## 5 Applications of 3D-SNOM
### Optimization of Nano-Optical Devices
These observations indicate that the 3D SNOM measurements above the sample surface actually measure a superposition of the \(E_{\parallel}\) and \(H_{\parallel}\) fields. Therefore, using a symmetrical c-SNOM probe it is possible to access all components of the electromagnetic spectrum above a PhCW using 3D SNOM mapping. The individual components can be further decoupled using [PERSON]'s equations combined with additional constraints such as symmetry in the sample or measurements with probes of different shapes [29]. Beyond the study of previously exiting waveguide architectures, 3D SNOM has also been used to aid in the design of other types of devices such as organic bulk heterojunction (BHJ) solar cells. The distribution of scattered light resulting of from a nanostructured surface can also be utilized to enhance the performance of BHJs. The prospects of this concept have been explored using a structured plasmonic architecture solar cell containing a layer of copper nanoparticles. The architecture is shown in Figure 13c,e where an SiO\({}_{2}\) thin film is placed as a spacer between the solar cell active layer and a layer of copper nanoparticles [48]. Figure 13a illustrates the integrated scattered light intensity in the SiO\({}_{2}\) modified device compared to a reference cell. A region of forward-scattered light exhibiting constructive interference is revealed from \(z\) = 0-550 nm above the nanoparticle layer when the spacer is present, as well as strong constructive interference at a greater distance from the nanoparticle layers. This enhancement region is the result of coupling between normally incident plane waves on the SiO\({}_{2}\) surface and waves travelling laterally, parallel to the surface. The superposition of these lateral and normal components leads to scattered light with increased intensity that is combined with the normally incident light in constructive interference. By contrast, when the spacer is not used destructive interference is observed in a band 160 nm from the sample surface. This has the effect of considerably depleting the near-field in the region where the solar cell active layer would be placed in a typical architecture. In the spacer-enhanced cell the amplified forward-scattering field is ideally located to fall in the active solar cell region. Figure 12: 3D SNOM detection of E and H fields simultaneously. (**a,b**) Calculated electric and magnetic fields in \(x\)-\(y\) planes above the PhCW surface up to a distance of 380 nm. (**c,d**) Measured 3D SNOM data (left) and data fitted to the calculated result (right). \(L_{x}\) and \(L_{y}\) represent two photodetectors arranged to preferentially detect \(E_{x}\) and \(E_{y}\) fields, respectively. Reproduced with permission from [29], Copyright Nature Photonics, 2014. The use of nanoparticle layers to enhance light collection was examined in solar cells fabricated with and without a SiO\({}_{2}\) spacer. Solar cell architectures of this type are shown schematically in Figure 13d,e consisting of a BHJ active layer prepared from a mixture of regioregular poly-(3-hexylthiophene-2,5-diyl) (P3 HT) and [6,6]-phenyl-C\({}_{61}\)-butyric acid methyl ester (PCBM) [48]. A layer of copper nanoparticles is incorporated on top of the indium tin oxide electrodes in both devices to serve as a forward scattering layer, and I-V curves were collected under inert nitrogen atmosphere as shown in Figure 13f,g for the reference and spacer-enhanced solar cells, respectively. Although the I-V characteristics of the two devices are very similar under no illumination, a significantly larger photocurrent was extracted from devices containing the spacer, more than twice the value obtainable in the unmodified reference cell. The increase in photocurrent alone is sufficient to improve the power conversion efficiency (PCE) from \(\eta\) = 1.38% with no spacer present to \(\eta\) = 3.08% on average with the spacer. The increased PCE is a result of increased intensity of light scattered by the layer of copper nanoparticles into the active layer of the solar cell.