Normally, light cannot be focused to a spot smaller than half its wavelength – something known as the diffraction limit. However, in recent years scientists have succeeded in focusing light down to the nanoscale by coupling it to plasmonic nanostructures in which conductive electrons can oscillate collectively at the surface of metals – called surface particle plasmons. The phenomenon is now a subject in its own right, called “nanoplasmonics”, based on tailored metallic nanostructures that could be used for making tiny optoelectronics devices in the future.

Naomi Halas and colleagues recently showed that magnetic plasmons could propagate over distances of several microns along a conjugated chain of artificial aromatic molecules called heptamers. To compare, electron plasmons can only travel a few hundred nanometres along linearly arranged chains of metallic nanoparticles, like those made of gold, for example.

To remind ourselves, electron plasmons are formed when electrons oscillate back and forth (like an electron dipole) while magnetic plasmons are formed when electrons oscillate in a circular fashion (like a magnetic dipole).

Unique ring currents

The new work had the researchers studying the optical properties of two complex nanoparticle nanoclusters – made of “fused” heptamers carefully arranged in such a way that they resemble the organic aromatic molecules chrysene and triphenylene. The heptamers are artificial molecules composed of ring-like components and generate unique ring currents that circulate around the structures when illuminated with light from a laser operating at 1500 nm.

In organic chemistry, rings are said to be fused if they share two or more atoms, explains team member Na Liu. “In our work, our fused heptamers share two gold nanoparticles that act as a mutual link for efficient current exchange between the two neighbouring heptamers,” she told

Building blocks for waveguiding networks

Halas and co-workers showed that the fused heptamers could be used as building blocks for magnetic plasmonic waveguiding networks. The researchers succeeded in making a steerer device that can direct plasmons around bends with large angles and a Y-splitter than can transport plasmons along two separate optical paths. The Y-splitter can also act as an interferometric device to switch plasmon propagation and on and off, says Halas.

“We also made a MachZehnder interferometer comprising two consecutive Y-splitters that can split and combine propagating plasmons,” added Liu.

According to the researchers, who reported their work in ACS Nano, the plasmon-based subwavelength waveguides could be used as important blueprints for designing a new generation of nanoscale photonic devices.