Near-field Raman Spectroscopy with or without Apertures

Near-field Raman Spectroscopy with or without Apertures 

If one includes the employment of evanescent light waves in spectroscopic studies in the near-field area. Knoll and coworkers are among the first who reported on near-field Raman spectroscopy , though this approach was termed at that time (the early 1990s) as surface plasmon field-enhanced Raman spectroscopy and microscopy [104], 

Near-field Raman spectroscopy was expected to become a very attractive approach because it offers chemical information from a subwavelength region and microscopic images of the distribution of species over a sample if it is viewed by its characteristic Raman fine. Most intriguing is that the spatial resolution is only hmited by the size of the aperture. 

The next step was to improve the stability of the system and - even more importantly - to improve the transmission of the fiber tips. Metalization of the tip is the key and produces superior properties, such as a near-field spot of less than 20 nm [107-110]. Zeisel et al. reported that metalization led to optical transmission of a fiber higher than that of conventional fibers by several orders of magnitude [111]. The authors showed images based on the fluorescence of dye-labeled polystyrene spheres and noted that fast and irreversible photobleaching takes place in the near field with enhanced intensity. In addition, they reported on surface-enhanced near-field Raman spectra of cresol fast violet and p-aminobenzoic acid adsorbed on a (rough) silver substrate, exhibiting a signal-to-noise ratio of 10 [111]. 

A slightly different approach was followed by Grausem and coworkers, who employed near-field Raman spectroscopy to study liquid CCI4. The near-field probe had an aperture of 50 nm. Because of the rapid decrease of the near field with distance from the aperture, only a small volume of liquid, containing only 0.4 attomol of CCI4, contributes to the Raman signal [112]. 

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