Monitoring the surface quality of silver plasmon waveguides with nonlinear photoemission electron microscopy and in-situ ion sputtering

We use photoemission electron microscopy (PEEM) with single- and two-photon excitation to study the properties of surface plasmon polaritons (SPPs) in tapered silver stripe waveguides. Arrays of waveguides with different widths, grating coupler geometries, and taper angles are fabricated using electron beam lithography (EBL) on Si and indium-tin-oxide (ITO)-covered glass substrates. The presence of propagating SPP modes is indicated by modulations of the non-linear photoemission intensity which run along the length of the waveguide - the result of interfering SPPs reflected from the edges. Surface roughness also results in the presence of random emission hot spots and strong edge diffraction, which hinders an analysis of the effect of waveguide geometry on SPP propagation length and nanofocusing. Accordingly, we develop a relatively simple, in-situ method of reducing the surface roughness by argon ion sputtering using low-energy (∼ 300 eV) ions. Surface roughness is qualitatively assessed by the ratio of the photoemission from the central interference fringes to that from the edges, and from the hot spot intensity. The improvement of the surface quality is a critical step for reducing radiative losses and therefore increasing the propagation length of plasmonic waveguides.