UP2: Optical antenna for ultrahigh resolution scanning probe microscopy

M. Raschke, C. Neacsu, M. Rang

The optical antenna properties of metallic tips to detect and concentrate light to highly confined regions is being investigated and employed for ultrahigh resolution near-field microscopy: In linear light scattering the plasmonic characteristics of the emission behavior of individual nanoscopic tips has been investigated and allowed for an understanding of the correlation of spectral dependence and local-field enhancement with structural parameter - providing important selection criteria for tips to be used as probes in scanning near-field microscopy [NSR].
In nonlinear light scattering, making use of the unique symmetry properties of the tips (8mm) allowed for the otherwise inseparable distinction between surface and bulk contributions in second-harmonic generation (SHG) [NRR]. This separation of different nonlinear source polarizations has been a long standing problem in surface nonlinear optics because of its fundamental importance for proper signal assignment. The study also allowed to derive general symmetry selection rules for SHG from nanostructures.
Near-field imaging on the basis of infrared vibrational contrast has been achieved and allowed for the identification of nanodomains formed by molecular selfassembly of block-copolymer surfaces (see Fig. 2). With a sub-10 nm spatial resolution corresponding to a sensitivity of as low as 103 C-H oscillators the results indicate that for the first time IR-spectroscopy providing access to intramolecular dimensions is within reach. Similarly the use of Raman spectroscopy for spectroscopic imaging is being explored.

Fig. 2. Apertureless near-field vibrational imaging of nanodomains formed by local phase separation of a block-copolymer thin film of Polystyrene-b-Poly-(2-vinylpyridine) (PS-b-P2VP). Contrast at 3.39 µm (2950 cm -1) is obtained due to spectral variations of the C-H stretch vibrational resonances between the different polymer constituents. Bright red regions correspond to P2VP domains in PS matrix (green/blue) .