Effect of the internal rib structure of the inclusions on the two-dimensional phononic crystal composed of periodic slotted tubes in air

Using a finite element method based on the Bloch theorem, a new phononic crystal composed of periodic slotted tubes with internal rib structure in air is investigated. Two parallel plates with slit are introduced into the inclusion as the internal rib structure and its effect on band gaps is studied. The band structure and acoustic modes of the PC are calculated. Results show that the starting frequency of the first band gap is rather lower than that of slotted tubes without rib structure. The internal rib structure plays an important role in both the lower and upper edges of the first band gap. Some rib structural parameters are also studied for their effects on the first band gap. Results show that the first gap can be modulated widely by these parameters and the Helmholtz resonator theory can be used to explain the relationship between the band gap and the parameters.     

Ultrawide low frequency band gap of phononic crystal in nacreous composite material

The band structure of a nacreous composite material is studied by two proposed models, where an ultrawide low frequency band gap is observed. The first model (tension-shear chain model) with two phases including brick and mortar is investigated to describe the wave propagation in the nacreous composite material, and the dispersion relation is calculated by transfer matrix method and Bloch theorem. The results show that the frequency ranges of the pass bands are quite narrow, because a special tension-shear chain motion in the nacreous composite material is formed by some very slow modes. Furthermore, the second model (two-dimensional finite element model) is presented to investigate its band gap by a multi-level substructure scheme. Our findings will be of great value to the design and synthesis of vibration isolation materials in a wide and low frequency range. Finally, the transmission characteristics are calculated to verify the results.     

The band gap and transmission characteristics investigation of local resonant quaternary phononic crystals with periodic coating

In this article, the investigation of the Lamb wave propagation in two-dimensional phononic crystals (PCs) composed of an array of periodic coating on a thin plate is presented. Compared with the traditional PCs usually consist of cylindrical scatters with uniform coatings in their exterior structure, the newly exterior coating structures with periodic alternant arrangement of two different materials are proposed. The band structures are calculated using finite element method. We discover that a complete band gap can be exhibited at low frequency. Furthermore, for a finite PCs plate, the computed transmission and resonance spectra shown an evident resonance nature which can be directly related to formation of the low-frequency gaps. The effects of different material parameters and arrangement mode of coating on the acoustic energy transmission and attenuation are also studied. Finally, the experimental transmission spectrum of the periodic coating PCs are also presented and compared with the numerical results. This study will provide useful support to the design of tuning band gaps and isolators in the low-frequency range.     

Band gap and band offsets of GaNAsBi lattice matched to GaAs substrate

In this paper, we calculate the band gap and the band discontinuities of a GaN _y AsBi _x structure lattice matched to GaAs substrate using the conduction and the valence band anticrossing models at the same time. The results obtained show a good agreement with experiment. The nitrogen and the bismuth concentrations leading to a wavelength emission of 1.55 μm have been determined (x = 3.5%, y = 2%). This structure shows a good electron confinement resulting in a high characteristic temperature.     

Effects of structural factors on filtering operation of photonic band gap air bridges with circular and square shape holes

Applying the finite element analysis on the photonic band-gap materials with a cavity at the center for two types of structures, circular and square shape holes with the same cross section, filtering operation of these structures is simulated and compared. Any variations in the structure parameters, such as cavity length, period and hole dimensions, change the transmission peak and frequency bandwidth of these structures. The effects of rotation of square holes and ellipticity of circular holes on filtering operation are studied. Other new structures such as tapered, shortened and non-uniform rotated structures are proposed and filtering characteristics of them are discussed. It is found that, by rotating the square holes around their axis, it is possible to have a blue-shift in the transmission peak wavelength without notable variations in the filter peak and bandwidth. It is concluded that the increase of elliptical holes diameters length causes the decrease of transmission peak and increase of bandwidth with blueshift of the peak wavelength. This shift is larger for one of the elliptical diameter values considered that is along the waveguide length.     

Triply coupled vibrational band gap in a periodic and nonsymmetrical axially loaded thin-walled Bernoulli–Euler beam including the warping effect

The propagation of triply coupled vibrations in a periodic, nonsymmetrical and axially loaded thin-walled Bernoulli–Euler beam composed of two kinds of materials is investigated with the transfer matrix method. The cross-section of the beam lacks symmetrical axes, and bending vibrations in the two perpendicular directions are coupled with torsional vibrations. Furthermore, the effect of warping stiffness is included. The band structures of the periodic beam, both including and excluding the warping effect, are obtained. The frequency response function of the finite periodic beam is simulated with the finite element method. These simulations show large vibration-based attenuation in the frequency range of the gap, as expected. By comparing the band structure of the beam with plane wave expansion method calculations that are available in the literature, one finds that including the warping effect leads to a more accurate simulation. The effects of warping stiffness and axial force on the band structure are also discussed.     

