Ultrathin sputter-deposited plasmonic silver nanostructures.

In this study, ultrathin silver plasmonic nanostructures are fabricated by sputter deposition on substrates patterned by nanoimprint lithography, without additional lift-off processes. Detailed investigation of silver growth on different substrates results in a structured, defect-free silver film with thickness down to 6 nm, deposited on a thin layer of doped zinc oxide. Variation of the aspect ratio of the nanostructure reduces grain formation at the flanks, allowing for well-separated disk and hole arrays, even though conventional magnetron sputtering is less directional than evaporation. The resulting disk-hole array features high average transmittance in the visible range of 71% and a strong plasmonic dipole resonance in the near-infrared region. It is shown that the ultrathin Ag film exhibits even lower optical losses in the NIR range compared to known bulk optical properties. The presented FDTD simulations agree well with experimental spectra and show that for defect-free, ultrathin Ag nanostructures, bulk optical properties of Ag are sufficient for a reliable simulation-based design.