Electromagnetic energy transport below the diffraction limit in periodic metal nanostructures

We investigate the possibility of using arrays of closely spaced metal nanoparticles as waveguides for electromagnetic energy below the diffraction limit of visible light. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission through chain networks such as corners and tee structures is possible at high efficiencies. Although radiation losses into the far field are negligible due to the near-field nature of the coupling, resistive heating leads to transmission losses of about 3 dB/500 nm for gold and silver particles. We confirmed the predictions of this analytical model using numeric finite difference time domain (FDTD) simulations. Also, we have fabricated gold nanoparticle arrays using electron beam lithography to study this type of electromagnetic energy transport. A modified illumination near field scanning optical microscope (NSOM) was used as a local excitation source of a nanoparticle in these arrays. Transport is studied by imaging the fluorescence of dye-filled latex beads positioned next to the nanoparticle arrays. We report on initial experiments of this kind.