Photonic band gap via quantum coherence in vortex lattices of Bose-Einstein condensates

We investigate the optical response of an atomic Bose-Einstein condensate with a vortex lattice. We find that it is possible for the vortex lattice to act as a photonic crystal and create photonic band gaps, by enhancing the refractive index of the condensate via a quantum coherent scheme. If high enough index contrast between the vortex core and the atomic sample is achieved, a photonic band gap arises depending on the healing length and the lattice spacing. A wide range of experimentally accessible parameters are examined and band gaps in the visible region of the electromagnetic spectrum are found. We also show how directional band gaps can be used to directly measure the rotation frequency of the condensate.     

Band gap formation and multiple scattering in photonic quasicrystals with a Penrose-type lattice

This Letter presents a study of the local density of states (LDOS) in photonic quasicrystals. We show that the LDOS of a Penrose-type quasicrystal exhibits small additional band gaps. Among the band gaps, some exhibit a behavior similar to that typical of photonic crystals, while others do not. The development of certain band gaps requires large-size quasicrystals. It is explained by the long-range interactions involved in their formation. Moreover, the frequencies where the band gaps occur are not necessarily explained using single scattering and should therefore involve multiple scattering.     

一种多频局域共振型声子晶体板的低频带隙与减振特性

设计了一种多频局域共振型声子晶体板结构, 该结构由一薄板上附加周期性排列的多个双悬臂梁式子结构而构成. 由于多个双悬臂梁式子结构的低频振动与薄板振动的相互耦合作用, 这种局域共振型板结构可产生多个低频弯曲波带隙(禁带); 带隙频率范围内的板弯曲波会被禁止传播, 利用带隙可以实现对薄板的多个目标频率处低频减振. 本文针对这种局域共振型板结构进行了简化, 并基于平面波展开法建立了其弯曲波带隙计算理论模型; 基于该模型, 结合具体算例进行了带隙特性理论分析. 设计、制备了一种存在两个低频弯曲波带隙的局域共振型板结构样件, 通过激光扫描测振仪测试证实该结构存在两个低频带隙, 在带隙频率范围的板弯曲振动被显著衰减.     

Approximations to wave propagation through a lattice of Dirichlet scatterers

A scheme of matched asymptotic expansions is used to obtain approximations to the dispersion relation when waves, governed by the Helmholtz equation, propagate through a two-dimensional lattice of scatterers on each of which a homogeneous Dirichlet boundary condition is imposed. The scatterers must be identical, but can be of any shape as long as each is small relative to the wavelength and the lattice periodicity. The results differ from those obtained using homogenisation in that there is no requirement that the wavelength be much longer than the lattice periodicity, and hence it is possible to describe band gaps.     

Effect of a Two-Dimensional Periodic Dielectric Background on Complete Photonic Band Gap in Complex Square Lattices

A two-dimensional photonic crystal model with a periodic square dielectric background is proposed. The photonic band modulation effects due to the two-dimensional periodic background are investigated in detail. It is found that periodic modulation of the dielectric background greatly alters photonic band structures, especially for the Epolarization modes. The number, width and position of the photonic band gaps sensitively depend on the dielectric constants of the two-dimensional periodic background. Complete band gaps are found, and the dependence of the widths of these gaps on the structural and material parameters of the two alternating rods/holes is studied.     

An Analysis of Structural-Acoustic Coupling Band Gaps in a Fluid-Filled Periodic Pipe

A periodic pipe system composed of steel pipes and rubber hoses with the same inner radius is designed based on the theory of phononic crystals. Using the transfer matrix method, the band structure of the periodic pipe is calculated considering the structural-acoustic coupling. The results show that longitudinal vibration band gaps and acoustic band gaps can coexist in the fluid-filled periodic pipe. The formation of the band gap mechanism is further analyzed. The band gaps are validated by the sound transmission loss and vibration-frequency response functions calculated using the finite element method. The effect of the damp on the band gap is analyzed by calculating the complex band structure. The periodic pipe system can be used not only in the field of vibration reduction but also for noise elimination.     

