# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 3 Material Platforms for Nonlinear Integrated Photonics
We note that AlN [121; 122] shows optical properties similar to SiN (refractive index, transparency, and \(n_{2}\)) but also possesses second-order nonlinearity. Considering AlGaAs, the structures used in NLIP can be divided into three-layer, two-layer, and multi-layer platforms depending on the number of epitaxially grown layers with different material compositions. Figure 6 shows examples of these three AlGaAs platforms; the waveguides sketched in Figure 6h,i are often employed for phase matching of the \(\chi^{(2)}\) processes. Figure 6: Schematics of AlGaAs platforms and waveguide geometries. (**a–c**) 3-layer platform with strip-loaded, nanowire, and half-core waveguides, respectively. (**d–f**) 2-layer platforms with suspended nanorib, suspended nanowire, and AlGaAs-OI waveguides, respectively. (**g–i**) Multi-layer platform with multi-quantum-well waveguide, modulated-\(\chi^{(2)}\) waveguide, and Bragg-reflector waveguide, respectively. Reproduced from [120] under Creative Commons License. In recent years, lithium niobate-on-insulator (LNOI) wafer fabrication process has been rapidly advancing [125; 126; 127; 128; 129]. This technology is revolutionizing the lithium niobate industry, enabling higher performance, lower costs, and entirely new devices and applications. Availability of LNOI wafers has sparked significant interest in the platform for integrated optical applications as LNOI offers the attractive material properties of lithium niobate while also offering stronger optical confinement and high optical element integration density that has driven the success of more mature silicon and silicon nitride (SiN) photonics platforms. Many of the key building blocks for highly integrated photonic integrated circuits (PICs) have been established on this platform, including low loss optical waveguides, electro-optical interfaces for ultra-fast modulation, and nonlinear optical elements and resonators. However, further work needs to be completed to make LNOI an attractive and competitive integrated optical platform: (i) optical interfacing to LNOI waveguides has to be improved, reducing fiber-to-chip coupling losses, for example by developing inverted tapers and waveguide tapers on LNOI; (ii) optical gain media need to be demonstrated in this platform, either via bonding already doped lithium niobate in the LNOI wafer fabrication process or by doping the LNOI wafer after fabrication using ion implantation techniques. A further approach could be heterogeneous integration of III-V lasers on LNOI waveguides, which needs to be investigated; (iii) development of photodetectors on LNOI waveguides requires further investigation [125; 126; 127; 128; 129]. Current trends in PIC material technologies, however, indicate that there is no waveguide material technology that can address the needs for all the potential applications of PICs. Furthermore, advanced PICs may require the best possible performance of many different photonic elements to achieve the desired functionality, which may not be possible with a single waveguide material technology. However, the fundamental size limit for photonic devices is significantly larger than that of electrical devices. Therefore, integrated photonics has, over the decades, developed at a considerably slower pace than integrated electronics, in integration density as well as total number of devices on a chip. The specific application requirements drive the choice of the most suitable substrate material as each material has its own specific advantageous features and limitations. A solution to overcome this limitation is to integrate different material technologies into a single PIC or package. This approach has the benefit that each material can be used to provide the photonic element functionality for which it is best suited without compromising the functionalities of the other elements in the system. Integration of the different material technologies can occur through two different routes: (i) hybrid integration and (ii) heterogeneous integration.