Observation of coupled plasmon-polariton modes of plasmon waveguides for electromagnetic energy transport below the diffraction limit

We investigate the possibility of using arrays of closely spaced metal nanoparticles as plasmon waveguides for electromagnetic energy below the diffraction limit of light. Far-field spectroscopy on arrays of closely spaced 50 run Au particles fabricated using electron beam lithography reveals the presence of near-field optical particle interactions that lead to shifts in the plasmon resonance frequencies for longitudinal and transverse excitations. We link this observation to a point-dipole model for energy transfer in plasmon waveguides and give an estimate of the expected group velocities and energy decay lengths for the fabricated structures. A near-field optical excitation and detection scheme for energy transport is proposed and demonstrated. The fabricated structures show a high propagation loss of about 3 dB /15 nm which renders a direct experimental observation of energy transfer impossible. The nature of the loss and ways to decrease it by an order of magnitude are discussed. We also present finite-difference time-domain simulations on the energy transfer properties of plasmon waveguides.