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U.S. scientists take plasmon lasers out of deep freeze

12-21-2010 14:46 BJT

LOS ANGELES, Dec. 20 (Xinhua) -- U.S. scientists have developed a new technique that allows plasmon lasers to operate at room temperature, overcoming a major barrier to practical utilization of the technology, the American Association for the Advancement of Science (AAAS) reported on Monday.

The achievement, made by researchers at the University of California in Berkeley, is a "major step towards applications" for plasmon lasers, the AAAS said on its website EurekAlert.com.

"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," Xiang Zhang, the research team's principal investigator and professor of mechanical engineering, said in remarks published by the report.

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 its natural diffraction limit of half a wavelength.

Last year, Zhang's team reported a plasmon laser that generated visible light in a space only five nanometers wide, or about the size of a single protein molecule, according to the report.

But efforts to exploit such advancements for commercial devices had failed.

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, researchers explained.

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.

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 configuration was made out of a cadmium sulfide square measuring 45 nanometers thick and one micrometer long placed on top of a silver surface and separated by a five nanometer gap of magnesium fluoride.

The scientists were able to enhance by 18-fold the emission 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 Renmin Ma, a post- doctoral researcher in Zhang's lab.

"Lasers with a mode size of a single protein are a key milestone 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 components of other functional building-blocks in integrated circuits, such as add-drop filters, direction couplers and modulators," Ma said.

Editor:Zheng Limin |Source: Xinhua

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