Using near-field optical microscopy to excite fluorescent nanospheres atop chains of rod-shaped silver nanoparticles, a team of scientists at California Institute of Technology in Pasadena and at the University of Southern California in Los Angeles has monitored the propagation of surface plasmon polaritons in the structures. The work, which appeared in the April issue of Nature Materials, confirms theoretical predictions of the energy decay length in these so-called plasmon waveguides and has implications for the fabrication of nanoscale photonic devices.

Closely spaced metal nanoparticles act like waveguides by transporting electromagnetic energy in the form of surface plasmon polaritons -- coupled optical excitations and oscillations of their conduction electrons -- from one particle to the next down the line, like a chain of transceivers. Because this is a near-field phenomenon, researchers in photonics have hoped that it could be exploited to beat the diffraction limit and to enable the construction of smaller and smaller devices. Unfortunately, it was predicted that these nanostructures would be relatively lossy.

In the new experiments, the scientists locally excited the chains of 90 x 30 x 30-nm silver particles, spaced 50 nm apart, with 570-nm laser light. An avalanche photodiode detected the 610-nm response from a 110-nm-diameter polystyrene bead filled with fluorescent dye and placed atop the chain some distance from the excitation source. They observed energy transport over 500 nm, which they calculated to decay at approximately 6 dB per 200 nm.