## 1 Introduction  
Over the last decade, silicon photonics has established itself as a mature technology for the fabrication of low cost, scalable, optical integrated circuits that can meet the requirements of future optical networks [1]. Silicon-on-Insulator (SOI) is widely acknowledged as the ideal fabrication platform for silicon photonic components, providing a Complementary Metal Oxide Semiconductor (CMOS)-compatible, high-index contrast system. At present, most fundamental signal processing functionalities on optical networks are still performed in the electrical domain, imposing successive optical-to-electrical-to-optical conversions across the network, thereby increasing the system complexity and cost and restricting the maximum bandwidth of the transmitted signals [2]. Nonlinear optical devices have already shown great potential for either substituting or complementing electronic processing for the implementation of certain signal processing functionalities, showing no practical limitations in terms of bandwidth [3]. Thanks to their high refractive index difference (>2) and the strong third-order nonlinear response of silicon [4; 5], SOI waveguides have been widely used to observe nonlinear optical effects. SOI devices have substantially contributed to the realization of efficient all optical nonlinear components; a number of fundamental functionalities have been demonstrated in the SOI nonlinear platform such as all optical wavelength conversion [6; 7; 8; 9], optical switching [10; 11], format conversion [12; 13], logic operation [14] and high speed all optical modulation [10; 15; 16]. However, the large nonlinear response in silicon is also accompanied by strong two-photon absorption (TPA) that causes accumulation of free carriers that induce a very high nonlinear loss [3]. This effect reduces the optical power that can be beneficially coupled into the waveguide, and subsequently the achievable nonlinear phase shift is restricted to fractions of \(\pi/2\) radians [3], practically limiting the use of the technology in telecom settings. Advanced modulation formats, such as multiple (M)-order QAM (Quadrature Amplitude Modulation), have become key to the design of modern high-capacity optical transport networks [17]. Integrated optical components that can operate on these modulation formats are required to enable bandwidth efficient, high speed transmission on a chip level, providing fundamental optical functionalities such as wavelength routing, wavelength conversion, format conversion, etc. Such functionalities have already been demonstrated in silicon-based nonlinear devices in recent years [13; 18; 19]. However, the devices featured in these works are typically limited by TPA-triggered processes, therefore they suffer from reduced efficiency that makes them unsuitable for practical applications. In order to overcome this limitation, alternative materials have been proposed by the scientific community as AlGaAs [20; 21], chalcogenides [22; 23], composite glasses [24] and nonlinear polymers [25]. Even though these materials have shown promise for achieving remarkable performance, fabrication constraints make their integration with the standard silicon platform difficult, requiring additional manufacturing steps, which are not desirable in high integration optical platforms. We have recently investigated two material platforms that can be easily integrated with existing silicon photonic infrastructures, namely amorphous silicon and silicon germanium. We have demonstrated wavelength conversion of complex modulation format signals [26; 27; 28], phase sensitive regeneration in integrated devices [29; 30] and supercontinuum generation in cm-long waveguides [31].