Rupert Oulton et al., Nature, Advance Online Publication (30 August 2009)

The skinny. Electronic waves called surface plasmons zip back and forth in the narrow channel under a nanowire. By amplifying such waves, researchers have fashioned an ultrathin laser.

Tiny Laser Could Light the Way to New Microchip Technology

Staff Writer

A new type of laser takes up only a small fraction of as much space as a conventional laser, a team of physicists reports. The nanometer-sized gizmo could provide a key tool for researchers trying to develop a new type of microchip technology called "plasmonics" that mixes electronics and optics.

A laser amplifies light into an exquisitely bright and uniform beam through a weird quantum mechanical process called "stimulated emission." The heart of the device consists of two mirrors facing each other, one of which is only partially reflective so that it lets some light through. To get the laser going, the operator "excites" a light-emitting material that sits between the two mirrors by zapping it with electricity or shining light onto it. Some of the atoms in the material then emit particles of light, or photons, which bounce back and forth between the mirrors. As they pass through the material, the photons stimulate more atoms to emit photons, triggering a torrent of light--the laser beam.

Nature places limits on how small a laser can be, however. If the mirrors are smaller in diameter than half the light's wavelength, then the light will not bounce neatly between the mirrors. Instead, it will "diffract" off them and spread out so that it leaks out the sides of the laser.

To get around this "diffraction limit," Xiang Zhang and a team led by physicist at the University of California, Berkeley, and Lawrence Berkeley National Laboratory exploited subtle interactions between light and metallic surfaces. When light hits a surface, it can set off a wave called a surface plasmon, which is a kind of combination of the light wave and a rippling of the electrons in the metal. Such a wave can be confined to a much smaller space than a pure light wave can. So to make a super-skinny laser, Zhang and colleagues laid down a micrometer-long, nanometers-wide wire of cadmium sulfide on a silver surface that had been coated with nanometers of magnesium fluoride. They then zapped the nanowire with light to cause it to emit photons. Most of these photons produced surface plasmons that zipped along beneath the wire and bounced back and forth between its ends. Just as in an ordinary laser, the plasmons stimulated the atoms in the cadmium sulfide to emit more light, which in turn produced more plasmons in a runaway process, the team reports online this week in Nature. Some of the energy of the plasmons emerged from the ends of the channel as laser light with a wavelength of 489 nanometers.

The channel in Zhang's device measures as little as 40 nanometers wide by 5 nanometers high, far smaller than the roughly 250-nanometer diameter of a conventional laser of a similar wavelength. Still, the team's device does not quite set the record for the smallest laser. On 26 August, Mikhail Noginov of Norfolk State University in Virginia and colleagues reported in Nature a similar advance using 44-nanometer spheres of silica with 14-nanometer gold cores. Noginov's team shined light on a suspension of such spheres and was able to excite slightly different plasmons that don't travel on the surfaces of the sphere. They found that in each sphere, the plasmons could stimulate the production of still more plasmons and, hence, the emission of light. That makes each individual dot the smallest laser so far.

So which minuscule laser is better? That depends on what you want to do, says Mark Stockman, a theoretical physicist at Georgia State University in Atlanta and a member of the team of researchers who predicted such plasmonic lasers might be possible. Some physicists and engineers are hoping to build nanocircuits that manipulate plasmonics to marry high-speed electronics and high-speed optics. Zhang's laser might make an ideal power source for such circuits, whereas Noginov's dots might serve as the logical circuit elements themselves, Stockman says. "Both are absolutely stunning achievements," he says, "but the applications may be somewhat different." But David Bergman, a theorist at Tel Aviv University in Israel and Stockman's collaborator, notes that neither gizmo is quite ready for prime time: "They constitute at this point proof of principle."

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