Characterization of long-range surface-plasmon-polariton waveguides

Measurements of the attenuation and excitation efficiency of the long-range surface-plasmon-polariton mode supported by waveguides comprised of one or many thin metal films of finite width embedded in dielectric were made in the near infrared $({\lambda }_0=1550\mathrm{nm})$ . Au films 31, 25, and 20 nm thick, and Ag films 20 nm thick were used to implement the structures. The lowest attenuations measured among the Au and Ag waveguides are 0.42 and $0.32\mathrm{dB}∕\mathrm{mm}$ , respectively, corresponding to propagation lengths of 10 340, and $13572\mu \mathrm{m}$ , respectively. These propagation lengths are longer than those of the single-interface surface-plasmon polariton in the corresponding semi-infinite structures by factors of 93 and 138, respectively. These factors are the largest reported to date for long-range surface-plasmon-polariton waves. The largest excitation efficiency measured among the set of Au structures is 98%. Theoretical results were obtained for all of the structures characterized experimentally using an accurate electromagnetic-field model. Theory and experiment agree to within about 5% over the 31- and 25-nm-thick Au structures, but a thickness-dependant permittivity must be assumed in order to achieve agreement to within 12% for the 20-nm Au structures.