# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
###### Abstract  
The combination of integrated optics technologies with nonlinear photonics, which has led to growth of nonlinear integrated photonics, has also opened the way to groundbreaking new devices and applications. In a companion paper also submitted for publication in this journal, we introduce the main physical processes involved in nonlinear photonics applications and discuss the fundamentals of this research area. The applications, on the other hand, have been made possible by availability of suitable materials with high nonlinear coefficients and/or by design of guided-wave structures that can enhance a material's nonlinear properties. A summary of the traditional and innovative nonlinear materials is presented there. Here, we discuss the fabrication processes and integration platforms, referring to semiconductors, glasses, lithium niobate, and two-dimensional materials. Various waveguide structures are presented. In addition, we report several examples of nonlinear photonic integrated devices to be employed in optical communications, all-optical signal processing and computing, or in quantum optics. We aimed at offering a broad overview, even if, certainly, not exhaustive. However, we hope that the overall work will provide guidance for newcomers to this field and some hints to interested researchers for more detailed investigation of the present and future development of this hot and rapidly growing field. photonics devices; nonlinear photonics; integrated photonics; photonic structures; optical materials; all-optical signal processing; all-optical computing; all-optical communications; all-optical digital devices; signal amplification and frequency conversion; all-optical signal regeneration; super-continuum generation; microcomb generation +
Footnote †: journal: _Micromachines_  
In this review, we focus on three main areas of application: all-optical computing, all-optical processing, and nonlinear photonics sources. In all-optical computing, the key strength provided by optical technologies is parallelism of information transfer and processing onto multiple wavelength channels. Parallel access to information points is permitted due to the capability to use light waves of distinct wavelengths within the same device. In addition, photon being the information carrier, there is no propagation delay in different parts of the optical system. Implementation of logic operations using a photon signal is a very challenging frontier of research because of the fundamental requirement of very efficient light-control. Ultrafast all-optical switch is a fundamental component for all-optical computing. It can be defined as a structure with a pump light-controlling the ON/OFF transition of the signal light. Optical devices, performing digital functions, are expected to fulfill the following criteria: (1) ultra-compactness; (2) low power consumption (\(\sim\)f)/bit); (3) high-speed operation (scalable beyond 100 Gb/s); (4) parallel operation on multiple wavelength channels, reducing the need for large fan-outs and redundant parallel processing structures; (5) preservation of information (e.g., phase) carried in the optical domain, usually lost in optical-electronic conversion; (6) the ability to transparently process a data channel regardless of its data rate or its modulation format; (7) scalability; and (8) cascading [13; 14; 15; 16; 17; 18]. Unfortunately, as underlined by [PERSON] et al. in 2020, at that date, no known structure behaved as an ideal switch [18]. The basic building block of an optical flip-flop device is the optical bistable switch. In this device, the output of the system takes on one of two possible states depending on the states of the inputs. Optical bistable operation permits optical read-write memory operation, opening the possibility of an integrated optical logic circuit on a single chip. A typical way of forming a bistable optical device is to place a saturable absorber inside a resonator.