Flexural vibration band gaps in a thin plate containing a periodic array of hemmed discs

Using a periodic expansion by means of the Bloch theorem, the flexural vibration band gaps are studied in a thin plate with two-dimensional ternary locally resonant structures, i.e. a thin epoxy plate containing a periodic square array of lead discs hemmed around by rubber. The full band gaps of flexural vibration in the thin plate are obtained within which sound and vibration will be forbidden. The numerical results are used to show how the width of the first full band gap depends on the radius ratio of lead disc to hemmed disc, filling fraction, lattice constant (distance between the centers of the nearest lead discs) and thickness of the thin plate. It is observed that the gap width can be changed a lot by modulating these physical parameters.     

Interplay of local resonances and Bragg band gaps in acoustic waveguides with periodic detuned resonators

This letter is concerned with acoustic wave propagation and transmission in acoustic waveguides with periodically grafted detuned Helmholtz resonators. The interplay of local resonances and Bragg band gaps in such periodic systems is examined. It is shown that, when the resonant frequencies of the resonators are tuned close to a Bragg band gap, the behavior of the Bragg band gap can be affected dramatically. Particularly, by introducing appropriately tuned resonators, the bandwidth of a Bragg band gap can be reduced to zero, leading to a very narrow pass band with great wave attenuation performance near both band edges. The band formation mechanisms of such periodic waveguides are further examined, providing explicit formulae to locate the band edge frequencies of all the band gaps, as well as the conditions to achieve very narrow pass bands in such periodic waveguides.     

Engineering of band gap and cavity mode in phononic crystal strip waveguides

The optimal parameters for the largest band gap were investigated in three typical phononic crystal strip waveguides. Single cavity mode was created inside the band gap region by proper design of a defect. The band structures and the displacement distributions were discussed with the variation of the defect. Results show possibilities to guide extremely slow phonon cavity mode in strip waveguide with chosen displacement components, frequencies and symmetries.     

超窄带和梳状多通道光子晶体滤波器设计

 设计了一种因掺杂而具有超窄带滤波特性的光子晶体滤波器和一种复周期结构的梳状多通道光子晶体滤波器。在两种均匀介质交替分布的周期结构中间插入一层折射率不同的介质膜后,通过传输矩阵法的数值计算,发现采用前后不对称的周期结构并调节参数,可以在1 550 nm处找到一个透射率接近100%的极窄的透过峰,因而可得到一种超窄带光子晶体滤波器。另外,在以高低两种折射率的介质交替分布,并以两种单独的单元周期合并组成一个复周期,然后进行周期重复构成的光子晶体中,利用通道数等于复周期数减1的规律,并配合各层厚度的调节,设计出在1 550 nm附近梳状8通道和16通道的光子晶体滤波器,各通道透过峰的透射率均接近100%,由此设计出一种梳状多通道光子晶体滤波器。     

基于六角结构二维光子晶体绝对带隙的优化设计研究

根据平面波展开法对二维光子晶体的能带结构进行计算，采用栅格结构连接电介质圆柱体对六角结构的二维光子晶体进行了优化. 通过计算栅格宽度和圆柱体半径对绝对带隙的影响，找到了一组可以获得大带隙二维光子晶体结构的最佳参数.优化后的光子晶体的大带隙对光子晶体制造工艺中介质圆柱体半径的偏离具有很好的稳定性，因此该结构的二维光子晶体具有很高的实用性. 关键词： 光子晶体 绝对带隙 六角结构     

Local band gap modulation of AlGaAs grown on patterned GaAs substrates

The growth of AlGaAs layers on patterned GaAs substrates by molecular-beam epitaxy is strongly determined by the interfacet surface migration of Ga adatoms, leading to a pronounced variation of the local Al concentration. Taking advantage of this migration process, we have overgrown mesa structures, that consist of intersecting stripes or partially overlapping squares, to obtain a local maximum or minimum, respectively, of the AlGaAs band gap on top of the mesas.     

