The topological phase of matter, originally proposed and first demonstrated in fermionic electronic systems, has drawn considerable research attention in the past decades due to its robust transport of edge states and its potential with respect to future quantum information, communication, and computation. Recently, searching for such a unique material phase in bosonic systems has become a hot research topic worldwide. So far, many bosonic topological models and methods for realizing them have been discovered in photonic systems, acoustic systems, mechanical systems, etc. These discoveries have certainly yielded vast opportunities in designing material phases and related properties in the topological domain. In this review, we first focus on some of the representative photonic topological models and employ the underlying Dirac model to analyze the edge states and geometric phase. On the basis of these models, three common types of two-dimensional topological photonic systems are discussed: 1) photonic quantum Hall effect with broken time-reversal symmetry; 2) photonic topological insulator and the associated pseudo-time-reversal symmetry-protected mechanism; 3) time/space periodically modulated photonic Floquet topological insulator. Finally, we provide a summary and extension of this emerging field, including a brief introduction to the Weyl point in three-dimensional systems.

A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron’s spin-1/2 (fermionic) time-reversal symmetry Tf2=−1Tf2=−1. However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon’s spin-1 (bosonic) time-reversal symmetry Tb2=1Tb2=1. In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp (Tp2=−1Tp2=−1), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb. This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.

Novel sandwich-like TiO2@C composite hollow spheres with {001} facets exposed are obtained through hydrothermal carbon-coating treatment for the first time. Based on investigations by transmission electron microscopy (TEM), scanning electron microscopy (FESEM), XRD, laser Raman spectra, and N2 adsorption–desorption isotherms, TiO2 shells are proved to be wrapped in porous carbon layers, which provide conductive support for TiO2, resulting the improvement in electronic conductivity and diffusion of Li+. Moreover, these TiO2 shells expose the reactive {001} facets, which facilitate Li+ insertion/extraction. Through combining the above advantages, TiO2@C composite hollow spheres show more superior rate capability and higher stability than TiO2 nanoparticles (Degussa P25), TiO2 nanosheets with {001} facets exposed, and TiO2 hollow spheres before carbon-coating. This method may be extended to synthesize other sandwich-like composite hollow spheres for energy storage.Graphical abstractHighlights► Sandwich-like TiO2@C hollow spheres with {001} facets exposed are obtained. ► TiO2 shells are wrapped in electron-conductive carbon layers. ► TiO2@C hollow spheres show superior rate capability and stability.

The nonstoichiometric LiFePO4/C core–shell composites (as microscale LiFePO4 cores coated with 3–5 nm thin carbon shells) are synthesized by a novel solid-state reaction method. All samples show outstanding coulombic efficiency (about 100%) and good battery cyclability, along with high tap density (>1.70 g cm−3). On this basis, the electrochemical properties of two different core–shell composites as Li1.02Fe0.99PO4/C and Li0.98Fe1.01PO4/C are compared. The results show that the rate performance of Li1.02Fe0.99PO4/C is obviously better than the rate performance of Li0.98Fe1.01PO4/C.Highlights► A novel method is reported for the first time to fabricate the nonstoichiometric LiFePO4/C core–shell composites which need less carbon to form the thin carbon shell. This method has both advantages of carbon deposition and liquid N2 quenching measurements. ► The nonstoichiometric LiFePO4/C core–shell composites as microscale LiFePO4 core coated with 3–5 nm thin carbon shell show outstanding coulombic efficiency, good battery cyclability whatever high or low current rates along with high tap density. ► On this basis, Li1.02Fe0.99PO4/C (Li-rich) and Li0.98Fe1.01PO4/C (Fe-rich) were compared with each other as two typical shapes of nonstoichiometric LiFePO4. The results show Li1.02Fe0.99PO4/C core–shell composite performs good rate performance which can meet the high power applications and its electrochemical properties are obviously better than that of Li0.98Fe1.01PO4/C.

A novel 10-H hexagonal Bi(FeCr)O3 (BFCO) perovskite nanosheet is synthesized by pulsed-laser ablation. Transmission electron microscopy (TEM) studies prove that the crystal structure of this phase is hexagonal-type with a P63/mmc space group. The atomic stacking sequence, revealed by high-resolution TEM, can be described as hhhcchhhcc in h–c notation. The magnetic properties of 10-H hexagonal BFCO perovskites are studied by both magnetic hysteresis loop measurements and first-principles calculations. The net magnetization of 10-H hexagonal BFCO comes from the anti-parallel spin moment of the Fe3+ and Cr3+ cations. The stability of the novel 10-H BFCO is also discussed by calculating its Goldschmidt factor and formation enthalpy. This work demonstrates the structural diversity in the Bi–Fe–Cr–O system, which may open the feasibility to explore the multi-functionality of this system.

A simple method to fabricate silica micro/nano-needle arrays (SNAs) is presented based on tube-etching mechanism. Using silica fibers as templates, highly aligned and free-standing needle arrays are created over large area by simple processes of polymer infiltration, cutting, chemical etching and polymer removal. Their sizes and orientations can be arbitrarily and precisely tuned by simply selecting fiber sizes and the cutting directions, respectively. This technique enables the needle arrays with special morphology to be fabricated in a greatly facile way, thereby offers them the potentials in various applications, such as optic, energy harvesting, sensors, etc. As a demonstration, the super hydrophobic property of PDMS treated SNAs is examined.Silica needle arrays are fabricated by tube arrays fabrication and etching method. They show super hydrophobic property after being treated with PDMS.

The influence of boundary conditions on the one-way edge modes in two-dimensional magneto-optical photonic crystals is studied theoretically by the supercell method. The numerical results reveal that tailoring the boundary could bring some new properties, but would not change the intrinsic one-way character for edge modes in the band gap generally. Changing the boundary conditions, more than one edge modes and waveguide modes can appear, which could couple and split into new ones. Two independent channels for one-way edge modes can be realized. The frequency of the edge Dirac point can be tunable.

Phononic crystals have been proposed about two decades ago and some important characteristics such as acoustic band structure and negative refraction have stimulated fundamental and practical studies in acoustic materials and devices since then. To carefully engineer a phononic crystal in an acoustic “atom” scale, acoustic metamaterials with their inherent deep subwavelength nature have triggered more exciting investigations on negative bulk modulus and/or negative mass density. Acoustic surface evanescent waves have also been recognized to play key roles to reach acoustic subwavelength imaging and enhanced transmission.

The influence of boundary conditions on the one-way edge modes in two-dimensional magneto-optical photonic crystals is studied theoretically by the supercell method. The numerical results reveal that tailoring the boundary could bring some new properties, but would not change the intrinsic one-way character for edge modes in the band gap generally. Changing the boundary conditions, more than one edge modes and waveguide modes can appear, which could couple and split into new ones. Two independent channels for one-way edge modes can be realized. The frequency of the edge Dirac point can be tunable.