# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 5 Conclusions  
Despite the difficulties, impressive progress has been made in on-chip nonlinear optical processing with favorable performance, paving the way to full integration of complete optical communication and processing systems on a monolithic or hybrid chip. As an example, an excellent result concerns the high device integration density achieved by using optimized racetrack microresonators based on ultra-low-loss multimode Si\({}_{3}\)N\({}_{4}\) photonic circuits. The optimized design uses adiabatic Euler bends in place of circular bends; it enables a device footprint as small as 0.21 mm\({}^{2}\), which can be very useful for several nonlinear photonic applications. Single-soliton generation of 19.8 GHz repetition rate was demonstrated; the soliton spectrum showed a 3-dB bandwidth of 16.3 nm, corresponding to a pulse duration of 156 fs [3]. Even better prospects exist for silicon carbide, which now appears able to fully compete with more mature nonlinear materials; as a proof, design and demonstration of octave-spanning microcombs have been reported in a 4H-SiC-on-insulator microring resonator with 36 \(\upmu\)m radius [136]. All-optical computing, on the other hand, still presents many challenges due to the difficulty of implementing efficient all-optical logic devices. Photonic crystals seem to offer a suitable approach to realization of these devices, but, up to now, groundbreaking results have not been achieved. Other approaches make use of nonlinear optical fibers and semiconductor optical amplifiers, but, in both cases, size is an obstacle to chip integration. Plasmonic logic gates would have a desirable nanometer size, but their large optical loss makes them also unsuitable for use in integrated chips. As has been shown by various examples, PICs fabricated on a silicon-on-insulator platform may exhibit low power consumption and high transmission efficiency but occupy a relatively large footprint. A solution has, therefore, still to be searched. However, based on the linear interference approach, it is worth reporting here a very recent result obtained using a design based on topology optimization [205] and leading to implementation of an SOI chip containing seven major logic gates (AND, OR, NOT, NAND, NOR, XOR, and XNOR) and a half adder, with a footprint of only 1.3 \(\times\) 4.5 \(\upmu\)m\({}^{2}\)[205]. Figure 16: EO comb generator using a microring resonator. (**a**) Sketch of a bulk EO comb generator based on an EO (\(\chi^{(2)}\)) phase modulator inside a Fabry–Pérot resonator. A cw laser beam enters the resonator and an optical frequency comb is generated at the output. (**b**) EO comb generation principle; the microwave modulating signal has frequency equal to FSR of the Fabry–Pérot resonator. (**c**) Integrated EO comb generator, where a microring replaces the Fabry–Pérot resonator. The cw signal from input waveguide is coupled into the microring and EO-modulated at a frequency matching the FSR of the ring. The generated comb is then coupled back to the waveguide. Reproduced with permission from [199] © Springer Nature Ltd. The evolving nano-devices would enable emerging advanced applications not only in optical processing and computing but also in metrology, single-molecule sensing, imaging, microscopy, mid-infrared photonics, terahertz generation, microwave photonics, and biomedicine. We expect that progress in nonlinear integrated photonics will occur along three paths: one involving new materials and new physics in nonlinear interactions; a second devoted to improving device performance using well-known and innovative guided-wave nonlinear optical schemes; and a third addressing new perspectives in quantum photonics. In this regard, being impossible to adequately cover the achievements in quantum optics in this short review, we just mention a few examples, all exploiting the exceptional characteristics of microring resonators, either on a silicon platform [206; 207] or a 4H-SiC-on-insulator platform [208].