Optical antenna for ultrahigh resolution scanning
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
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) .