Direct Measurement of the Interference between Surface Plasmons and an Exciting Laser

Carsten Henkel Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany Joachim Jelken Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany Burkhard Stiller Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany Dieter Neher Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany Thomas Papke Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany Sekhar N. Yadavalli Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany Svetlana Santer Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany

We have prepared gold films with defects in the nanometer range (grooves, slits, gratings etc). When illuminated with a laser beam (spot diameter several microns), the near field shows fringes whose period is consistent with the interference between a surface plasmon and the laser field. This allows for the holographic analysis of the plasmon field. We have detected Moire and checkerboard patterns from two plasmons scattered by grooves forming an angle.

The detection is done in two ways: in one setup, we enter into the near field with a sharp tip operated in the tapping mode. To the tip an optical fibre is attached that guides the light picked up from the near field to the detector. The second setup is based on a photo-sensitive organic film coated on top of the metallic layer. The film contains ado-benzene groups that isomerize in the near field. As a consequence, the film deforms in a permanent way. The corresponding change in topography is linked to the spatially modulated intensity and polarization and is read out with an atomic force microscope [1].

The experimental data compare well with a theoretical model based on the actual plasmonic modes in the metal film including the substrate and an eventual dielectric coating. This shows that the wavelength of the plasmonic hologram can be used as a sensitive probe of the dielectric environment of the metallic structure.

[1] T. Papke, N. Sekhar Yadavalli, C. Henkel, and S. Santer, "Mapping a plasmonic hologram with photosensitive polymer films: standing versus propagating waves", ACS Appl. Mater. Interfaces 6 (2014) 14 174.

henkel@uni-potsdam.de