## 5 Applications of 3D-SNOM
### Optimization of Nano-Optical Devices  
The first use of 3D SNOM to probe a scattered light field of an optoelectronic device was accomplished by means of a focusing waveguide grating coupler (FWGC) as an experimental system [61]. This study was undertaken to evaluate SNOM as a complimentary tool to characterize FWGCs, which normally are difficult and time consuming to accurately analyze, requiring a full 3D calculation to predict their waveguiding properties. The focal spot produced by such FWGCs is nanoscale is size near the grating surface, and therefore can only be imaged using non-diffraction limited techniques. In this case, SNOM was capable of imaging the real performance of the FWGC by imaging the spot at different heights about the sample, for comparison to a much simpler 2D Bloch wave analysis which was able to fully characterize the FWGC [61]. This initial investigation showed the utility of 3D SNOM beyond scanning the near-field very close to the surface, and how it can be used to study light scattering antennas and waveguides. 3D-SNOM has since been used to investigate resonant illumination of nano antennae consisting of ordered nano-holes as shown in Figure 10a [49; 55]. Nano hole antennas can possess interesting optoelectronic properties used to scatter light in a controlled way. The nano hole antenna shown in Figure 10a consists of a 40 nm thick gold film perforated with 17 holes of 150 nm in diameter arrayed in a line separated by 20 nm spaces and illuminated with 660 nm laser light [49]. Panel (b) shows the result of a 2D digital heterodyne holography scan above the sample surface to image scattered light. Although DHH is capable of 3D imaging similar to 3D SNOM, it is a far-field technique and therefore the resulting image is diffraction limited. This can be seen from the corresponding  
Figure 9: IR s-SNOM image of the cross-section of the VO\({}_{2}\) film at 335 K. The white lines represent the boundary between crystal domains as seen in Figure 8. The cross section shows 3 distinct regions, which indicate the nucleation of metal atoms where a more intense IR s-SNOM signal corresponds to a higher IR conductivity. The image indicates that metal nucleation happens preferentially at the film/substrate interface where the epitaxial strain is strongest. Reproduced with permission from [56], Copyright Applied Physics Letters, 2014. SNOM image showing much higher resolution details of the nano holes. This nanostructure has also been investigated in three dimensions to probe the light scattered above the nanoholes as shown in Figure 10c. The uniform light intensity near the surface indicates coupling between the nano holes. The interference pattern observed in the SNOM image is a consequence of light originating from the surface that is back scattered from the tip itself and reflected from the Au surface to be collected by the tip. Nano antennas or apertures with more exotic shapes such as the bowtie antenna [50; 51] have been sought after for their light scattering properties. Bowtie shaped antennas are designed to produce enhanced electric fields in the aperture between the two bowtie halves. Figure 11 shows the optical amplitude distribution in the space above a bowtie antenna of this type, using s-SNOM imaging. This bowtie antenna was manufactured using focused ion beam milling to produce an aperture with an outline of 150 nm and a gap size of 20 nm in a 60 nm gold film to [50]. From Figure 11a, the electric (\(E_{z}\)) field is concentrated along the edges of the bowtie-shaped aperture gap. The curvature of the side walls of the bowtie aperture gap allows the near-field to spread along the curvatures and reduce its spatial confinement to the aperture. From the 3D SNOM scans the near-field can be seen to extend up to 20 nm from the film surface, at which point it drops off sharply in intensity. The optical phase image (Figure 11b) clearly identifies a 180\({}^{\circ}\) phase shift that is characteristic of bowtie apertures, and extends past the near-field boundary seen in the amplitude image [50; 51].