类蜂窝状结构完全带隙二维光子晶体

通过在二维三角晶格中引入两个完全一样的介质圆柱构成了类蜂窝状结构光子晶体,并对其光子能带进行了频域计算。借助数值方法分析了介质柱位置改变对光子能带的影响,计算结果表明,这种类蜂窝状结构二维光子晶体可以产生很宽的带隙,而且在一定填充率下,可以通过调整介质柱的位置使完全光子带隙达到最大化。     

Temperature effects on the band gaps of Lamb waves in a one-dimensional phononic-crystal plate (L)

This study investigates the temperature-tuned band gaps of Lamb waves in a one-dimensional phononic-crystal plate, which is formed by alternating strips of ferroelectric ceramic Ba(0.7)Sr(0.3)TiO(3) and epoxy. The sensitive and continuous temperature-tunability of Lamb wave band gaps is demonstrated using the analyses of the band structures and the transmission spectra. The width and position of Lamb wave band gaps shift prominently with variation of temperature in the range of 26 °C-50 °C. For example, the width of the second band gap increases from 0.066 to 0.111 MHz as the temperature is increased from 26 °C to 50 °C. The strong shift promises that the structure could be suitable for temperature-tuned multi-frequency Lamb wave filters.     

Absolute band gaps of a two-dimensional triangular-lattice dielectric photonic crystal with different shapes

Absolute band gaps of a two-dimensional triangular-lattice photonic crystal are calculated with the finite-difference time-domain method in this paper.Through calculating the photonic band structures of the triangular-lattice photonic crystal consisting of Ge rods immersed in air with different shapes,it is found that a large absolute band gap of 0.098 (2c/a) can be obtained for the structures with hollow triangular Ge rods immersed in air,corresponding to 19.8% of the middle frequency.The influence of the different factors on the width of the absolute band gaps is also discussed.     

Surface acoustic waves in 2D-phononic crystal of laminated pillars on a semi-infinite ZnO substrate

In this paper a phononic crystal of laminated pillars which are composed of two materials on a semi-infinite ZnO substrate is proposed. The unit cell can be regarded as a double-resonator model, which explicates the variation of band gaps. It is revealed that the band structure of the PC is highly influenced by the proportion of PZT-4 in laminated pillars. Besides, the high-frequency band gaps are the result of combined actions of the local resonance and the Bragg scattering, and the former dominates. Furthermore, the influences of the structural damping are investigated. It is observed that the bandgap-ratio remain unchanged and the band gaps translate to the right with isotropic structural loss factor increasing. Additionally, by introducing linear defects inside the laminated pillars, elastic energy at surface is strongly confined within defect pillars, and the coupling between defective pillars and SAW is governed by the physical parameter of resonators.     

Coherently induced double photonic band gaps in a driven N-type atomic system

In an N-type atomic system with double electromagnetically induced transparency, when the traveling-wave coupling field is replaced by a standing-wave, double fully developed photonic band gaps open up due to space-modulated refractive index within the two electromagnetically induced transparency widows. The dynamically induced band gaps can be tuned coherently, and have applications in manipulating the propagation dynamics of light pulses.     

Investigation of the pressure dependence of band gaps for silver halides within a first-principles method

The pressure dependence of the direct and indirect band gaps in rocksalt silver halides has been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for the exchange-correlation potential. It is found that indirect band gaps (L→X and L→Γ) exhibit different responses to application of pressure. Similar trends are found for the indirect band gaps of AgCl and AgBr while the trend in AgI (L→Γ) band gap is different. In all the compounds, the effect of pressure on the direct band gaps (Γ→Γ, X→X and L→L) show qualitatively similar results. The fundamental indirect band gap (L→Γ) pressure coefficients are −4.19 meV (GPa)⁻¹ and −3.81 meV (GPa)⁻¹ for AgCl and AgBr while for AgI (L→X) it is −61.50 meV (GPa)⁻¹. The band gap pressure coefficient as well as the volume deformation potential for the various band gaps of the compounds have also been investigated.     

