## 5 Applications of 3D-SNOM  
Two-dimensional mapping of nanostructured surfaces at the sub-wavelength limit has been slowly maturing for decades, and eventually led to many significant breakthroughs including tip design and coupling to other systems such as IR spectroscopy. 3D light mapping has by comparison not been as greatly utilized. Early work in this field employed aperture tips composed of tapered optical fibers to collect images at variable distances from the surface without the use of advanced AFM based force modulation [46; 47]. In one case the researchers were able to detect Weiner fringes which are an interference pattern arising from interference between incident and reflected light [46]. It was found that the fringe pattern can be collected through the nanoscale aperture of an optical fiber, and can be used to reconstruct an image of the sample surface able to resolve features \(\sim\)100 nm in size. Alternatively, images can be collected in a plane above a sample surface to image scattered light from nanostructures such as a microgratings [47]. This work demonstrated that SNOM can be used to map light scattering patterns arising from devices in three dimensions using a nano aperture tip. These early studies lead to mapping of light and electric near-fields in three dimensions that can provide unique insight for device applications where the spatial structure of plasmonic and magnetic fields as well as forward scattered light fields can greatly affect device operation and performance. In the following section, applications of 3D SNOM compared to traditional 2D scanning are discussed, with examples of the use of 3D SNOM to study and improve the design of organic solar cells, nano-aperture antennas, nanoparticle arrays, and photonic crystal waveguides [28; 29; 34; 48; 49; 50; 51; 52]. Figure 5 illustrates the equipment setup that can be used for 3D SNOM measurements. This method employs a variation of collection mode where illumination is directed from the substrate  
Figure 4: S-SNOM IR and FTIR images. (**a**) AFM topography of a pentacene thin film; (**b**) the corresponding IR-SNOM image excited with 907 cm\({}^{-1}\) wavelength light, scale bar is 2 \(\upmu\)m; (**c**) Nano-FTIR spectra taken at two different locations representing the thin film and bulk phases of a pentacene film (40-nm thick) deposited on an SiO\({}_{2}\) substrate. Lorentzian fits (curves) to the data reveal that the bulk phase (green line, yellow ellipsoids in the image) displays a higher resonance frequency than the surrounding thin film phase (blue line, red phase in the images). Reproduced with permission from [45], Copyright Nature communications, 2014.  
side through an inverted microscope using a laser light source in the case transparent or nearly transparent samples [48]. Alternatively the sample may illuminated from the surface side, but laser illumination from the backside of the substrate has been found to greatly reduce the background of scattered far-field light and improve the near-field signal intensity. A SNOM image in three dimensions can be generated using a SNOM tip mounted on an AFM system and scanned in the near-field of the sample surface at variable distances \(z\) away from the surface. The tip is raster scanned over the sample surface in the \(x\)-\(y\) plane while moving at fixed intervals in the \(z\)-direction using a piezoelectric scanner. Light is detected through an upright confocal optical microscope focused on the aperture at the end of the tip for each point (\(x,y,z\)) in the scan. The intensity of scattered light collected by the tip aperture transmitted via optical fiber to be measured a photomultiplier tube. Using scans consisting of (\(x,y\)) slices in the \(z\) direction, the entire near-field and its boundary with the far-field can be mapped. The extent of the mapping capabilities depends on the specific arrangement of a given SNOM instrument.