Nanoparticle plasmon waveguides

Metallic nanoparticles of a size significantly smaller than the wavelength of light can sustain strong dipolar excitations in the form of localized surface plasmon resonances. Electromagnetic coupling between particles tightly arranged in a one-dimensional array therefore leads to the propagation of electromagnetic energy. Such metal nanoparticle plasmon waveguides can show a high mode confinement well below the diffraction limit upon excitation in the vicinity of the particle resonances, however with associated large losses. An elementary treatment of this problem is presented, alongside initial experimental investigations of the coupling phenomenon. Many open question regarding the nature of the coupling and the length scale over which energy propagation occurs remain, making the further study of these systems highly topical. Possible applications lie in energy concentration and focusing over micron-scale distances in the visible part of the spectrum. In addition, a short glimpse at opportunities to employ metal nanoparticles as the building blocks for two-dimensional photonic-crystal-like plasmon waveguides is provided. While in this case the lateral mode confinement is of the order of the wavelength, low losses allow intruiging applications for surface waveguides and sensors at near-infrared frequencies, including efficient coupling to optical fibres.