## 5 Conclusions  
In this paper, we have reviewed some recent developments on compact nonlinear devices for all optical signal processing. We have shown results achieved using different integrated optics platforms, and focused mainly on those achieved in amorphous silicon and silicon germanium. We have shown wavelength conversion of complex modulation signals, such as 16-QAM and 64-QAM, demonstrating that both amorphous silicon and silicon germanium platforms are now sufficiently mature to provide a reliable fabrication platform for the development of practical devices for future communication networks based on advanced modulation formats. Amorphous silicon exhibits a very high nonlinear Kerr parameter (Re[\(\gamma\)] \(\sim\)800 (Wm)\({}^{-1}\)) with a reduced TPA effect with respect to c-Si devices, at telecom wavelengths. On the other hand, linear loss are still relatively high, making this platform not suitable for the realization of long waveguides (\(>\)1 cm). The silicon germanium technology offers a unique platform that is suitable for the development of nonlinear devices at both communications and mid-IR wavelengths. Furthermore, the possibility to control the refractive index profile by engineering the Ge concentration in the SiGe alloy allows obtaining a very high refractive index contrast (\(>\)2.5) between the waveguide core (SiGe) and the cladding (SiO\({}_{2}\) or air), greatly enhancing the waveguide Kerr response. TPA is also higher in SiGe waveguides with respect to c-Si and a-Si waveguides, however it has been demonstrated that nonlinear optical processing can be performed in these structures by using pump power levels that are still below the TPA-threshold at the communication wavelength region. In the final section, we have reviewed work towards the development of novel materials that can provide enhanced performance with respect to pure silicon, along with a full CMOS-BEOL compatibility that can facilitate integration with existing silicon photonics components. In this context, silicon-rich silicon nitride and tantalum pentoxide are both seen as promising platforms, showing enhanced nonlinear response along with a complete absence of TPA effects at the telecom wavelength region. **Acknowledgments:** The authors would like to acknowledge [PERSON] and [PERSON] for their contributions towards the realization and the study of the amorphous silicon waveguides; [PERSON], [PERSON] and [PERSON] for their contributions on the design and modeling of silicon germanium waveguides; [PERSON], [PERSON], and [PERSON] for the fabrication of the silicon germanium waveguides; and [PERSON] and [PERSON] for the design and fabrication of tantalum pentoxide waveguides. This work was supported by the European Communities Seventh Frame-work Programme FP7/2007-2013 under Grant 288304 (STREP CLARITY) and the Photonics Hyperhighway Programme Grant (EPSRC grant EP/I01196X) and by the Engineering and Physical Sciences Research Council (EPSRC) under the grant \"Silicon Photonics for Future Systems\" (SPS) ([[http://doi.org/10.5258/SOTON/403875](http://doi.org/10.5258/SOTON/403875)]([http://doi.org/10.5258/SOTON/403875](http://doi.org/10.5258/SOTON/403875))). Figure 13: Cross section of the Ta\({}_{2}\)O\({}_{5}\) RIB waveguide; and **(inset)** SEM image of the fabricated final device covered by 1.6 \(\upmu\)m of SiO\({}_{2}\) (width = 5 \(\upmu\)m) [71]. **Author Contributions:** [PERSON] and [PERSON] performed the experimental characterization of the amorphous silicon waveguides. [PERSON] analyzed the data regarding the amorphous silicon experiments. [PERSON] and [PERSON] performed the experiments on the SiGe waveguides and consequently analyzed the data. [PERSON] has developed the Si-rich SiN material described in here. [PERSON] has performed the experiments on the Ta\({}_{2}\)O\({}_{5}\) waveguides and analyzed the extracted data. [PERSON], [PERSON] and [PERSON] wrote the paper. [PERSON] provided the overall supervision and technical leadership. All authors reviewed the manuscript.