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An all-Si photonic structure emulating the quantum-valley-Hall effect is proposed. We show that it acts as a photonic topological insulator (PTI), and that an interface between two such PTIs can support edge states that are free from scattering. The conservation of the valley degree of freedom enables efficient in- and out-coupling of light between the free space and the photonic structure. The topological protection of the edge waves can be utilized for designing arrays of resonant time-delay photonic cavities that do not suffer from reflections and cross-talk.

The rapid development of topological photonics and acoustics calls for accurate understanding of band topology in classical waves, which is not yet achieved in many situations. Here, we present the Wilson-loop approach for exact numerical calculation of the topological invariants for several photonic/sonic crystals. We demonstrate that these topological photonic/sonic crystals are topological crystalline insulators with fragile topology, a feature which has been ignored in previous studies. We further discuss the bulk-edge correspondence in these systems with emphasis on symmetry broken on the edges.

Topological photonics seeks to control the behaviour of the light through the design of protected topological modes in photonic structures. While this approach originated from studying the behaviour of electrons in solid-state materials, it has since blossomed into a field that is at the very forefront of the search for new topological types of matter. This can have real implications for future technologies by harnessing the robustness of topological photonics for applications in photonics devices. This roadmap surveys some of the main emerging areas of research within topological photonics, with a special attention to questions in fundamental science, which photonics is in an ideal position to address. Each section provides an overview of the current and future challenges within a part of the field, highlighting the most exciting opportunities for future research and developments.

Non-Hermitian photonics and topological photonics, as new research fields in optics, have attracted much attention in recent years, accompanying by a great deal of new physical concepts and novel effects emerging. The two fields are gradually crossed during the development process and the non-Hermitian topological photonics was born. Non-Hermitian topological photonics not only constantly produces various novel physical effects, but also shows great potential in optical device applications. It becomes an important part of the modern physics and optics, penetrating into different research fields. On one hand, photonics system can introduce artificially-constructed gain and loss to study non-Hermitian physics. Photonics platform is an important methods and ways to verify novel physical phenomena and promote the development of non-Hermitian physics. On the other hand, the non-Hermitian topological photonics provides a new dimension for manipulating topological states. Active and dissipate materials are common in photonic systems; therefore, by using light pump and dissipation of photonic systems, it is expected to promote further development of topological photonics in device applications. In this review article, we focus on the recent advances and applications on non-Hermitian topological photonics, including the non-Hermitian topological phase transition and skin effect, as well as the applications emerging prosperously in reconfigurable, nonlinear and quantum optical systems. The possible future research directions of non-Hermitian topological photonics are also discussed at the end. Non-Hermitian topological photonics can have great potential in technological revolution and have the capacity of leading the development of both physics and technology industry.

The emergence of topological photonics has revolutionized the paradigm of photonic device design, with its core principle being the utilization of topological invariants to achieve robust control over light propagation and localization. In recent years, this concept has been successfully introduced into fiber optics, giving rise to topological photonic crystal fibers (TPCFs). The Dirac-vortex TPCFs, based on the Jackiw-Rossi zero mode, are realized by introducing a generalized Kekulé modulation in their cross section. This approach exhibits remarkable properties, including a controllable number of modes, a large bandwidth for single-polarization single-mode operation, and robustness against structural disorder. In this paper, we give a comprehensive overview of recent advances in Dirac-vortex TPCFs, including the physical mechanisms with its origin of topological photonic crystals and photonic crystal fibers, theoretical design, and experimental realization. We also discuss opportunities and challenges of Dirac-vortex TPCFs for future applications.

Photonic topological states have revolutionized the understanding of the propagation and scattering of light. The recent discovery of higher‐order photonic topological insulators opens an emergent horizon for 0D topological corner states. However, the previous realizations of higher‐order topological insulators in electromagnetic‐wave systems suffer from either a limited operational frequency range due to the lumped components involved or a bulky structure with a large footprint, which are unfavorable for achieving compact photonic devices. To overcome these limitations, a planar surface‐wave photonic crystal realization of 2D higher‐order topological insulators is hereby demonstrated experimentally. The surface‐wave photonic crystals exhibit a very large bulk bandgap (a bandwidth of 28%) due to multiple Bragg scatterings and host 1D gapped edge states described by massive Dirac equations. The topology of those higher‐dimensional photonic bands leads to the emergence of in‐gap 0D corner states, which provide a route toward robust cavity modes for scalable compact photonic devices. Topological corner states are experimentally demonstrated in planar surface‐wave photonic crystals, which have advantages such as large bandgaps and small lattice constants. Such corner states arise from the topology of the gapped edge states that live in the bulk bandgap of photonic crystal. This work provides a pathway toward robust cavity modes for scalable compact electromagnetic‐wave/photonic devices.

