# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 4 Nonlinear Photonics Devices
### All-Optical Digital Devices: Switches, Gates, Flip-Flop Units, and Optical Transistors  
Two methods are widely used in all-optical switches. The first one is based on linear interference between two light signals, whose relative optical phase difference determines the logic operation functions. Implementation of this method is simple, but, due to the lack of precise control of the optical phase difference, inherent instability, with low-intensity contrast of output logic states '1' and '0', is experienced. The second method is based on third-order nonlinear optical effects, i.e., on the effect of intensity-dependent refractive index. The advantage of this method is its strong universality, and, in principle, complex all-optical devices could be realized based on it. The main obstacle limiting its application is the intrinsic material bottleneck limitation due to the contradiction between the huge third-order nonlinear coefficient and the ultrafast response time (i.e., the larger the third-order nonlinear coefficient, the slower the response time). An important enabling technology to implement all-optical switching operation is provided by high Q/V\({}_{\rm m}\) resonators, Q being the quality factor and V\({}_{\rm m}\) the mode volume of the cavity. The field intensity enhancement inside the cavity makes it possible to utilize optical nonlinearities at a low input power (a few hundred \(\mu\)W to mW level), achieving picosecond-femtosecond response time and lower energy consumption [13; 138]. Microring resonators are among the most widely used configurations for realization of all-optical switch due to their simple design and easy experimental realization [14; 16; 22; 139]. The switching principle, based on nonlinear effects, relies on a shift in resonant wavelength. It is the following: the refractive index variation in the nonlinear material changes the resonant frequency of the microring and/or coupling between the waveguide and the microring. As a consequence, switch control of the signal light output can be enabled. As an example, Figure 9 presents a schematic illustration of an all-optical switch based on a microring resonator. Figure 8: Schematic of a Si-PIC packaged with a multi-channel quasi-planar-coupled (QPC) fiber-array, a hybrid-integrated laser source based on a micro-optic bench (MOB), a vertically integrated electronic integrated circuit (EIC), and a thermo-electric cooler. Electrical connections between the PIC and the printed circuit board (PCB) are made by wire-bonds, while the connections between the PIC and EIC are made using copper pillar bumps (CPBs). Reproduced from [137] under Creative Commons (CC-BY) license. Photonic crystal cavities are also an important tool in all-optical switching, mostly due to their ability to confine light to extremely small mode volumes [20; 140; 141; 142]. Optical bistable switching by using a high-Q two-dimensional photonic crystal nanocavity was demonstrated experimentally using thermo-optic [20] and carrier [140] effects. In order to make possible an all-optical switch, PC transmission spectrum can change when the pump light is injected into the system and the propagation state can change from allowed to forbidden or from forbidden to allowed. An all-optical switch based on a photonic crystal nanocavity was demonstrated in 2015 [141]. The working principle of the device was based on the resonance wavelength change in the cavity by either the Kerr effect or free carriers. In 2020, [PERSON] et al. [142] demonstrated a picosecond all-optical switch using an InP/InAsP nanowire integrated in a silicon photonic crystal. The line-defect photonic crystal had a Q-factor of 25,000. The reaction time of the switch was 150 ps, while the switching energy was several hundred fJ. Photonic crystal chips also lend themselves for large-scale and dense integration of optical memories; this result is obtained by introducing a wavelength-addressable serial integration scheme and exploiting the wavelength-division-multiplexing capability [140].