Topological photonics is an emerging field of research, which is inspired by the discovery of topological insulators. The introduction of topology endows traditional photonic systems with brand new properties, including unidirectional propagating edge states and robustness against impurities or defect without backscattering. With the further development of theoretical research on topological photonics, many new photonic devices have been proposed and experimentally demonstrated, showing its broad prospects in constructing high‐performance integrated photonic chips. Topology, as a new research perspective in photonics, is expected to bring about remarkable changes to the field of integrated photonic devices and optical interconnection. This review summarizes the fundamental realization principles of topology in photonic systems, comparison of topology in photonics and electronics, applications of topological photonics in integrated photonic devices and related optical effects. Finally, a brief outlook on the challenges and future development direction in the pursuit of the application of topological photonics in integrated photonic devices is provided.

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study of topological physics for bulk-edge correspondence. It is difficult to directly observe the topological properties of the bulk band, but instead one can explore it by probing the TES. [28][29][30] Although the topological edge state in 2D-and 3D-topological systems can transport unidirectionally, this requires complex design and fabrication processes; hence, very few of these structures have an experimentally observed TES at optical or communication frequencies. [30,31] Chan [1] et al. first used the Zak phase [32] to predict the photonic TES in the 1D PC heterostructure system. The method they proposed was convenient for designing a TES in different bandgaps of the 1D PC heterostructure. [2,4,5] The edge local TES can induce photons to transmit through the PCs at frequencies in the bandgap, which offers a new way to control photon transportation. However, the use of a PC heterostructure to control photon transportation in 1D PCs means that the light should propagate normal to the films, which is unsuitable for integrated photonic circuits.To extend applications of photonic topology in integrated photonic circuits, we propose the use of 1D-grating heterostructures to generate topological edge states. The grating resembles the 1D PC according to the similarity of the Bragg scattering effect when photons propagate along lattice periods. The 1D-grating heterostructure has advantages in terms of both ease of design and the relatively simple fabrication processes compared with those for 2D-and 3D-topological systems. Furthermore, the film-parallel transportation, communication wavelength range design and the compact size might allow for practical integration of 1D photonic topology with on-chip platforms. We first designed grating heterostructures from a 1D PC heterostructure and showed their similarity and the feasibility of constructing a TES in the grating heterostructure. We then fabricated the Si/SiO 2 grating heterostructure to verify the TES transmission peak in an experiment. Finally, we realized a thermally tunable topological grating heterostructure with a metalinsulator transition material vanadium dioxide (VO 2 ), enabling control of the TES and photon propagation in the bandgap. From 1D PC to GratingWe start from the TES in a two joint 1D-perfect photonic crystal and then show that the grating structures on the chip An on-chip integrated one-dimension topological insulator in the optical communication range is realized directly in an integrated photonic circuit. The system takes on a configuration of a 220 nm thick 1D photonic crystal heterostructure sandwiched between two gold films. A photonic topological edge state centered at 1550 nm is obtained for the chip-integrated onedimension topological insulator made of a silicon/SiO 2 photonic crystal heterostructure with a feature size of only 2.25 µm integrated with a silicon waveguide. On/off switching of the photonic topological edge state was also achieved in a 1D topological insulator made of a VO 2 /SiO 2 photonic crystal heterostructure based o...

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Time-domain dynamic evolution properties of topological states play an important role in both fundamental physics study and practical applications of topological photonics. However, owing to the absence of available ultrafast time-domain dynamic characterization methods, studies have mostly focused on the frequency-domain-based properties, and there are few reports demonstrating the time-domain-based properties. Here, we measured the dynamic near-field responses of plasmonic topological structures of gold nanochains with the configuration of the Su−Schrieffer−Heeger model by using ultrahigh spatialtemporal resolution photoemission electron microscopy. The dephasing time of plasmonic topological edge states increases with increasing the bulk lattice number that has a threshold requirement and finally reaches saturation. We directly revealed through simulation that there is a transient bulk state in the evolution of topological edge states, that is, the energy undergoes relaxation from oscillation between the bulk lattice and the edge. This work shows a new perspective of time-domain dynamic topological photonics.

On‐chip‐triggered all‐optical switching is a key component of ultrahigh‐speed and ultrawide‐band information processing chips. This switching technique, the operating states of which are triggered by a remote control light, paves the way for the realization of cascaded and complicated logic processing circuits and quantum solid chips. Here, a strategy is reported to realize on‐chip remotely‐triggered, ultralow‐power, ultrafast, and nanoscale all‐optical switching with high switching efficiency in integrated photonic circuits. It is based on control‐light induced dynamic modulation of the coupling properties of two remotely‐coupled silicon photonic crystal nanocavities, and extremely large optical nonlinearity enhancement associated with epsilon‐near‐zero multi‐component nanocomposite achieved through dispersion engineering. Compared with previous reports of on‐chip direct‐triggered all‐optical switching, the threshold control intensity, 560 kW/cm2, is reduced by four orders of magnitude, while maintaining ultrafast switching time of 15 ps. This not only provides a strategy to construct photonic materials with ultrafast and large third‐order nonlinearity, but also offers an on‐chip platform for the fundamental study of nonlinear optics.

We achieved an ultralow-power Fano-like diode in integrated photonic circuits through two cascaded and uncoupled photonic-crystal microcavities, coated with a nonlinear layer constructed from polycrystalline indium-tin oxide doped with gold nanoparticles. The multicomponent nanocomposite layer helps to obtain an extremely large nonlinearity enhancement on account of the quantum confinement effect, hot-electron injection, and local-field enhancement effect. Thus, the strong nonlinearity enhancement, asymmetric Fano-like spectrum lineshape, and asymmetric light confinement for the forward and backward incidence cases guarantee the nonreciprocal transmission of signal light. A compact size of 7 μm is maintained for the photonic-crystal microcavities and the transmission contrast reached 15 dB. Besides, we achieved 1.5 kW cm−2 operating threshold intensity (cut down by a thousand times). This work assists chip-integrated optical computing on the basis of nonlinear photonic-crystal microcavities.