In this project, the complementary techniques of PEEM and SNOM have been utilized to investigate the optical properties of plasmonic nano structures. Using two techniques enables us to study the femtosecond electron dynamics considering the radiative damping channel (SNOM) as well as the electron-hole pair generation (PEEM). This gives the opportunity to elucidate the plasmonic mechanisms in detail and to identify their physical origin. As time-reolved SNOM being a very complex and demanding technique, first the polarization dependent transmission behaviour of near-field apertures (Fig. 1-3) was explored identifying differences of the near-field compared with the far-field. Simultaneously, a substantially improved normal incidence PEEM set-up was started up. (Fig. 4) Measurement of particular plasmonic structures with both techniques revealed that differences in images attributed to the radiating and non-radiating decay may occur.

We obtained first promising results for a gold nano ring and ring segments, as shown in Fig. 6-9. The same structure array was measured with the new established normal incidence PEEM setup (Fig. 6-7) and the SNOM (Fig. 8-9) varying the polarization as indicated by white arrows. The resonantly excited gold rings show clearly a dipolar mode for the SNOM as well as the PEEM measurements. Therefore this system seems predestinated for comparative dynamical studies of plasmonic excitations.

Batch fabrication is a precondition to exploit plasmonic structures for real applications. We have introduced a concept combining electron beam lithography with ion beam milling for massive parallel fabrication of plasmonic structures, in particular c-shaped apertures and inverse bowtie antennas. In a single run 280,000 c-shape apertures were fabricated (Fig. 5). This gives the freedom for a rich parameter variation which is impractible in case of hand crafting techniques like focused ion beam (FIB) etching. In fact, the proposed method is the point of origin for the second project period because it paves the way for a new strategy to define plasmonic structures by tailoring the underlying material properties without removing the material. This demands a thorough investigation of the underlying material properties.

Enhanced scattering may arise after FIB milling of evaporated or sputtered Au samples because of the angle dependent sputtering induced enhanced surface roughness. Therefore, synthesis of Au nanoflakes has been implemented with support by B. Hecht (Uni. Würzburg). After optimization of the process, a method was applied mounting single flakes on SNOM tips to make them suitable as basic material for nanooptical probes. At the same time flakes have been used to set-up samples with almost no topography but strong optical contrast (Au / dielectrics).

Captions:

Fig. 1 Hollow SNOM tip treated by IBE milling to establish a platform for nanoflake deposition. Fig. 2 and 3: triangular shaped Au flake with a fibbed aperture of ca. 25 nm diameter. Fig. 4: Normal incidence PEEM. Fig. 5: SEM image of a section of 35 c-shape apertures from 280,000 apertures. Fig. 6: PEEM image of a gold ring/ ringsegment array under UV light illumination with a mercury lamp using a photon energy of 4.9 eV. Fig. 7: Three photon photoemission signal of the ring/ ringsegment array. The signal is obtained under normal incidence at a central wavelength of 800nm, clearly showing a dipolar excitation. Fig. 8: SNOM topography image of the same array shown in Fig. 6. Fig. 9: Corresponding near-field image with an excitation wavelength of λ=800 nm measured by SNOM. The arrow indicates the in-plane polarization of the electric field vector of the incident light. Fig. 10: Measurement of a corrugated silver surface taken under normal incidence. A modulation of the electron yield occurs due to the direct imaging of surface plasmon phase fronts. Fig. 11: Measurement at the same sample position as in Fig. 10, but under an angle of incidence of 65°. The beating pattern is clearly visible.

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