We propose a design for efficient end-fire coupling of surface plasmon polaritons in a metal-insulator-metal (MIM) waveguide with an optical fiber as part of a simple photoplastic connector. The design was analyzed and optimized using the three-dimensional finite-difference time-domain method. The calculated excitation efficiency coefficient of the waveguide is 83.7% ( 0.77 dB) at a wavelength of 405 nm. This design enables simple connection of an optical fiber to a MIM waveguide and highly efficient local excitation of the waveguide. Moreover, the length of the metallic elements of the waveguide, and thus the dissipative losses, can be reduced. The proposed design may be useful in plasmonic-type waveguide applications such as near-field investigation of live cells and other objects with super-resolution.

We investigate the superposition properties of the dipole and quadrupole plasmon modes in the near field both experimentally, by using photoemission electron microscopy (PEEM), and theoretically. In particular, the asymmetric near-field distributions on gold (Au) nanodisks and nanoblocks under oblique incidence with different polarizations are investigated in detail. The results of PEEM measurements show that the evolutions of the asymmetric near-field distributions are different between the excitation with s-polarized and p-polarized light. The experimental results can be reproduced very well by numerical simulations and interpreted as the superposition of the dipole and quadrupole modes with the help of analytic calculations. Moreover, we hypothesize that the electrons collected by PEEM are mainly from the plasmonic hot spots located at the plane in the interface between the Au particles and the substrate in the PEEM experiments.