Tiny lasers hold key to nano-circuitry

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Scanning electron microscope images (b and c) show that the gold core and the thickness of the silica shell were about 14 nanometers and 15 nanometers, respectively. (Photo: Birck Nanotechnology Center, Purdue University)

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Researchers at Cornell, Purdue and Norfolk State University have reported the successful creation of a 'spaser', a new kind of nanoscale laser that breaks dimensional limits previously thought to be insurmountable, leading the way to significantly faster and more efficient computer processing and data transfer rates.

Laser technology has been playing an increasingly important role in electronics during the past few decades even though, researchers agree, some of the most useful and game-changing applications will only become a reality once the problem of downscaling is solved, so that more can be integrated in a single chip.

Just a few weeks ago, researchers had found a way to create lasers much smaller than the 1,500 nanometers wavelength that is commonly used in nanophotonics, which involves precisely manipulating a limited number of photons. Now, the findings by the team represent yet another giant step forward in laser integration.

The device, called 'spaser', is the first of its kind to emit radiation in the visible light spectrum, with a 530 nanometers wavelength that is only about a third of the one usually employed in nanophotonics. The nanolasers consist of spheres with a diameter of only 44 nanometers, a tremendous improvement over common nanolasers.

In order to work correctly, in fact, lasers need to employ a resonator — a component that contributes to amplify and synchronize an initial signal so that the output is a high-energy beam concentrated in a small surface. Optical resonators must typically be at least half the size of the wavelength involved, which would put the size of a 530 nanometers wavelength laser at about 270 nanometers.

The researchers, however, have overcome this obstacle by using quantum particles known as surface plasmons instead of photons. The surface plasmon lasers, or 'spasers', contain a gold core surrounded by a shell filled with green dye. When photons hit the spheres, plasmons generated by the gold core are amplified by the dye and are converted to photons in the visible light spectrum.

"This work represents an important milestone that may prove to be the start of a revolution in nanophotonics, with applications in imaging and sensing at a scale that is much smaller than the wavelength of visible light," said Timothy D. Sands, the Mary Jo and Robert L. Kirk Director of the Birck Nanotechnology Center in Purdue's Discovery Park.

One of the many interesting applications of spasers, apart from the already cited photonics-based computer processing and telecommunication, the researchers suggested, could be in microscopic imaging ten times as powerful as today's, which would allow us to observe objects as small as DNA cells for the first time.

The team also announced that one of its next focuses will be to try to use an electrical source instead of a light source for their lasers, which would ease the transition to faster laser-based computation.

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