Interferometric near-field characterization of plasmonic slot waveguides in single- and poly-crystalline gold films.

Single-crystalline gold films show superior plasmonic properties compared to their poly-crystalline counterparts. However, this advantage comes at the cost of a more complex preparation process. It is thus crucial to validate whether the impact of the material quality on the performance of the respective plasmonic device justifies this additional effort. In order to address this question for the case of plasmonic slot waveguides, we present interferometric near-field measurements at telecommunication wavelengths on slot waveguides in single- and poly-crystalline gold films. We observe significantly larger propagation lengths in the case of single-crystalline gold films for slot widths below 100 nm. In contrast for larger widths, both gold films give rise to comparable propagation lengths.

Freestanding metasurfaces for optical frequencies.

We present freestanding metasurfaces operating at optical frequencies with a total thickness of only 40 nm. The metasurfaces are fabricated by focused ion beam milling of nanovoids in a carbon film followed by thermal evaporation of gold and plasma ashing of the carbon film. As a first example, we demonstrate a metasurface lens based on resonant V-shaped nanovoids with a focal length of 1 mm. The second example is a metasurface phase plate consisting of appropriately oriented rectangular nanovoids that transform a Gaussian input beam into a Laguerre-Gaussian LG-1,0 mode.