The studies of topological phases of matter have been extended from condensed matter physics to photonic systems, resulting in fascinating designs of robust photonic devices. Recently, higher-order topological insulators (HOTIs) have been investigated as a novel topological phase of matter beyond the conventional bulk-boundary correspondence. Previous studies of HOTIs have been mainly focused on the topological multipole systems with negative coupling between lattice sites. Here we experimentally demonstrate that second-order topological insulating phases without negative coupling can be realized in two-dimensional dielectric photonic crystals (PCs). We visualize both one-dimensional topological edge states and zero-dimensional topological corner states by using nearfield scanning technique. To characterize the topological properties of PCs, we define a topological invariant based on the bulk polarizations. Our findings open new research frontiers for searching HOTIs in dielectric PCs and provide a new mechanism for light-manipulating in a hierarchical way.Introduction.-One of the most enchanting developments of condensed matter physics over the past few decades has been the discovery of topological phases of matter primarily found in electronic systems [1,2] and recently extended to bosonic systems such as photonics [3][4][5][6][7][8][9][10][11][12][13][14] and phononics [16][17][18][19][20][21]. A key feature of the topological insulators is the backscattering-immune edge states which are robust against perturbations and provide potential designs of various topological devices [3-6, 18, 20]. Typically, n-dimensional (nD) topological insulators (TIs) have (n − 1)D edge states which is defined as the bulk-boundary correspondence (BBC) [15]. However, a new kind of TIs defined as the higher-order topological insulators (HOTIs), have been recently proposed in tight-binding models in electronic systems which go beyond the traditional BBC description [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Concretely, the mth-order TIs have nD gapped bulk states and (n − 1)D, (n − 2)D, ..., (n − m − 1)D gapped edge states while having (n − m)D gapless edge states. The arising of these lower-dimensional topological edge states can either stem from the quantization of quadrupole moments such as the topological quadrupole insulators [22] which have been realized in mechanics [23], microwave systems [24] and topolectrical circuits [25], or stem from the quantization of the dipole moments [22] such as the HOTIs in 2D breathing kagome lattice [29] which have been realized in sonic crystals [33-35] and a waveguide array [32].

To avoid the complex core surface functionalization or pretreatment that is necessary in order to coat latex and silica colloids with a uniform, complete metal shell, the solvent‐assisted route has been explored to prepare a complete metal (Ag or Au) shell with controlled thickness on polystyrene (PS) colloids and the electroless plating approach, based on electrostatic attraction, has been explored to prepare a complete silver shell with controlled thickness on silica colloids. Without any additional surface treatment, the as‐prepared complex core–shell colloids can be crystallized directly into long‐range‐ordered structures with photonic bandgaps, as reported here for the first time. These ordered structures may find potential applications as substrates or physical systems for the enhancement of Raman scattering studies, besides applications as photonic crystals. The optical plasmon resonance of the composite core–shell colloids changes with metal shell thickness, the wavelength varying over hundreds of nanometers. Our coating routes are facile and versatile, and can be extended to coat PS and silica colloids with any other metal whose ion or complex can be reduced in solution.

2D materials exhibit superior properties in electronic and optoelectronic fields. The wide demand for high‐performance optoelectronic devices promotes the exploration of diversified 2D materials. Recently, 2D covalent organic frameworks (COFs) have emerged as next‐generation layered materials with predesigned π‐electronic skeletons and highly ordered topological structures, which are promising for tailoring their optoelectronic properties. However, COFs are usually produced as solid powders due to anisotropic growth, making them unreliable to integrate into devices. Here, by selecting tetraphenylethylene monomers with photoelectric activity, elaborately designed photosensitive 2D‐COFs with highly ordered donor‐acceptor topologies are in situ synthesized on graphene, ultimately forming COF‐graphene heterostructures. Ultrasensitive photodetectors are successfully fabricated with the COFETBC–TAPT‐graphene heterostructure and exhibited an excellent overall performance with a photoresponsivity of ≈3.2 × 107 A W−1 at 473 nm and a time response of ≈1.14 ms. Moreover, due to the high surface area and the polarity selectivity of COFs, the photosensing properties of the photodetectors can be reversibly regulated by specific target molecules. The research provides new strategies for building advanced functional devices with programmable material structures and diversified regulation methods, paving the way for a generation of high‐performance applications in optoelectronics and many other fields.

25 With a facile physical vapor transport method, we can prepare vertical nanowire arrays from DAAQ with controllable dimensions. Here we report patterned growth of these vertical nanowire arrays on substrates with predefined geometrical and chemical features. The nanowires can also be directly grown on sharp metal or AFM tips, and colloidal particles. Fluorescence microscopy and localized excitation experiments showed that the vertical DAAQ nanowires can act as miniaturized optical waveguides, which have much lower optical loss than the horizontal ones. In addition, the nanowire arrays were directly grown onto an optical fiber probe thus allowing real-time fluorescence measurement of these nanowires on exposure to chemical vapors. RESULTS AND DISCUSSIONSelf-seeded growth. The vapor transport growth of the DAAQ nanowires is considered to be controlled by vapor-solid (VS) process 6 as illustrated in Figure 1A. Figure 1B shows an SEM image of the early stage product of a DAAQ nanowire array grown

