Scanning Plasmon Near-field Microscopy SPNM

Scanning Plasmon Near-field Microscopy (SPNM) 

Scanning near-field optical microscopy can be used to obtain detailed images of localized surface plasmons, e.g., around lithographically fabricated gold dots (Hecht et al. 1996 Kim et al. 2003). The resolution, however, is limited by the small detection sensitivity, which limits the minimum aperture size of the scanning optical fiber. Usually one obtains a resolution of the order of 100 nm. 

One way of drastically increasing the lateral resolution down to a few nanometers is given by the combination of a STM tip (radius of curvature around 10 nm) and resonantly excited extended surface plasmons (Specht et al. 1992). For this technique, surface plasmons are excited in a thin metallic film on a prism (Fig. 9.11). They are monitored by the intensity change of the totally internally reflected exciting laser beam, which shows a pronounced minimum as a function of the angle of incidence at the angle where surface plasmons are excited (see also Fig. 3.8). 

The depth of this minimum is modified as the STM tip penetrates the evanescent field above the surface, and this modification is measured as the tip is scanned two-dimensionally over the surface. The method is especially sensitive due to radiationless energy transfer from the tip to the film, which has a cubic distance dependence. In contrast to plain STM imaging, SPNM 

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