Plasmon lasers at room temperature

A new technique allows plasmon lasers to operate at room temperature, a “major step toward applications” for the lasers, according to Xiang Zhang, principal investigator, who is also a University of California professor of mechanical engineering and a faculty scientist at Lawrence Berkeley National Laboratory.

Zhang also directs the UC Berkeley Center for Scalable and Integrated Nanomanufacturing, established through the National Science Foundation’s Nanoscale Science and Engineering Centers program. His team’s achievement was described Dec. 19, 2010, in an advance online publication of the journal Nature Materials.

A new plasmon laser can generate subdiffraction light at room temperature. Left, a schematic shows a cadmium-sulfide square atop a silver substrate separated by a 5-nm gap of magnesium fluoride. 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. At right, a scanning electron microscope image of the plasmon laser. Courtesy of Renmin Ma and Rupert Oulton, UC Berkeley.

Plasmon lasers couple electromagnetic waves with electrons that oscillate at the surface of metals to squeeze light into nanoscale spaces far past its natural diffraction limit of half a wavelength. Last year, Zhang and his team reported a plasmon laser that generated visible light in a space only 5 nm wide, or about the size of a single protein molecule. But they could not exploit their discoveries for commercial devices because of the extreme cooling required: down to cryogenic temperatures as low as 10 K. For previous designs, the cooling was vital to increase amplification of the remaining light energy to enable sustained laser operation; without it, most of the light produced by the laser leaked out.

Inspired by a whispering gallery, the scientists used a total internal reflection technique to bounce surface plasmons back inside a nanosquare device. This enhanced the emission rate of light by eighteenfold and confined the light to a space of about 20 nm. By controlling the loss of radiation, it was no longer necessary to encase the device in a vacuum cooled with liquid helium; the laser could function at room temperature.