Point defect states of a hollow cylinder in two-dimensional phononic crystal

Point defect states in two-dimensional phononic crystal of a hollow mercury cylinder in a water host are studied.An improved plane expansion method combined with the supercell technique is used to calculate the band gaps and the pressure distribution at the defect position.The sonic pressure of defect modes shows that the waves are localized at or near the defect.As the filing fraction increases,more defect modes appear in the band gaps.     

Empirical pseudopotential band structure parameters of 4H-SiC using a genetic algorithm fitting routine

In this paper, the viability of using a genetic algorithm to find band structure parameters for empirical pseudopotential method (EPM) calculations is demonstrated by applying a genetic algorithm to find the EPM parameters for 4H-SiC. The form of the pseudopotential for 4H-SiC and the 19 form factors found by the genetic algorithm to fit the band structure to experimentally measured indirect energy gap and direct optical gaps are given. In addition, the effective masses for the conduction band minimum are extracted from the calculated band structure. It is shown that the genetic algorithm provides an effective, automated way to find parameters that give reasonably good fits to both the band gaps and the effective masses simultaneously.     

On the production of excited hydroxyl radicals in the H+O3-atomic flame

The existence of surface states in band gaps of a nearly free electron band structure is investigated. While in gaps at the Brillouin zone boundary one finds such a state only for positive potential matrix element, there a surface state always exists for band gaps inside the Brillouin zone.     

旋转对二维正方晶格介质柱内空结构光子晶体禁带的影响

采用平面波展开的方法计算了三种旋转操作下二维正方晶格各向异性材料(Te)介质柱内空结构光子晶体TE,TM模式能带.讨论了三种旋转操作对TE,TM模式带隙及完全光子禁带的影响.发现TM模式高频带隙与结构的旋转对称性有着密切的关系.而TE模式的带隙不仅受到晶体旋转对称性的影响同时也受到介质在x-y平面分布情况的影响.     

旋转对二维正方晶格介质柱内空结构光子晶体禁带的影响

采用平面波展开的方法计算了三种旋转操作下二维正方晶格各向异性材料(Te)介质柱内空结构光子晶体TE,TM模式能带.讨论了三种旋转操作对TE,TM模式带隙及完全光子禁带的影响.发现TM模式高频带隙与结构的旋转对称性有着密切的关系.而TE模式的带隙不仅受到晶体旋转对称性的影响同时也受到介质在x-y平面分布情况的影响. 关键词： 二维光子晶体 内空结构 旋转操作 光子带隙     

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.     

The Brillouin zones and band gaps of a two-dimensional phononic crystal with parallelogram lattice structure

We present a detailed theoretical study on the acoustic band structure of two-dimensional(2D)phononic crystal.The 2D phononic crystal with parallelogram lattice structure is considered to be formed by rigid solid rods embedded in air.For the circular rods,some of the extrema of the acoustic bands appear in the usual high-symmetry points and,in contrast,we find that some of them are located in other specific lines.For the case of elliptic rods,our results indicate that it is necessary to study the whole first Brillouin zone to obtain rightly the band structure and corresponding band gaps.Furthermore,we evaluate the first and second band gaps using the plane wave expansion method and find that these gaps can be tuned by adjusting the side lengths ratio R,inclined angleθand filling fraction F of the parallelogram lattice with circular rods.The results show that the largest value of the first band gap appears atθ=90°and F=0.7854.In contrast,the largest value of the second band gap is atθ=60°and F=0.9068.Our results indicate that the improvement of matching degree between scatterers and lattice pattern,rather than the reduction of structural symmetry,is mainly responsible for the enhancement of the band gaps in the 2D phononic crystal.     

Extension of photonic band gaps in one-dimensional photonic crystals

A method is proposed for extending photonic band gaps by constructing periodic structures from two or more consecutively located photonic crystals with different lattice constants or filling factors. The photonic band gaps with a maximum extension are predicted by superposing the photonic band gap maps on one another. It is demonstrated that both the lowest and higher order band gaps can be merged in photonic crystals with a high refractive index contrast.