The idea of photonic crystals and photonic band gap was first introduced by both Yablonovitch [1] and John in 1987 [2]. Photonic crystals are man-made periodic optical media in which the dispersion of light is strongly modified due to the scattering of periodically arranged dielectric or metal inclusions in the unit cell. Photonic band gaps, a frequency range in which light cannot propagate, can form as a consequence of Bragg scattering or the resonance of the inclusions in the unit cell. The existence of band gaps means that photonic crystals can serve as low-loss distributed feedback mirrors and as such, they can confine light and can be used to realize high fidelity resonant cavities that can facilitate the observation of quantum electronics phenomena. The application of such ideas to realize strong coupling between photon and exciton is achieved using planar dielectric Si periodic structures [3]. When combined with a gain material, photonic crystals are obviously good platforms to realize lasing and indeed photonic crystal based lasers have attracted great interest in past three decades. The technical challenges and progress in distributed feedback organic lasers based on photonic crystals are discussed and reviewed by Fu and Zhai [4]. For practical applications, nonlinear photonic crystals with different superlattices has been successfully used in quasi-phase matching and nonlinear diffraction harmonic generation. This is reviewed by Li and Ma [5].

Photonic crystals offer a platform for manipulating light at the mesoscopic scale owing to the unique photonic bandgap properties originating from spatially periodic dielectric distributions. Not only various integrated photonic devices have been realized based on photonic crystals, such as photonic crystal laser and logic devices, but also many physical effects including negative refractive and optical cloak have been realized in photonic crystals. Based on the photonic band structure, topological photonics has been an emerging research hotspot nowadays. Topological photonics provide two new modulation degrees of freedom, i.e., topological state degrees of freedom and energy valley degrees of freedom. It can be expected that topological photonics will not only boost the fundamental study of physical effects and phenomena, but also improve the research of high-performance photonic devices. Following the research trend in the field of photonics, the journal of Frontiers of Optoelectronics produces a special issue on Photonic Crystal and Topological Photonics in order to promote the research in the area of topological photonics and development of photonic devices based on topological features. In this special issue, Prof. Chan from The Hong Kong University of Science and Technology gives an in-depth comment on the development and research direction of photonic crystal and topological photonics. There are also three research articles and three review articles in this special issue. This special issue will improve the fundamental and application research of the field of photonic crystal and topological photonics.

Optical cavity polaritons, originated from strong coupling between the excitons in materials and photons in the confined cavities field, have recently emerged as their applications in the high-speed lowpower polaritons devices, low-threshold lasing and so on. However, the traditional exciton polaritons based on metal plasmonic structures or Fabry-Perot cavities suffer from the disadvantages of large intrinsic losses or hard to integrate and nanofabricate. This greatly limits the applications of exciton poalritons. Thus, here we implement a compact low-loss dielectric photonic – organic nanostructure by placing a 2-nm-thick PVA doped with TDBC film on top of a planar Si asymmetric nanogratings to reveal the exciton polaritons modes. We find a distinct anti-crossing dispersion behavior appears with a 117.16 meV Rabi splitting when varying the period of Si nanogratings. Polaritons dispersion and mode anti-crossing behaviors are also observed when considering the independence of the height of Si, width of Si nanowire B, and distance between the two Si nanowires in one period. This work offers an opportunity to realize low-loss novel polaritons applications.

Novel optical properties in graded photonic super-crystals can be further explored if new types of graded photonic super-crystals are fabricated. In this paper, we report holographic fabrication of graded photonic super-crystal with eight graded lattice clusters surrounding the central non-gradient lattices through pixel-by-pixel phase engineering in a spatial light modulator. The prospect of applications of octagon graded photonic super-crystal in topological photonics is discussed through photonic band gap engineering and coupled ring resonators.

Chern number is one of the most important criteria by which the existence of a topological photonic state among various photonic crystals can be judged; however, few reports have presented a universal numerical calculation method to directly calculate the Chern numbers of different topological photonic crystals and have denoted the influence of different structural parameters. Herein, we demonstrate a direct and universal method based on the finite element method to calculate the Chern number of the typical topological photonic crystals by dividing the Brillouin zone into small zones, establishing new properties to obtain the discrete Chern number, and simultaneously drawing the Berry curvature of the first Brillouin zone. We also explore the manner in which the topological properties are influenced by the different structure types, air duty ratios, and rotating operations of the unit cells; meanwhile, we obtain large Chern numbers from – 2 to 4. Furthermore, we can tune the topological phase change via different rotation operations of triangular dielectric pillars. This study provides a highly efficient and simple method for calculating the Chern numbers and plays a major role in the prediction of novel topological photonic states.

Considerable research efforts have been devoted to the investigation of distributed feedback (DFB) organic lasing in photonic crystals in recent decades. It is still a big challenge to realize DFB lasing in complex photonic crystals. This review discusses the recent progress on the DFB organic laser based on one-, two-, and three-dimensional photonic crystals. The photophysics of gain materials and the fabrication of laser cavities are also introduced. At last, future development trends of the lasers are prospected.

Since the lasers at fixed wavelengths are unable to meet the requirements of the development of modern science and technology, nonlinear optics is significant for overcoming the obstacle. Investigation on frequency conversion in ferroelectric nonlinear photonic crystals with different superlattices has been being one of the popular research directions in this field. In this paper, some mature fabrication methods of nonlinear photonic crystals are concluded, for example, the electric poling method at room temperature and the femtosecond direct laser writing technique. Then the development of nonlinear photonic crystals with one-dimensional, two-dimensional and three-dimensional superlattices which are used in quasi-phase matching and nonlinear diffraction harmonic generation is introduced. In the meantime, several creative applications of nonlinear photonic crystals are summarized, showing the great value of them in an extensive practical area, such as communication, detection, imaging, and so on.

The field of topological photonic crystals has attracted growing interest since the inception of optical analog of quantum Hall effect proposed in 2008. Photonic band structures embraced topological phases of matter, have spawned a novel platform for studying topological phase transitions and designing topological optical devices. Here, we present a brief review of topological photonic crystals based on different material platforms, including all-dielectric systems, metallic materials, optical resonators, coupled waveguide systems, and other platforms. Furthermore, this review summarizes recent progress on topological photonic crystals, such as higherorder topological photonic crystals, non-Hermitian photonic crystals, and nonlinear photonic crystals. These studies indicate that topological photonic crystals as versatile platforms have enormous potential applications in maneuvering the flow of light.