Multiprobe Measurements of Dark and Bright Modes in Plasmonic Metasurfaces
A common effect associated with extremely thin metal films such as metasurfaces is their ability to transmit radiation, called extraordinary transmission (EOT). The opposite effect of extraordinary suppressed transmission (EOST) can be accomplished by antenna-like structure patterning on the surface.
In this important work, authors Dobmann et al. use a Multiprobe MultiView 4000 to selectively excite different resonant modes in the metasurface and monitor them with NSOM. They use one NSOM probe to position and inject light, while a second NSOM probe scans in the vicinity of the illumination probe and detects the light. The authors observe that far-field irradiation excites the antenna-like, or "bright modes" that are localized on the metal ridges and ultimately suppress transmission almost completely. A second type of mode also exists where the bound or "dark" surface plasmon polaritons launched from the NSOM tip propagate well across the metasurface, preferentially perpendicular to the grating lines. This work shows the power of NSOM as a critical tool to investigate dark plasmons as they can only be observed in the near-field.
[3D overlays of NSOM image on topography. On the left,with resonant polarization the electric field is localized on the metal ridges that act as antennas. On the right, for non-resonant polarization the field is transmitted through the grating gaps.]
Full paper can be found at: S. Dobmann et al., Advanced Optical Materials, p.7, 2014