# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 3 Material Platforms for Nonlinear Integrated Photonics  
As mentioned in the companion paper [62], numerous materials for integrated nonlinear optics have been investigated. Among them are Si and related materials, such as SiN, a-Si, and SiC; glasses, such as silica, high-index glass, and chalcogenide glasses; III-V semiconductors, in particular AlGaAs and lithium niobate (LN); and recently investigated materials, such as tantalum pentoxide (Ta\({}_{2}\)O\({}_{5}\)) and vanadium dioxide (VO\({}_{2}\)) [62] and references therein. Correspondingly, several material platforms have been developed to achieve the goal of a dense components' integration. Each platform has its advantages and disadvantages and offers variable nonlinear efficiencies and integration densities depending on the values of the nonlinear coefficients and refractive index contrast. An additional limiting factor is related to losses that are determined by both the quality of the grown material and the maturity of the fabrication process, which affect, for instance, the roughness and absorption of the surface passivation layer. Although in the longer term the growth and fabrication quality for all these materials will eventually level up, there still remains one important limiting factor that clearly separates them regarding high power densities, namely two photon absorption (TPA). TPA is determined by the band gap of the materials and by the working wavelength, which is often in the telecom range due to the historically developed infrastructure of sources, detectors, etc. In this regard, small-band-gap semiconductors, such as Si [113; 114; 115; 116; 117] and GaAs [118; 119], are fundamentally limited and there seems to be no way around this problem other than to change the working wavelength and all the surrounding infrastructure. Silicon photonics research and development has much progressed, and both component performance and integration complexity have made significant steps forward in the past decade [113; 114; 115; 116; 117]. Nowadays, silicon-based platforms, in particular silicon-on  
Figure 5: Schematic illustration of meta-atom, 1D chain, 2D metasurface, and 3D metamaterial. Inserts are the representation of the parameter space for permittivity \(\varepsilon\) and permeability \(\mu\) and the typical examples of applications of metamaterials. Adapted from [102] under Creative Commons License 3.0.  
insulator (SOI), are among the most mature for PIC realization. The silicon-on-insulator (SOI) platform is a fabrication approach in which a thin silicon layer on top of an insulator layer resides on a silicon substrate. The functional optical elements are situated in the thin top-silicon layer, and the insulator is typically made from SiO\({}_{2}\). The SOI platform has become the foundation of silicon photonics for several reasons, including strong optical confinement of silicon due to the significant refractive index difference between silicon and SiO\({}_{2}\), which enables very compact optical devices, and demonstration of much more compact optical waveguides with lower optical propagation loss, better processing yield, low cost, etc. Among the numerous nonlinear waveguide platforms that have been explored, the group of materials capable of combining both passive waveguides, for light steering and nonlinear manipulation, and active functionalities (laser sources, modulators, and detectors) monolithically on the same chip is III-V semiconductors [118; 119]. This has been a strong driving force stimulating development of nonlinear optical devices based on III-V semiconductors and addressing challenges associated with loss mitigation and nonlinearity enhancement in these platforms. The main III-V integrated nonlinear photonic platforms considered to date are GaAs [76] and its AlGaAs [120] derivative, InP and InGaAsP, III-nitrides AlN [121; 122], and GaN [123], as well as GaP [124] and its ternary derivative InGaP.