Significance Nonreciprocal photonic devices including optical isolators and circulators, are widely used in information photonic systems and are indispensable in fiber optical communication and fiber optical sensing systems. Commercial nonreciprocal photonic devices rely on magneto-optical effects such as the Faraday rotation effect. In these devices, rare earth garnet single crystals grown by liquid phase epitaxy (LPE) are used to introduce nonreciprocal polarization to incident light. In combination with polarizers or waveplates, these devices provide optical isolation or circulation functionalities in free-space or fiber in-line structures. The development of photonic integrated circuits (PICs) allows for the parallel integration of photonic devices using semiconductor fabrication technologies. In PICs, reflection or scattering from the interfaces of different devices leads to an urgent demand for integrated nonreciprocal photonic devices. However, significant challenges have been encountered. Firstly, the magnetic material, rare-earth iron garnet (RIG), used in bulk nonreciprocal photonic devices, is lattice and thermal mismatched with PICs, making it very difficult to grow high-quality epitaxial films on semiconductor substrates. Secondly, the fabrication technology, LPE, is not compatible with the semiconductor fabrication process, which predominately uses vapor phase deposition. In addition, the presence of Fe elements prevents front-end-of-line (FEOL) integration of such materials, necessitating the development of a back-end-ofline (BEOL) compatible process. Thirdly, the Faraday rotation device structure faces significant issues due to waveguide structure-induced birefringence, leading to incomplete mode conversion between TE and TM modes due to different propagation constants. These problems pose significant challenges to the integration of nonreciprocal photonic devices on PICs. To date, nonreciprocal photonic devices are absent from all PIC technologies, including SiO2 waveguide-based photonic light circuits (PLCs), III-V material-based PICs, and silicon photonic PICs, making it one of the fundamental difficulties in integrated photonics. Driven by the fast development of complex silicon photonic systems and the strong need for optical nonreciprocity in PICs, different mechanisms have been proposed theoretically and studied experimentally over the past decade to achieve optical nonreciprocity in silicon photonic waveguides. Nonreciprocal photonics has become a very active research field. The research to date can be broadly categorized into three mechanisms: 1) The magneto-optical effect. This research direction focuses on developing new magneto-optical materials grown on silicon with strong Faraday rotation and low propagation loss. It also involves developing new fabrication technologies other than LPE to be compatible with semiconductor fabrication processes and BEOL. For device development, the goal is to utilize Faraday rotation or other magneto-optical effects, such as nonreciprocal phase shift, to construct nonreciprocal photonic devices. 2) The nonlinear optical effect. This direction involves utilizing nonreciprocal photonic materials and nanophotonic device structures to allow different electromagnetic field distributions and nonlinear photonic effects in the forward and backward propagation directions. The focus is on exploring nonmagnetic but strong nonlinear photonic materials and structures to achieve strong nonreciprocity, low loss, wide bandwidth, and low power-dependent device structures. 3) The spatio-temporal modulation. This approach modulates the index of silicon photonic waveguides as a function of time and space to achieve optical nonreciprocity. Researchers explore efficient materials and device structures to achieve strong nonreciprocity with low insertion loss and low driving power. All three research directions have made significant progress in the past decade, realizing nonreciprocal photonic devices with comparable or even superior performance to their bulk counterparts. In this paper, we summarize the operation principles and major achievements in the integration of nonreciprocal photonic devices on PICs, particularly on silicon photonics. We introduce both the mechanisms and experimental progress in this field. Progress We begin with magneto-optical nonreciprocal photonic devices. In Fig. 1, the working principle and structure of the Faraday optical isolator and circulator are demonstrated. In Fig. 2, the operation principle of nonreciprocal mode conversion type devices is introduced. In Fig. 3, major experimental achievements based on this principle are summarized. These results include efficient waveguide Faraday rotators and isolators fabricated by reducing waveguide birefringence or using quasi-phase match structures. In Fig. 4, the operation principle of nonreciprocal phase shift device is introduced. The experimental progress on such devices is summarized in Figs. 5 and 6, including devices fabricated by wafer bonding or deposition of RIG thin films on silicon. The mechanism of nonreciprocal photonic devices based on nonlinear photonic effects is introduced in Fig. 7. In Fig. 8, recent experimental developments of nonlinear nonreciprocal photonic devices are introduced, including devices based on nonlinear photonic effects in silicon and silicon nitride (SiN), as well as devices based on parametric amplification, nonreciprocal four-wave mixing, and PT symmetric non-Hermitian systems. In Fig. 9, the principle of spatio-temporal modulation induced optical nonreciprocity is discussed. Experimental work along this direction is summarized in Fig. 10, categorized by modulation mechanisms, including electro-optical modulation and optomechanical modulation. The achievements, advantages, and disadvantages of different mechanisms are summarized at the end of this review. Conclusions and Prospects Integrated nonreciprocal photonic devices are not only miniaturized optical isolators and circulators but also new media to introduce optical nonreciprocity in integrated photonic systems, potentially providing new functionalities to lasers, amplifiers, waveguides, modulators, and integrated photonic networks. Optical nonreciprocity may change the design principles and introduce new freedoms to integrated photonic systems. With the urgent need and rapid development of nonreciprocal photonics, we believe this long-sought goal of integrated photonics will realize breakthroughs and wide applications in the near future.
