## 4 Novel Materials for All Optical Processing Nonlinear Devices
### Engineered Silicon Rich Silicon Nitride  
Silicon nitride has become a material of great interest in the field of nonlinear optics in recent years [59; 60; 61; 62]. A variety of nonlinear applications have already been demonstrated, such as wavelength conversion [60], supercontinuum generation [59] and harmonic generation [63]. Silicon nitride is a CMOS compatible composite that can be deposited on a SiO\({}_{2}\) layer, providing a suitable alternative to c-Si. Thanks to its optical properties, by carefully controlling the material fabrication conditions,  
Figure 11: Dispersion and nonlinear coefficient curves for the structure under investigation as a function of wavelength. Inset: Scanning Electron Microscope (SEM) image of the 1 \(\upmu\)m wide SiGe strip waveguide before and after encapsulation in a 12 \(\upmu\)m Si cladding layer [27]. very low loss waveguides can be obtained. Furthermore, no TPA is observed in the C-band wavelength region, making this platform suitable for both low and high power applications. However, the use of this platform as a nonlinear tool is still hindered by fabrication difficulties that prevent the possibility of growing thick SiN layers on SiO\({}_{2}\) (>200 nm) [24], thus drastically reducing the ability of confine light tightly within the core material, and in turn limiting the overall nonlinear response of the device. Moreover, the silicon nitride Kerr coefficient is substantially lower than that in silicon [64] (two orders of magnitude lower), further reducing the strength of the nonlinear effects. Recently, a number of research groups have proposed the use of silicon-enriched silicon nitride compounds, aiming to increase the device nonlinear response [59; 65; 66; 67; 68]. This direction, however, introduces additional losses in the waveguides [66; 68], that may compromise their suitability for practical applications. We have recently studied systematically the impact of introducing silicon within a silicon nitride matrix using only CMOS Back End Of Line (BEOL) fabrication processes, on both linear and nonlinear optical properties [68; 69]. We outline here the most important outcomes of this study, demonstrating that by carefully selecting the fabrication conditions a TPA free, low loss, highly nonlinear waveguide configuration can be obtained, overcoming most of the issues imposed by the introduction of silicon when designing a nonlinear device. In order to evaluate the effect of enriching silicon nitride with silicon, three different material compositions were developed and deposited on thermal SiO\({}_{2}\) wafers. The wafer composition is schematically shown in Figure 12.  
The resulting refractive indices of the three fabricated wafers (identified as Wafer_01, Wafer_02 and Wafer_03 hereafter) are shown in Table 6. Various test structures (including straight waveguides of different lengths and widths) were defined onto the wafers by using a standard dry etch technique. In order to evaluate the linear and nonlinear properties of the fabricated waveguides as a function of the introduced silicon excess, three different parameters were considered: (1) waveguide linear loss \(\alpha_{0}\); (2) Kerr response coefficient Re[\(\gamma\)]; and (3) TPA coefficient Im[\(\gamma\)], all evaluated across the whole C-band wavelength region. The waveguide linear losses were evaluated by using a standard cutback technique. The optical transmission of waveguides of varying lengths was evaluated, providing the \(\alpha_{0}\) coefficient for each width configuration. Re[\(\gamma\)] and Im[\(\gamma\)] were both evaluated utilizing techniques shown in Section 2, by using CW-FWM and pulsed transmission experiments, respectively. The obtained results for a representative waveguide (Width = 1000 nm, Height = 300 nm) are shown in Table 6.  
It is clear from Table 6 that higher linear losses were observed when the silicon content contained in the compound was increased; it is noted however, a modest increase was observed in Wafer_02, while a significantly higher value was measured for Wafer_03.