Reflection SNOM

Direct UV-vis and IR-SNOM absorption spectra require white illumination, and IR-SNOM is experimentally complicated. Absorption effects are, of course, part of the enhancement factor in monochromatic reflection SNOM. Emission SNOM requires suitable cutoff filters or interference and notch filters for eliminating the primary light, or a diffracting spectrometer. 

Scanning Near-Field Optical Microscopy (SNOM) Aperture SNOM (ASNOM) Collection ASNOM (C-ASNOM) Emission ASNOM (E-ASNOM) Evanescent Field SNOM (EF-SNOM) Nonaperture ASNOM (NA-SNOM) Shear Force Microscopy Transmission Mode (TSNOM) Reflection Mode Luminescence Mode... 

Like conventional optical microscopy, the SNOM can be performed in transmission or in reflection. The most common method is the transmission SNOM in which a thin, transparent sample is excited by the tip (i.e., illumina-... 

Moreover, among SNOM modifications operating in the collection mode, there is a so-called photon scanning tunnel mode, where light is incident at the angle of total internal reflection (Figure 7.9d). The optical near field localized in the neighborhood of the specimen surface is detected with a near-field probe. 

The first application of the SNOM for the MO studies happened in 1992 [62], when it was demonstrated that near-field MO observation can be obtained in the same manner as conventional far-field observation— that is, by using two cross-polarizers. Betzig et al. [62] visualized 100-nm magnetic domains and claimed spatial resolution of 30-50 nm. The possibility of MO domain imaging was confirmed in both the transmission regime (Faraday geometry) [63,64] and the reflection regime (Kerr microscopy) [65-67]. 

We examine here the passive probe model [71], which ignores the effect of the probe on the SNOM image and assumes that the signal detected is proportional to the near-field intensity at the nanostructure surface in the absence of the probe. This hypothesis may be valid either if the field scattered by the tip is very small or if it is not reflected back by the sample. Thus, from this qualitative analysis, we may expect the probe to be passive either if the tip is very small or if the sample has a low reflectivity. Therefore, a metallic tip close to a metallic sample may not satisfy the assumption of a passive probe, whereas a tiny metallic tip above a dielectric (or magnetic) might be considered as a passive probe. 

We concentrate here on the simplest and most versatile SNOM technique with illuminated, sharp, uncoated tips in shear-force distance with reflection back to the uncoated fiber, because only this technique is able to deal with delicate surfaces that exhibit low or high corrugation in supramolecular chemistry or for the characterization of single and aggregated nanoparticles. 

As aperture SNOM is amply described in Ref. 1 and, as the scattered SNOM techniques with modulation are also not very useful for the present topics, their elaborate equipment are not repeated here. However, the versatile reflection-back-to-the-fiber SNOM is simply a shear-force AFM with an optical addition. Figure 3 depicts a block diagram. ... 

Figure 3 Block diagram of an apratureless reflection-back-to-the-fiber SNOM setup with shear-force distance controf and cross-polarization to minimize stray light (1) beam sphtto and crossed polarizers (2) shear-force arranganent (3) sample mounted on a piezo stage.

The basic requirement for apertureless SNOM is the reflectance enhancement in the shear-force gap. The studied material strongly influences the efficiency of the shear-force damping (100 x d/do where d is the set amplitude and do the free amplitude of vibration). Thus, for example, 50% damping will be obtained at different distances on different materials, and the reflectance enhancement factor F (F = I/Iq where I is the total intensity and 7o the far-field background intensity) depends on the distance. Fortunately, there are numerous material properties, such as refraction index, gloss, crystal packing, water layer, and distance, that influence the value of F. The far-field intensity lo should... 

The spectrum (Figure 19c) required only a short Raman collection, because shear-force controlled apertureless internal reflection Raman SNOM also creates the characteristic enhancement of the Raman reflection back to the sharp tapered fiber. Further, resonant Raman SNOM spectra of silicon (519.7 cm ) under an old (5 nm) or a freshly grown silica layer, and nomesonant Raman SNOM of these layers (500 cm ) as well as of gallium nitride (nonresonant El (TO) and E2 Raman modes at 560.8 and 570.4 cm ) on alumina (subtraction of the Raman response of support and fiber) with the shear-force apertureless technique (using 488 nm light) at total collection times of <10 min have been reported in Ref. 25. 

AFM tips (diameter <50nm metalized with silver) on transparent samples were illuminated by a focused laser beam from below through the support and the sample to reach a 30 times increase of the scattered Raman light. This apertureless scatter SERS SNOM succeeded also with an etched gold wire in shear-force distance with 40-fold increase of the Raman signal, but this shifted the Raman lines with respect to those in the bulk Raman spectra. Nanopipette probes for apertureless SERS SNOM with gold or silver particles held in the aperture are also available. However, none of these elaborate techniques approaches the capabilities and versatility of the easiest apertureless SNOM with sharp pulled tips and enhanced internal reflection (Figure lb). 

Further, rather elaborate SNOM techniques do not profit from the internal reflection enhancement that enables artifact-free strictly local SNOM even at high topographies. The present SNOM will be the technique of choice if ease of operation, reliability, economics, and versatility for practical applications with real-world samples are the criteria. 

A focusing paper (G. Kaupp, The enhancement effect at local reflectance and emission back to apermreless SNOM tips in the shear-force gap) has been accepted to appear in The Open Surface Sci. J 2011, 12 p. 

SNOM can be applied in a range of modes including transmission, reflectance, and emission. Such techniques are likely to be coupled to electrochemical studies in the future. The spatial resolution of such imaging makes the possibility of interrogating potential control of single molecules a reality. 

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