## 5 Applications of 3D-SNOM
### Optimization of Nano-Optical Devices
Figure 10: Resonant illumination of nano hole antenna. (**a**) Nano hole antenna experimental configuration; (**b**) 2D scan of the nano hole surface using far-field DHH imaging and phase as well as c-SNOM; (**c**) Scattered light intensity in a plane perpendicular to the sample surface. Reproduced with permission from [49], Copyright Optics Communications, 2016. The 2D optical amplitude and phase in an \(x\)-\(y\) plane directly above the film surface corresponding to \(z=0\) are constructed from the volume data and shown in Figure 11c,d. In Figure 11c, two separate hot spots of the \(E_{z}\) field with FWHMs of 90 nm are clearly identified above the apex of each half of the bowtie, with a minima in the field at the center corresponding to the gap of the bowtie aperture. The phase shift seen in the \(x\)-\(z\) scan is reproduced at the bowtie antenna surface, showing that this effect extends spatially in three dimensions. The full three-dimensional field can be reconstructed from these scans; Figure 11e shows the intersection of the \(x\)-\(y\), \(x\)-\(z\), and \(y\)-\(z\) data planes. In addition to nano antennae, photonic crystal waveguides (PhCWs) possess evanescent wave fields that have been investigated in great detail using SNOM imaging. In these devices, 3D SNOM can be utilized to examine the six components of the electric and magnetic fields. Measurements at increasing distances \(z\) above the crystal can be used to differentiate between parallel electric (\(E_{\parallel}\)) and magnetic (\(H_{\parallel}\)) fields. In the \(x\), \(y\) plane of symmetry, only transverse components (\(E_{x}\), \(E_{y}\), \(H_{z}\)) of the electromagnetic field are non-zero and have been detected using SNOM [29]. However, as the distance from the surface increases the relative amplitudes of the electric and magnetic fields and their spatial profiles changes. This divergence occurs because a given mode is composed of many Bloch harmonics in superposition, each of which experiences a different decay in the \(z\)-direction, which can be investigated using 3D SNOM. Figure 12 shows the result of 3D SNOM investigations of a silicon PhCW with 120 nm radius holes separated by 420 nm spaces. This structure is illuminated by 1570 nm light along the plane of the waveguide, which is maintained in the waveguide by the photonic bandgap of the surrounding holes, and confined to the bulk of the silicon by total internal reflection. The PhCW acts a waveguide to confine light incident on the edge of the silicon PhCW between two rows of periodic holes that create a periodicity in the dielectric properties of the silicon resulting in confinement of the light between the holes. Each individual small panel in Figure 12 is a 2D scan of a portion of the waveguide, collected at a height above the surface given by the scale on the left of the figure. Panels (c) and (d) of Figure 12 show the measured light field above the sample where \(L_{x}\) and \(L_{y}\) represent separate detectors that are set to  
Figure 11: 3D SNOM amplitude and phase images of a bowtie antenna. The amplitude (**a**) and phase (**b**) images in an \(x\)-\(z\) plane showing a cross-section of the bowtie aperture. Panels (**c**) and (**d**) are 2D amplitude and phase images, respectively, sized 400 nm \(\times\) 100 nm and taken at the bowtie antenna surface, \(z=0\); (**e**) Combined scans showing the 3D light field of the antenna where the white scale bar represents 50 nm. Reproduced with permission from [50], Copyright Optics Express, 2015.  
preferentially measure the \(E_{x}\) or \(E_{y}\) field components respectively. The \(L_{x}\) signal presents a minimum along the center of the waveguide (arrow at the bottom of panel c) that is similar to the calculated \(E_{x}\) in panel a as expected. However, the signal obtained from \(L_{y}\) greatly resembles the calculated \(H_{x}\) field rather than \(E_{y}\) as expected.