One of the problems to be solved in the development of nanoscale photonic devices is how to direct light from its source to its destination efficiently. Point-like light sources emit light in all directions, which means that a considerable amount of light is lost if emission is relied on in such devices. Unfortunately, the classical waveguides used on the macroscale to collect and direct this light do not work on the nanoscale, where devices are smaller than the wavelength of the light to be guided.

Li Zhou, Qu-Quan Wang and co-workers from Wuhan University in China have now developed a system in which light can be transferred along silver nanowires over distances as far as 560 nm.1 The devices have many potential applications says Qu-Quan Wang. “This efficient energy transfer could find applications in full-color light-emitting diode displays, or in the subwavelength imaging of nanostructures.”

Fig. 1: Propagation of surface plasmon resonances (from left to right) along an array of silver nanowires. The standing wave is clearly visible.

Unable to rely on the classical principles of guiding light, as used in optical fiber for example, the researchers used surface plasmons — collective motions on the surface of metals. Surface plasmons interact strongly with light, and plasmonic nanostructures are thus able to transport light over considerable distances, even though the diameter of the nanostructures is much smaller than the wavelength used.

The researchers’ plasmonic device is made of silver nanowires that connect semiconductor quantum dots. A light source at one end of the array excites surface plasmons that travel along the wires. Back reflections from the other end of the wires then set up plasmonic standing waves. If the length of the nanowires is designed such that the maximum of the standing wave occurs at the end of the nanowire, the emission of light from the wires can be very strong, forming a highly efficient waveguide.

Although the transport of light over their silver wires for distances of up to 560 nm is already impressive, the researchers are now working to extend the length of the nanowires. Arrays could then potentially be designed to carry light of multiple wavelengths, says Wang. “These longer arrays could be used for multi-color imaging, in particular as a way of achieving superlensing of objects smaller than the wavelength for a broad range of visible wavelengths.”