31 August 2009

Nano-scale plasmon laser breaks size barrier

by Kate Melville

Researchers at the University of California, Berkeley (UC), have reached a new milestone in laser physics with the creation of the world's smallest semiconductor laser, capable of generating visible light in a space smaller than a single protein molecule.

Described in the journal Nature, the research breaks new ground as the scientists not only successfully squeezed light into such a tiny space, but they also found a novel way to keep that light energy from dissipating as it moved along, thereby achieving laser coherence. "This work shatters traditional notions of laser limits, and makes a major advance toward applications in the biomedical, communications and computing fields," said Xiang Zhang, director of UC Berkeley's Nanoscale Science and Engineering Center.

The achievement should help enable the development of such innovations as nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power.

The Nature article notes that while it was traditionally accepted that laser light cannot be focused beyond the size of half its wavelength, scientists have found a way to compress light down to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as surface plasmons. But the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.

Zhang and his research team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire - 1,000 times thinner than a human hair - with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished. With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.

"When you are working at such small scales, you do not have much space to play around with," said co-researcher Rupert Oulton. "In our design, the nanowire acts as both a confinement mechanism and an amplifier. It's pulling double duty."

Trapping and sustaining light in radically tight quarters creates such extreme conditions that the very interaction of light and matter is strongly altered. An increase in the spontaneous emission rate of light is a telltale sign of this altered interaction; in this study, the researchers measured a six-fold increase in the spontaneous emission rate of light in a gap size of 5 nanometers.

"What is particularly exciting about the plasmonic lasers we demonstrated here is that they are solid state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale," said Volker Sorger, a Ph.D. student in Zhang's lab.

"Plasmon lasers represent an exciting class of coherent light sources capable of extremely small confinement. This work can bridge the worlds of electronics and optics at truly molecular length scales," Zhang noted in conclusion.

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Source: University of California - Berkeley
Pic courtesy Xiang Zhang