The Smallest Laser Ever Made

Surface-plasmon lasers could enable a new generation of computers based on nanophotonics.

By Katherine Bourzac

Researchers have demonstrated the smallest laser ever, consisting of a nanoparticle just 44 nanometers across. The device is dubbed a "spaser" because it generates a form of radiation called surface plasmons. The technique allows light to be confined in very small spaces, and some physicists believe that spasers could form the basis of future optical computers just as transistors are the basis of today's electronics.

Tiny laser: This simulation shows the intensity of light around a new type of laser, called a spaser, when operating in a plasmon-producing mode. The concentration of plasmons is most intense at the gold sphere that makes up its core. The inner black circle indicates the position of the sphere, which is coated with a dye-embedded silica shell, marked by the outer black line. Credit: Nature

While the best consumer electronics operate at speeds of about 10 gigahertz, Mikhail Noginov, professor of physics in the Center for Materials Research at Norfolk State University in Norfolk, VA, notes that optical devices can operate at hundreds of terahertz. Optical devices are, however, difficult to miniaturize because photons can't be confined to areas much smaller than half their wavelength. But devices that interact with light in the form of surface plasmons can confine it within much tighter spots.

"There's currently a big effort, mostly theoretical, towards designing a new generation of nanoelectronics based on plasmonics," says Noginov. Unlike other previous plasmonic devices, spasers are an active element that can produce and amplify these waves. Noginov co-led the development of the new spaser with Ulrich Wiesner of Cornell University and Vladimir Shalaev and Evgenii Narimanov of Purdue University. The work is described today in the journal Nature.

The spaser made by Noginov and his collaborators consists of a single nanoparticle just 44 nanometers in diameter, with different parts that perform functions analogous to those in a conventional laser. In a normal laser, photons bounce between two mirrors through a gain medium that amplifies the light. The light in a spaser bounces around on the surface of a gold sphere in the nanoparticle's core in the form of plasmons.

The challenge, says Noginov, is to make sure that this energy does not dissipate rapidly from the metal surface. His team accomplished this by coating the gold with a layer of silica embedded with dye. This layer acts as a gain medium. Light from the spaser can remain confined as plasmons or it can be made to leave the particle surface as photons in the visible-light range. Like a laser, the spaser must be "pumped" to supply the necessary energy. Noginov's group accomplishes this by bombarding the particle with pulses of light.

The size of a conventional laser is dictated by the wavelength of the light it uses, and the distance between the reflective surfaces can't be smaller than half the wavelength of the light--in the case of visible light, about 200 nanometers. The "beauty" of the spaser is that it gets around this limitation by using plasmons, says Noginov. Spasers could probably be made as small as one nanometer. Any smaller than that, Noginov explains, and the nanoparticles' functionality breaks down.

Noginov and his collaborators are not the first to make a nanolaser. This July, researchers led by Cun-Zheng Ning, professor of electrical engineering at Arizona State University, and Martin Hill of Eindhoven University in the Netherlands created a nanolaser about 100 nanometers wide, using different materials. Ning and Hill's nanolaser was the first to overcome the wavelength constraints on the size of lasers. The work published today, however, is the first example of a spaser.

"The spaser works about a thousand times faster than the fastest transistor, while having the same nanoscale size," says Mark Stockman, professor of physics at Georgia State University. "This opens up the possibility to build ultrafast amplifiers, logic elements, and microprocessors working about a thousand times faster than conventional silicon-based microprocessors."

Stockman predicted the phaser in 2003 with David Bergman, professor of physics at Tel Aviv University in Israel. The creation of the spaser, says Bergman, "is a beautiful piece of work."

Spasers are likely to find their first application not in optical computing but in places where conventional lasers are used today, says Noginov. Indeed, "a more near-term application is in the magnetic data-storage industry," says Sakhrat Khizroev, professor of electrical engineering at the University of California, Riverside, who is also developing nanolasers. The magnetic data-storage media used for today's hard disks are reaching their physical limits; one way of extending its capabilities is to heat the media with very small spots of light during recording, which could be done with nanolasers, says Khizroev. However, the researchers caution, any applications are probably years away.

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