Gold nanoparticles (left) have been used to produce laser light (right). Credit: Noginov, M. et al.

The world's smallest laser, contained in a silica sphere just 44 nanometres across, has been unveiled. At about 10 times smaller than the wavelength of light, however, this is no ordinary laser, it is the first ever 'spaser'.

Whereas a laser amplifies light, using a mirrored cavity to intensify it, a spaser amplifies surface plasmons — tiny oscillations in the density of free electrons on the surface of metals, which, in turn, produce light waves.

The spaser could be used as a light source for scanning near-field optical microscopes, which can resolve details beyond the reach of standard light microscopy, and in nanolithography, to etch patterns much smaller than the width of a human hair. The device also opens the door to nanoscale circuits that could process information thousands of times faster than the microelectronic chips inside today's computers.

"This work has utmost significance," says Mark Stockman of Georgia State University in Atlanta, who with David Bergman of Tel Aviv University in Israel proposed the spaser concept in 20031. "The spaser is the smallest possible quantum amplifier and generator of optical fields on the nanoscale — without it, nanoplasmonics is like microelectronics would have been without a transistor."

Beating the limit

Shining light on to metal nanoparticles produces surface plasmons with the same frequency as the light. But unlike light, which can't be focused down to spot that is less than around half a wavelength wide by conventional means, plasmons are spread over much shorter distances and so can beat this 'diffraction limit'.

The problem has been that surface plasmon oscillations ebb away too quickly to be of practical use.

Now, Mikhail Noginov, a material scientist at Norfolk State University in Virginia and his colleagues have been able to stimulate the emission of surface plasmons on a gold nanoparticle and amplify them so that laser-like light is produced2.

The gold nanoparticles are encased in silica shells containing Oregon Green 488, an organic dye. Shining light on the nanoparticles excites or 'pumps' the dye molecules and they transfer energy to the surrounding electrons to produce surface plasmon oscillations. The electromagnetic waves that result from these oscillating electrical charges produce greenish laser light with a wavelength of 531 nanonmetres.

The nanoparticles, however, radiate light in all directions, rather than producing a tight laser beam. Co-author Vladimir Shalaev of Purdue University in West Lafayette, Indiana, adds that although the peaks and troughs of the light waves from the spaser should be in step or 'coherent' — a key property of laser light — the team has not yet verified this.

Organic step

Nikolay Zheludev, a physicist at the Optoelectronics Research Centre at Southampton University, UK, who, together with his colleagues has recently demonstrated a tunable nanoscale light source pumped by free electrons3, says the work is exciting. "I can think of applications in tagging large biochemical assays and in security marking, where the spaser's narrow spectral output gives better tagging capacity than existing semiconductor quantum dot emitters."

Such applications are not far off, says the US team. But Noginov thinks the spaser's ability to generate coherent surface plasmons may be even more important than its uses as a nanolaser, and could herald a new generation of ultrafast nanoelectronics. Researchers have made plasmonic circuit elements that serve as wires but the spaser should now enable the development of amplifiers and generators.

For the spaser to have realistic applications in computing, however, researchers need to find a way to make it work electrically using a semiconductor, rather than using light to pump an organic dye. That would allow the spaser to be integrated with photonic nanocircuitry. Stockman thinks that such devices are about a year away. "There is already a nanolaser with electrical pumping4, and its extension to the spaser is very realistic," he says.