'Spaser' Shakes Up the Nanoworld

Hank Hogan

Researchers at Georgia State University in Atlanta and Tel Aviv University in Israel have proposed a device based on surface plasmons to shake things up in very small systems. In a manner analogous to the way a laser operates, the "spaser" (surface plasmon amplification by stimulated emission of radiation) would amplify a specific surface plasmon excitation mode using a metallic particle as a resonant cavity.

Excited optically, electrically or chemically, the quantum dots in the proposed spaser would generate surface plasmons. The metallic nanostructure would amplify and accumulate the plasmons, as in a laser's resonant cavity.

Surface plasmons are highly localized energy excitations on the surface of materials. Although small in volume, they can cause big effects, making them suitable for probing nanostructures. Today surface plasmons are generated with a laser or by other resonant optical methods.

Unlike a laser, the spaser itself would be a nanoscale device. As theorized, it would consist of quantum dots surrounding metallic nanoparticles. When excited optically, electrically or chemically, the quantum dots would interact with their surroundings and generate surface plasmons amplified by and accumulated in the metallic nanoparticle, much like a resonant cavity in a laser.
The spaser would offer a number of advantages over current techniques, explained Mark I.

Stockman, a professor of physics and astronomy at Georgia State, who developed the concept with David J. Bergman. Because of its size, the energy would be concentrated in a small area and in a specific and single mode. A laser, in contrast, spreads its energy over the focal volume and over many plasmon modes. This is inefficient and noisy, making precise nanoscale measurements difficult. And again unlike a laser, a spaser would not be limited to creating luminous surface plasmon modes. So-called dark surface plasmon modes exist, and they also could be used to probe nanostructures with no stray radiation.

The device exists only in theory, but Stockman and Bergman are working with Victor Klimov's research group at Los Alamos National Laboratory in New Mexico to implement it experimentally.