(a) AFM topographical image of a particular metallic nano-structure produced by e-beam lithography on an SiOx substrate. Baselength of letter "K" = 310 nm.	(b) Composite image of the same structure as on the left: AFM (height) and aSNOM signal (color) for s-polarized excitation of 833 nm incident in direction of view.
Fig. 1 Making out localized plasmonic hotspots with aSNOM.

The critical task for any nano-optical work is the collection and discrimination of light emitted from a volume of matter only a few nm in size. For example, in Fig. 1 the hotspot size is less than a fifth of the wavelength and a conventional confocal microscope - subject to the Abbe-limit - would not be able to image it clearly.

In general spectro-microscopy, the resolution is typically limited to some fraction of the wavelength. From the variety of possible nano-optical microscopy techniques we employ the (arguably) most promising for ultimate lateral resolution at surfaces and interfaces: In contrast to scanning nearfield optical microscopy (SNOM), apertureless SNOM (aSNOM) is based upon localized field enhancement at the apex of an extremely sharp needle.

In our home-built aSNOM we use off-the-shelf AFM Si tips that have an effective apex radius of well below 10 nm, which represents well the lateral resolutions we achieve routinely - demonstrated for example with SERS hotspots. To further convince ourselves of the proper interpretation of recorded signal, we have undertaken a series of of full 3-dimensional simulations of the entire imaging process as the probing tip scans over metallic inclusions in a dielectric matrix.

One of our recent major improvements to the experimental setup concerns the elegant technique of cross-polarized aSNOM (incident and scattered radiation are orthogonally polarized). Interference effects and/or coupling between sample and probe are eliminated to first order as we could show, e.g. in Nano Lett. (2008) 8, 3155-3159. The aSNOM signal we record can be directly related to the normal E-field component of the plasmonic sample mode that was excited.

Our cross-polarized aSNOM approach allows to study plasmonic eigenmodes nearly unperturbed by the probing tip, which is a crucial consideration for any microscopic technique that claims to provide accurate, neutral data. An elementary demonstration we give with the example of circular nano-disks (made from gold, 20 nm high) from Nano Lett. (2008) 8, 3155-3159: See Fig. 2.

(a) Topography	(b) Magnitude |E|
(c) Phase φ = arg(E(t))	(d) Instantaneous field strength ℜ{E(t)}
Fig. 2 Multipolar eigenmodes in plasmonic nanodisks. For phase and instantaneous field strength, animated movies show the temporal evolution.

Two species of disks have been selected by diameter to respond with dipolar or quadrupolar eigenmodes when excited by s-polarized radiation of 875 nm (here incident at 70 deg from the normal in the downward direction). Especially the temporal animation (according to ℜ{|E|⋅exp(iφ+iωt)}) to show the temporal evolution of the phase front or the instantaneous real part of the E-field, the beautiful, non-trivial eigenmode patterns.

Such localized plasmonic eigenmodes are particularly interesting in the context of sensor application, thanks to their characteristic spectral signatures, which are relatively easy to detect in the farfield. At the same time they are very sensitive to local changes in the dielectric environment. Here, a combinatorial optimization approach combines nicely with the microscopic capabilities of aSNOM.

Cross-polarized aSNOM is also well-suited to study propagating plasmon phenomena, as we have shown in, e.g., in phys. stat. sol. (b), 245, p. 2255-2260 (2008). For example, in Fig. 3 a variety of phenomena are observed, including propagating 2-dimensional plasmons in open areas, guided modes between slot arrays, localized excitation of the slots, and circular emission of waves from a defect.

(a) Topography	(b) Magnitude |E|	(c) Phase φ = arg(E(t))	(d) Instantaneous field strength ℜ{E(t)}
Fig. 3 Localized and propagating plasmonic modes around slots in a 100 nm thick gold film, which we recorded using our cross-polarized aSNOM approach. In the lower part of the images (11x11 um2), a temporary loss of excitation laser power occurred. For phase and instantaneous field strength, animated movies show the temporal evolution.