Scattering SNOM

The TERS strategy is not suitable for IR absorption spectroscopy (IRAS). This is because a broadband light source is used in IRAS and it is difficult to create a constant enhancement over the entire spectral range. Another problem stems from the much longer wavelength of IR compared to visible light. In other words, the far-field excitation area will be much larger than in the case of TERS, and the SNR will deteriorate. In order to overcome these problems, s-SNOM is commonly used for near-field IRAS [32]. 

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However, a-SNOM faces two fundamental difficulties (i) the power emitted from the aperture is usually on the order of nanoWatts, and too weak for collecting a spectrum with a short acquisition time (ii) its spatial resolution is limited to 50 nm in practice, and is insufficient for true nanoscale analysis [6]. In order to overcome these problems, alternatives have been developed during the past 10 years, namely, tip-enhanced Raman spectroscopy (TERS) [7, 8] (Figure 15.1b) and scattering SNOM (s-SNOM), which is mainly used in the IR spectral range (Figure 15.1c) [9]. In this chapter, we focus primarily on these two methods. 

Figure 15.1 Schematic drawings of different SNOM methods, (a) Aperture SNOM (a-SNOM) (b) Tip-enhanced Raman spectroscopy (c) Scattering SNOM (s-SNOM).

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. ... 

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. 

In September 2000, Hayazawa and coworkers reported the next study on apertureless SNOM using a silver-coated cantilever and a dye-coated silver film on a glass slide [137]. The dye was rhodamine 6g (Rh-6g). A 40-fold enhancement of Raman scattering was observed with a 4 8-nm laser excitation an enhancement of fluorescence was also noticed (see Fig. 10.16) [137]. The authors observed bleaching behavior for rhodamine 6g, but did not mention whether or not the bleaching rate was tip-enhanced. Instead, they pretreated this system with 20-min illumination until a stationary state was reached. 

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). 

Scattering near-field optical microscopy (SNOM) Focuses a laser beam to a sharp tip that is close to a surface and collects the scattered near-field light with separation from the far-field light by modulation techniques. 

SNOM See scattering near-field optical microscopy. 

Fig. 10 General concept of the tip-enhanced region for scattering-type near-field optical microscopy (s-SNOM) for spectroscopic imaging. (From [80] with permission). 

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