Eye-protection glasses against YAG laser injury based on the band gap reflection of one-dimensional photonic crystal

Eye-protection glasses against YAG laser injury based on band gap reflection of one-dimensional photonic crystal (PC) is designed and manufactured in this paper. The laser beam (wavelength 1.06 μm) is reflected by the one-dimensional PC (with the transmission 10⁻⁷) and absorbed by the phosphatic glass substrate (with the transmission 1% for 1.06 μm), so the transmission of the device for wavelengths of1.06 μm can reach 10⁻⁹. The glasses have enough capabilities to protect the eyes from injury of ns-YAG lasers whose energy density is 1 J/cm² for all incident angles, and also to avoid a second injury to others from the reflected laser beams. The transmission of the glasses is beyond 70% for the visible lights. The testing data of the eye-protection glasses agree well with the theoretical predictions.     

三角晶格有耗色散光子晶体的能带结构分析

为探讨有耗色散媒质光子晶体的特性,引入一种计算有耗色散光子晶体能带结构的方法,基于有限元法将能带结构的计算简化为求解关于Block波矢的二次特征值问题,可以有效地得到色散材料光子晶体的能带结构和特征模.分析了三角晶格介质光子晶体能带结构并与现有方法对比,结果表明两种方法在TM模和TE模下得到的能带结构完全相同,验证了该方法的有效性.分析了无耗及有耗色散光子晶体的能带结构,发现无耗光子晶体场强集中于色散媒质与空气的接触面,并呈现出明显的表面等离激元特性,具有对称性,而有耗光子晶体场强减小,表面等离激元变弱,对称性被破坏.相关结果可为有耗色散光子晶体以及表面等离激元的研究提供参考.     

Effect of annealing on the surface and band gap alignment of CdZnS thin films

Effects of the annealing temperature on structural, optical and surface properties of chemically deposited cadmium zinc sulfide (CdZnS) films were investigated. X-ray diffraction (XRD) results showed that the grown CdZnS thin films formed were polycrystalline with hexagonal structure. Atomic force microscopy (AFM) studies showed that the surface roughness of the CdZnS thin films was about 60-400 nm. Grain sizes of the CdZnS thin films varied between 70 and 300 nm as a function of annealing temperature. The root mean square surface roughness of the selected area, particular point, average roughness profile, topographical area of roughness were measured using the reported AFM software. The band gaps of CdZnS thin films were determined from absorbance measurements in the visible range as 300 nm and 1100 nm, respectively, using Tauc theory.     

Electronic states and the resonant optical non-linearity of exciton in a narrow band InSb quantum dot

Binding energy, interband emission energy and the non-linear optical properties of exciton in an InSb/InGa_xSb_1−x quantum dot are computed as functions of dot radius and the Ga content. Optical properties are obtained using the compact density matrix approach. The dependence of non-linear optical processes on the dot sizes is investigated for different Ga concentrations. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of the exciton are calculated for different concentrations of gallium content. It is found that gallium concentration has great influence on the optical properties of InSb/InGa_xSb_1−x dots.     

大带隙二维正方介质柱三角晶格光子晶体

关键词：     

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应用多光子非线性Compton散射模型和时域有限差分法,对Compton散射对时变非磁化等离子体光子晶体禁带的影响进行了研究,提出了将多光子非线性Compton散射电磁波和入射电磁波作为等离子体光子晶体产生光子禁带的新机制,给出了电磁场的Maxwell方程组和叠代方程的修正方程,并进行了数值模拟。结果表明,Compton散射使等离子体禁带宽随等离子体上升时间的增大比散射前有明显减小而最后趋于定值,均匀等离子体透射率峰值比线性等离子体增大得更多,利用Compton散射可实现对光子禁带的控制。     

溅射制备Ge,Nb共掺杂窄光学带隙和低电阻率的TiO2薄膜

采用射频磁控溅射技术制备了Ge,Nb共掺杂的锐钛矿结构TiO₂薄膜,详细探讨了薄膜的结构、电阻率及光学带隙等性质随Ge,Nb掺杂量、溅射功率和热处理温度等参数的变化,发现Ge,Nb共掺杂可以同时调节TiO₂薄膜的光学带隙和电阻率.体积分数约为6%Nb和20%Ge的共掺杂TiO₂薄膜电阻率由10⁴Ω/cm减小至10^-1Ω/cm,光学带隙由3.2 eV减小至1.9 eV.退火后掺杂TiO₂薄膜不仅显示更低的电阻率,还表现出更强的可见-红外光吸收.结果表明,改变Ge,Nb的掺杂量和退火条件能够制备出电阻率和带隙都可调的TiO₂薄膜.