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Boffins build World's tiniest 'laser'

Photons in, surface plasmons out

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Scientists in the US have developed what they claim is the world's smallest laser. The device, which is just 44nm in diameter, could pave the way for the development of chips that operate using light rather than electrons.

The device isn't technically a laser, but it works using the same principle. It uses a property of metals in which the density of electrons fluctuates at the edges of their crystal lattices. These fluctuations can be treated as if they are particles. Phsyicists call them 'surface plasmons'.

Zap the surface with a beam of light and the surface plasmons can be made to run along the surface of the metal as if the light itself was moving along it. Since the plasmons can be made to emit light further on, it's as if the light was transmitted by the metal.

Mikhail Noginov, professor of physics at the Center for Materials Research at Norfolk State University in Norfolk, Virginia, used this property to produce a device, known colloquially as a 'spaser', that actively propagates a cascade of surface plasmons in much the same way that a laser amplifies light. The spaser comprises a gold sphere coated with a layer of silica embedded with dye. When a beam of light pulses strike the ball, it pumps surface plasmons across the sphere's surface. The interaction of the plasmons as they bounce around the surface amplifies the effect.

Last month, researchers at Arizona State University in the US and Eindhoven University in the Netherlands created an optical laser that was 100nm in size, but Noginov's is less than half the size of that one. It's also the first spaser.

Size is key. Lasers are limited in size to half the wavelength of the light they use. In the case of visible light, that's 200nm, though you can go smaller as you push into the ultra-violet part of the spectrum. But Noginov's spaser gets the size down well beyond this.

Noginov told the journal Nature he believes spasers can be made to operate down to 1nm, but even at 44nm they're reaching a size that matches the transistors within electron-driven chips.

Optical chips are seen as a potential successor to electronic microcircuits because they're able to operate at clock frequencies more than a thousand times higher than today's silicon chips can manage. The hard part, however, is scaling the laser light sources down to the measurement scales silicon chips use.

Noginov's work shows that surface plasmons rather than laser light may prove to be the basis for high-speed optical processors that operate on a scale to match or exceed the current and future generations of silicon chips. ®

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