in mechanical engineering, and Guy Bartal, a former research scientist in Zhang's lab. For more information, visit: www.berkeley.edu