Relationship between atomic structure and crystal type studied by artificial neural network

We have used chemical bond parameters and pattern recognition method to investigatethe regularities of the crystal type of alloy phase,and achieved good results.Theparameters used,however,are semi-empirical paramters,which are not very strict fromtheoretical viewpoint.In this letter,we use the numbers describing atomic structure(thenumbers of valence electrons Z1,Z2,the principal quantum numbers of valence electrons n1,

Impact of cooling condition on the crystal structure and surface quality of preferred c-axis-oriented AlN films for SAW devices

AlN films with preferred c-axis orientation are deposited on Si substrates using the radio frequency(RF) magnetron sputtering method.The post-processing is carried out under the cooling conditions including high vacuum,low vacuum under deposition gas ambient and low vacuum under dynamic N2 ambient.Structures and morphologies of the films are analyzed by X-ray diffraction(XRD) and atomic force microscopy(AFM).The hardness and Young's modulus are investigated by the nanoindenter.The experimental results indicate that the(100) and(110) peak intensities decrease in the XRD spectra and the root-mean-square of roughness(Rrms) of the film decreases gradually with the increase of the cooling rate.The maximum values of the hardness and Young modulus are obtained by cooling in low vacuum under deposition gas ambient.The reason for orientation variation of the films is explained from the perspective of the Al-N bond formation.

ON THE RELATIONSHIP BETWEEN THE EMBEDDED-ATOMMETHOD FUNCTION OF TITANIUM AND TEMPERATURE

Temperature-dependent Embedded-Atom-Method (EAM) functions of hcp titanium have been determined by fitting to the lattice constants and elastic constants, measured at various temperatures as well as by considering the universal Rose's relation. The EAM functions thus obtained satisfy the stable structure requirement and Cauchy discrepancy. The results show that the EAM functions do have temperature-dependence, and in the meanwhile it also provides EAM functions of titanium for the study of Ti-based metallic materials at various temperatures. (Author abstract) 16 Refs.

Wavelength-selective coupling of dual-core photonic crystal fiber and its application

We investigate the coupling characteristics of photonic crystal fiber composed of two cores with different index profiles. The index curves of the fundamental modes of the two cores can cross at a desired wavelength by choosing appropriate index profiles of the cores. As a result, wavelength-selective coupling is achieved. The designed coupler can be applied as an oDtical fiber bandstop filter with 26-nm bandwidth.

Photonic bands,gap maps,and intrinsic losses in three-component 2D photonic crystal slabs

We obtain the photonic bands and intrinsic losses for the triangular lattice three-component two- dimensional （2D） photonic crystal （PhC） slabs by expanding the electromagnetic field on the basis of waveguide modes of an effective homogeneous waveguide. The introduction of the third component into the 2D PhC slabs influences the photonic band structure and the intrinsic losses of the system. We examine the dependences of the band gap width and gap edge position on the interlayer dielectric constant and interlayer thickness. It is found that the gap edges shift to lower frequencies and the intrinsic losses of each band decrease with the increasing interlayer thickness or dielectric constant. During the design of the real PhC system, the effect of unintentional native oxide surface layer on the optical properties of 2D PhC slabs has to be taken into consideration. At the same time, intentional oxidization of macroporous PhC structure can be utilized to optimize the design.

Performance testing of log pile photonic crystal fast-fabricated by direct femtosecond laser writing

Great efforts has been made on fabricating photonic crystals （PCs） with photonic band gaps （PBGs） during the past decade. Three-dimensional （3D） log pile PC was fabricated fast by direct femtosecond laser writing in ORMOCER. Qualitative analysis of the errors of PC was investigated using the Image Pro Plus. Surface qualities such as bending, distortion, and surface roughness were shown, and the band gap in the infrared wavelength region was observed. Meanwhile, the theory was experimentally verified that the center of PBG diminishes as the crystal lattice period reduces. Therefore, it is possible to fabricate PCs whose band gap range is from the near-infrared to visible wave band.

A compact T-branch beam splitter based on anomalous reflection in two-dimensional photonic crystals

We project a compact T-branch beam splitter with a micron scale using a two-dimensional （2D） photonic crystal （PC）. For TE polarization, one light beam can be split into two sub-beams along opposite directions. The propagating directions of the two splitting beams remain unchanged when the incident angle varies in a certain range. Coupled-mode theory is used to analyze the truncating interface structure in order to investigate the energy loss of the splitter. Simulation results and theoretical analysis show that choosing an appropriate location of the truncating interface （PC-air interface） is very important for obtaining high efficiency due to the effect of defect modes. The most advantage of this kind of beam splitter is being fabricated and integrated easily.

Design of broadband nearly-zero flattened dispersion highly nonlinear photonic crystal fiber

We propose a new structure of broadband nearly-zero flattened dispersion highly nonlinear photonic crystal fiber （PCF）. Through optimizing the diameters of the first two inner rings of air-holes and the GeO2 doping concentration of the core, the nonlinear coefficient is up to 47 W^-1.km^-1 at the wavelength of 1.55 um and nearly-zero flattened dispersion of ±0.5 ps/（nm·km） is achieved in the telecommunication window （1460 - 1625 nm）. Due to the use of GeO2-doped core, this innovative structure can offer not only a large nonlinear coefficient and broadband nearly-zero flattened dispersion but also low leakage losses.

