# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 3 Material Platforms for Nonlinear Integrated Photonics
The former is a process that connects two or more PICs or photonic device chips, usually from different material technologies into one single package. This process is, in general, performed at the packaging stage after fabrication of the PIC and photonic device chips. The latter, instead, is a process that combines two or more material technologies into a single PIC chip. This process is generally performed at the early- to mid stages of fabrication of the PIC chip, as in the case of unpatterned III-V thin-films integrated onto pre-processed silicon photonic wafers [130; 131; 132]. In order to address the shortcomings of the SOI platform, several novel waveguide platforms have been developing based on heterogeneous integration of other material systems on silicon substrates, with the common requirement of remaining compatible with the complementary metal-oxide-semiconductor (CMOS) technology [132]. As a more recent example, photonic components in SiC (specifically waveguides, 1D and 2D photonic crystal cavities, microdisk, and microring resonators), based on thin layers of SiC on insulator (SiCOI), have been implemented [133; 134]. High nonlinearity and low loss were demonstrated in waveguides and ring resonators fabricated in amorphous SiC (a-SiC) grown directly on silica, using plasma-enhanced chemical vapor deposition, with loss as low as 3 dB/cm, providing a very scalable material growth [133]. The intrinsic quality factor of the microring resonator was around 160,000 [133]. Figure 7 shows the fabrication process flow of microring devices (a) and the SEM micrographs of a microring (b) and of the coupling area to a channel waveguide (c). Much higher quality factors have been achieved by using the 4H polytype of silicon carbide (4H-SiC). For instance, a microring fabricated in high-purity semi-insulating 4H-SiC exhibited a Q = 1.1 \(\times\) 10\({}^{6}\), corresponding to a waveguide loss of 0.38 dB/cm [135]. The high field enhancement of this microring (55 \(\upmu\)m diameter) enabled demonstration of optical parametric oscillation and optical microcombs. More recently, generation of an octave-spanning microcomb, covering a wavelength range from 1100 nm to 2400 nm, was achieved by using a 36 \(\upmu\)m radius microring resonator fabricated in 4H-SiC on a silicon dioxide layer, with intrinsic quality factor above one million [136]. As a rule, photonic packaging of PICs is much more challenging and much more expensive (at least to date) than electronic packaging since it requires robust high-precision alignment of optical components and stringent real-time temperature control. This is one of the reasons why huge development of commercial integrated photonic devices is still lacking. Figure 8 may provide an idea of the technical complexity of the optical, electrical, thermal, and mechanical interconnections inside and outside a PIC [137]. Figure 7: Fabrication of a microring in a 350 nm thick a-SiC film with silica cladding of 3 \(\upmu\)m (bottom) and 2 \(\upmu\)m (top). (**a**) Schematic process flow; (**b**) SEM micrograph of the microring having 100 \(\upmu\)m diameter; (**c**) higher magnification SEM image of the coupling area between the ring and a ridge waveguide. Reprinted with permission from [PERSON] et al., ACS Photonics [134]. Copyright 2019, American Chemical Society.