Researchers at the University of California, Berkeley, have developed a new technique that allows plasmon（胞质基因，等离子体振子） lasers to operate at room temperature, overcoming a major barrier to practical utilization¹ of the technology. The achievement, described Dec. 19 in an advanced online publication of the journal Nature Materials, is a "major step towards applications" for plasmon lasers, said the research team's principal investigator², Xiang Zhang, UC Berkeley professor of mechanical engineering and faculty³ scientist at Lawrence Berkeley National Laboratory.

"Plasmon lasers can make possible single-molecule⁴ biodetectors, photonic（光激性的） 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（振荡，摆动） at the surface of metals to squeeze light into nanoscale spaces far past its natural diffraction limit of half a wavelength⁵. Last year, Zhang's team reported a plasmon laser that generated visible light in a space only 5 nanometers wide, or about the size of a single protein molecule.

But efforts to exploit such advancements⁶ for commercial devices had hit a wall of ice.

"To operate properly, plasmon lasers need to be sealed in a vacuum chamber⁷ cooled to cryogenic temperatures（制冷温度，低温） as low as 10 kelvins, or minus 441 degrees Fahrenheit⁸, so they have not been usable for practical applications," said Renmin Ma, a post-doctoral researcher in Zhang's lab and co-lead author of the Nature Materials paper.

In previous designs, most of the light produced by the laser leaked out, which required researchers to increase amplification⁹ 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 dome¹⁰ 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 standing¹¹ side by side. (Some notable examples of whispering galleries include the U.S. Capitol's Statuary Hall, New York's Grand Central Terminal, and the rotunda¹² 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 nano-square device. The configuration¹³ was made out of a cadmium sulfide（硫化镉） square measuring 45 nanometers thick and 1 micrometer long placed on top of a silver surface and separated by a 5 nanometer gap of magnesium¹⁴ fluoride.

The scientists were able to enhance by 18-fold the emission¹⁵ rate of light, and confine the light to a space of about 20 nanometers, 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 means that very weak signals might be observable," said Ma. "Lasers with a mode size of a single protein are a key milestone¹⁶ toward applications in ultra-compact 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 components¹⁷ of other functional¹⁸ building-blocks in integrated circuits, such as add-drop filters, direction couplers（耦合器，连接器） and modulators（调节器） ."