Nonzero Berry curvature is central to the existence of topological edge states in electronic and photonic valley‐Hall systems. While manipulating the Berry curvature in condensed matter systems is challenging, valley‐Hall topological photonics offer unprecedented control, where the broken spatial inversion symmetry alters the Berry curvature. Herein, an all‐silicon Berry antenna is presented, using a continuously varying geometry corresponding to a gradual change in Berry curvature. The on‐chip topological edge mode with a tunable field extent is achieved to enhance effective antenna aperture, creating a high‐gain on‐chip photonic antenna with perfectly planar wavefronts. Experimentally, a maximum gain of 17 dBi that supports 20 Gbps chip‐to‐chip wireless communication is demonstrated, with active optical tunability of the antenna gain with modulation depths of 8 dBi. This Berry antenna paves the way for the development of complementary metal‐oxide‐semiconductor (CMOS) compatible topological Berry devices, with potential applications in integrated micro‐/nano‐photonics, next‐generation wireless communications (6G to Xth generation), and terahertz detection and ranging. The on‐chip topological Berry antenna for 6G to Xth‐generation wireless communication is designed via integrating various Berry curvatures for an enhanced antenna aperture and radiation gain. This THz antenna has demonstrated a maximum gain of 17 dBi and chip‐to‐chip wireless communications at 20 Gbps over a distance of 15 cm.

Topologically protected states are important in realizing robust optical behaviors that are quite insensitive to local defects or perturbations, which provide a promising solution for robust photonic integrations. Here, we propose to implement fast topological beam splitters and routers via the adiabatic passage of edge and interface states in the cross-linking configuration of Su-Schrieffer-Heeger (SSH) chains with interface defects. The channel state does not immerse into the band continuum during the adiabatic cycle, making the adiabatic restriction less stringent and the transport process more efficient. Based on the accelerated topological pumping, the beam splitters and routers exhibit improved robustness against losses of the system yet degraded resilience to fluctuation of coupling strengths and on-site energies compared with the conventional topological splitting and routing schemes. In addition, we confirm that the model demonstrates good scalability when the system size is varied. The simulation results of topological beam splitting in coupled waveguide arrays are in good consistency with theoretical analysis. This topological design provides a robust way to control photons, which may suggest further application of topological devices with unique properties and functionalities for integrated photonics.

Let there be light–to change the world we want to be! Over the past several decades, and ever since the birth of the first laser, mankind has witnessed the development of the science of light, as light-based technologies have revolutionarily changed our lives. Needless to say, photonics has now penetrated into many aspects of science and technology, turning into an important and dynamically changing field of increasing interdisciplinary interest. In this inaugural issue of eLight, we highlight a few emerging trends in photonics that we think are likely to have major impact at least in the upcoming decade, spanning from integrated quantum photonics and quantum computing, through topological/non-Hermitian photonics and topological insulator lasers, to AI-empowered nanophotonics and photonic machine learning. This Perspective is by no means an attempt to summarize all the latest advances in photonics, yet we wish our subjective vision could fuel inspiration and foster excitement in scientific research especially for young researchers who love the science of light.

We present an all-dielectric photonic crystal structure that supports two-dimensionally confined helical topological edge states. The topological properties of the system are controlled by the crystal parameters. An interface between two regions of differing band topologies gives rise to topological edge states confined in a dielectric slab that propagate around sharp corners without backscattering. Three-dimensional finite-difference time-domain calculations show these edges to be confined in the out-of-plane direction by total internal reflection. Such nanoscale photonic crystal architectures could enable strong interactions between photonic edge states and quantum emitters.