Near-field characterization of Bragg mirrors engraved in surface plasmon waveguides

Surface plasmon waveguides (SPW’s) are metal ridges featuring widths in the micrometer range and thicknesses of a few tens of nanometers. A focused ion beam has been used to carve microscatterers into gold SPW’s and the near-field distributions around these microstructures are observed by means of photon scanning tunneling microscopy (PSTM). On the basis of near-field images, we show that a finite length periodic arrangement of narrow slits can reflect a surface plasmon mode propagating along a SPW. The reflection efficiency of the micrograting is found to depend upon the number of slits, the period of the grating, and the incident wavelength. The optimum reflection efficiency is obtained for a period of the micrograting equal to half the incident wavelength in vacuum. The PSTM images of the plasmon mirrors taken at different wavelengths allow us to measure the experimental dispersion curve of the SPW in the near-infrared. From this dispersion curve, we found that, in analogy with a surface plasmon (SP) excited on extended thin films, the group velocity of a SPW mode is close to the speed of light. For a given frequency in the near-infrared, the effective index of the SP mode supported by a $2.5\text{‐}\mu \mathrm{m}$-wide SPW is also found to be significantly larger than the effective index of an extended thin film SP. Finally, we show that the optical properties of microgratings engraved into a SPW can be qualitatively approached by a standard Bragg mirror model.