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Tailoring optical response using periodic nanostructures is one of the key issues in the current research on functional composite materials. [1][2][3][4][5] The anomalous light transmission through metallic films that have a regular array of submicrometer holes [3][4][5][6] has stimulated much interest. This interest stems from both the underlying physics and also the perceived potential for applications in nanophotonics, [7] quantum-information processing, [8] nanolithography, [9] and surface-enhanced Raman scattering.[10]Extraordinary transmission of light through an optically opaque metal film perforated with a 2D array of subwavelength holes was first reported by Ebbesen et al. [5] This unusual phenomenon can be understood as a result of diffractive coupling to evanescent surface plasmon polaritons (SPPs) that leads to a strong concentration of light at the metal surface, which then weakly tunnels through the holes in the film, reradiating by the inverse process on the exit side. [4,[11][12][13] In order to explore the SPP properties of microstructured metal films, extensive efforts have been made to study their spectral response and dependence on geometrical parameters, such as the type of lattice symmetry, metal film thickness, and adjacent dielectric media. [14] Recent studies show that the hole shape has a significant effect on the optical transmission. [15][16][17][18][19] Nearly all the metallic films studied have been on a flat substrate and the hole arrays were made using focused ion-beam milling, [5,15,17,19] and electron-beam lithography [8] or interferometric lithography combined with reactive ion etching. [16,18] Here we use nanosphere lithography [20] as the sample production technique. This approach has several advantages over the conventional lithographic and machining techniques, including the relative ease of casting large, high-quality, ordered nanomaterials and the low cost of implementation. Ordered arrays of gold half shells and nanocaps have been constructed by controlled gold vapor deposition with thicknesses less than 20 nm by using a 2D colloidal crystal (CC) as a substrate. [21,22] Baumberg's group has fabricated metallic nanocavity arrays by electrodeposition within the pores of CC templates and observed the excitation of the SPPs in metallic cavities that led to rich features in reflectivity spectra.[23] Very recently, Landström et al. have shown that the transmission spectra through a metal film formed on a 2D CC substrate are quite similar to those observed through subwavelength hole arrays in metal films. [24] In this communication, we report a study on the infrared transmission properties of gold films patterned on 2D CCs. The fabricated metallodielectric structures have a strong surface corrugation as well as a 2D periodic pore array. We show that the SPPs on these curved surfaces display unusual dispersion properties, compared to those of metal films on flat substrates studied before. The dielectric property of the template spheres is also found to have a substantial effect o...

The effects of dilution of tetraethyl orthosilicate (TEOS) with ethanol on the shape and monodispersity of silica particles were investigated. The results indicated that the dilution of TEOS with ethanol can depress the formation of new nuclei and the aggregation or adhesion of particles and make the distillation of TEOS unnecessary to achieve monodispersed silica spheres. A seeded growth technique using continuous drop addition of TEOS diluted with 4× volume of ethanol was developed to improve monodispersity and spheric shape and increase the size of silica particles. The monodisperse silica particles (150 nm ± 5%−1.2 μm ± 1%) with fine spheric shape were successfully synthesized by the seeded growth technique. Using the homemade 280 nm ± 2.8% silica spheres, we prepared opals of high quality which showed periodically ordered packing and a photonic band-gap effect.

Much attention has been focused recently on the preparation of macroporous materials by templating with colloidal crystals. This interest stems from the important properties of such microstructures, which have a wide range of applications including sensors, catalysts, and photonic crystals. Various macroporous and mesoporous materials have been made via this process, including ceramics, [1±9] carbon, [10,11] chalcogenides, [12±14] NaCl, [3] polymers, [15±19] and silica±gold. [20] The extension of such a material patterning method to metals is particularly attractive, as metals with ordered microstructures exhibit unique optical, electrical, magnetic, and catalytic properties. For example, surface enhancement of Raman scattering in porous gold films has been demonstrated by Tessire et al. [21] In a recent study, Baumberg and coworkers reported the observation of confined plasmons in gold nanocavities. [22] Although the adaptation of the templating method to the preparation of porous metal films is limited, several strategies have been reported, including precipitation/chemical conversion, [23,24] direct penetration of metal nanocrystals, [25] electroless deposition, [26,27] and electrochemical reduction methods. [28] When monodispersed silica spheres are used as the sacrificial elements, annealing of the colloidal crystal is usually performed at high temperatures. This results in the formation of small necks between neighboring spheres, which makes the template stable during the material patterning process.[26±28] A complete filtration of metal into the free spaces of the template has been successfully demonstrated using these approaches, which result in macroporous metal films that are an exact replica of the sacrificial templates. In this communication we report for the first time the preparation of ordered arrays of hollow metal spheres by colloidal crystal templating. However, instead of using annealed silica colloidal crystals as templates, [26±28] we use polymer colloidal crystals. Moreover, the template is confined between two substrates in order to preserve its ordering in the material-patterning process, as described by Xia and coworkers. [29] We combine a seeded growth technique for metal coatings on isolated colloids in solution and the confined template-directed synthesis method for material patterning. The samples we obtain are were two-dimensional (2D) and three-dimensional (3D) ordered hollow silver spheres prepared by templating against the monolayer or the multilayer of hexagonal close packed polymer beads, respectively. The schematic procedure of our experiment is depicted in Figure 1. The starting material is an ordered colloidal crystal consisting of polystyrene (PS) beads, which was prepared using a modification of the micromolding method reported by Kim et al. [30] Specifically, a microchannel was first formed between two glass slides that were separated with two identical spacers. Upon dipping into a colloidal dispersion solution, the microchannel was spontaneously filled with the colloi...