Bragg reflection in a quantum periodic structure

We investigate the reflected field for few-cycle ultra-short laser pulses propagating through resonant media embedded within wavelength-scale structures. Full-wave Maxwell–Bloch equations are solved numerically by using the finite-difference time-domain method. The results show that the spectral feature of the reflected spectrum is determined by the Bragg reflection condition, and that the periodic structure of a dense atomic system can be regarded as a one-dimensional photonic crystal and even as a highly reflective multilayer film. Our study explains the suppression of the frequency shifts in the reflected spectrum based on the Bragg reflection theory and provides a method to control the frequency and frequency intervals of the spectral spikes in the reflected spectrum.

Photonic crystal waveguides and their applications Invited Paper

Two-dimensional （2D） slab photonic crystal waveguides （PCWGs） on silicon-on-insulator （SOI） wafer were designed and fabricated. Full photonic band gap, band gap guided mode, and index guided mode were observed by measuring the transmission spectra. Mini-stop-bands in the PCWG were simulated with different structure parameters. Coupling characteristics of PCWG were investigated theoretically considering the imperfections during the fabrication process. It was found that suppressing power reservation effect can realize both short coupling length and high coupling efficiency.

Three-dimensional photonic crystals containing designed defects achieved with two-photon photopolymerization

Two-photon photopolymerization （TPP） with femtosecond laser is a promising method to fabricate threedimensional （3D） photonic crystals （PCs）. Based on the TPP principle, the micro-fabrication system has been built. The 3D woodpile PCs with rod space of 2000 nm are fabricated easily and different defects are introduced in order to form the cross-waveguide and the micro-laser structure PCs. Simulation results of the optical field intensity distributions using finite-difference time domain （FDTD） method are given, which support the designs and implementation of the PC of two types in theory.

Highly birefringent extruded elliptical-hole photonic crystal fibers with single defect and double defects

Highly birefringent elliptical-hole photonic crystal fibers （PCFs） with single defect and double defects are proposed, which are supposed to be achieved by extruding normal circular-hole PCFs based on a triangular-lattice photonic crystal structure. Comparative research on the birefringence and the confinement loss of the proposed PCFs with single defect and double defects is presented. Simulation results show that the proposed PCFs with single defect and double defects can be with high birefringence （even up to the order of 10^-2）. The confinement loss increases when the ellipticity of the air hole of the PCFs increases, which nevertheless can be overcome bv increasing the ring number or the air holes in the fiber cladding.

Investigation of biosensor built with photonic crystal microcavity

The ultra-compact biosensor based on the two-dimensional （2D） photonic crystal （PhC） microcavity is investigated. The performances of the sensor are analyzed theoretically using the Fabry-Perot （F-P） cavity model and simulated using the finite-difference time-domain （FDTD） method. The simulation results go along with the theoretical analysis.

Compact and broadband waveguide taper based on partial bandgap photonic crystals

Partial bandgap characteristics of parallelogram lattice photonic crystals are proposed to suppress the radiation modes in a compact dielectric waveguide taper so as to obtain high transmittance in a large wavelength range. Band structure of the photonic crystals shows that there exists a partial bandgap, The photonie crystals with partial bandgap are then used as the cladding of a waveguide taper to reduce the radiation loss efficiently. In comparison with the conventional dielectric taper and the complete bandgap photonic crystal taper, the partial bandgap photonic crystal taper has a high transmittance of above 85% with a wide band of 170 nm.

Fabrication of fiber-embedded multi-core photonic crystal fibers

A novel fabrication method of multi-core photonic crystal fibers is proposed on the basis of a fiber-embedded technique. A taper tower is used to modify the structures of the fiber preform, and four steps of fiber fabrication and different structures of fiber samples are given. The mode structures and beating characteristics of a photonic crystal fiber sample with two successive cores are investigated in detail with the help of a supercontinuum light source, a charge-coupled device （CCD） camera, and an optical spectrum analyzer. The test results show a clear beating phenomenon between two orthotropic polarization modes with a 2.8-nm peak interval in wavelength.

Study of the substructure in nanometer copper thin films treated by laser shock processing

We study nanometer copper thin films prepared by magnetron sputtering and treated with laser shock processing （LSP）. We observe the formation of firstborn twin crystals and some complete twin crystals in the copper thin films. After LSP, scanning electron microscope （SEM） images show obvious plastic deformation of the copper grain on the film surface, dramatically increased grain size, and the appearance of a large number of twin crystals. Moreover, the width of the crystals is a few dozen nanometers, and the cross angle is more than or close to 90°. Many vacancy defects appear during the sliding of atomic plane, which leads to a faulty structure; however, no obvious dislocation is observed. These substructures play a significant role in improving the mechanical performance of nanometer copper thin films.

Focuses of material science development in recent years

Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates elements of applied physics and chemistry. With significant media attention focused on nanoscience and nanotechnology in recent years, materials science has been propelled to the forefront at many universities. Materials science encompasses various classes of materials, including electronic materials, functional ceramics, magnesium, material and processes for flat-panel displays, eco/environmental materials, sustainable energy materials, transportation materials, electronic packaging materials, etc.