二维旋转直柱光子晶体的频率带隙结构分析

设计了一种二维方形旋转正四边形直柱光子晶体,利用平面波展开方法计算了其光子频率带结构,发现在低频和高频区域,该类光子晶体的光子频率禁带明显增大.计算了空气中Al材料的旋转四边形直柱光子晶体的带结构和态密度,当填充比等于0.5时存在绝对带隙,旋转角度为45°时绝对带隙最大,旋转角度为0时,光子频率禁带位于高频区域.利用FDTD方法检验了计算结果,并分析了旋转角度为45°时,正四边形直柱光子晶体的波导特性以及TM模的电场分布.     

Resonance properties of two-dimensional photonic crystals

The resonance properties of two-dimensional photonic crystals, which are manifested by the localization of radiation inside these objects at frequencies corresponding to their bandgap boundaries, have been theoretically studied. This effect is compared to resonances in photonic crystals with violated internal structure, which are related to the generation of defect modes.     

Symmetry properties of two-dimensional anisotropic photonic crystals

A theoretical study of two-dimensional photonic crystals made of anisotropic material is presented. Detailed computation principles including a treatment of the TE and TM polarizations are given for a photonic crystal made of either uniaxially or biaxially anisotropic materials. These two polarizations can be decoupled as long as any one of the principal axes of the anisotropic material is perpendicular to the periodic plane of the photonic crystal. The symmetry loss due to the anisotropy of the material and the variation of the Brillouin zones relative to the tensor orientations are also analyzed. Furthermore, the symmetry properties of the two-dimensional photonic band structure are studied, and the resulting effect on the photonic bandgap and the dispersion properties of photonic crystal are analyzed as a function of the orientation of the anisotropic material.     

Nonlinear optical responses in two-dimensional photonic crystals

A two-dimensional (2D) highly nonlinear lithium niobate (LN) photonic crystal (PhC) waveguide is fabricated with the aim of studying its nonlinear optical properties. We show a large enhancement of the second-harmonic generation (SHG) in the 2D LN PhCs, originating from resonance between the external pump laser field and a photonic band mode. The SHG enhancement results agree well with the experimental photonic band structure obtained by an angle-dependent optical reflectivity and the theoretical band structure generated by three-dimensional finite-difference time-domain calculations. These results open new possibilities for the use of 2D LN PhC waveguide in integrated nonlinear optical applications.     

Tuning the flow of light in two-dimensional metallic photonic crystals based on Faraday effect

In this paper, we investigate the tunability of two-dimensional metallic photonic crystals by an external magnetic field. Our theoretical analysis is based on the frequency-dependent plane wave expansion method. The numerical results show that the external magnetic field has a pronounced effect on the permittivity of the metals. Therefore, the photonic band structures can be strongly tuned and controlled by adjusting the external magnetic field. Our structure is a good candidate for many applications such as filters, optical switches and modulators.     

All-optical controllable channel-drop filters in two-dimensional square-lattice photonic crystals

A novel all-optical controllable channel-drop filter in photonic crystals (PC) of square lattice is presented. We show that using a resonant-cavity-based add-drop filter with a wavelength-selective reflection feedback and a single-control switching module which is based on nonlinear PC microcavities, the dropped channel can be routed to the drop port or returned to the bus waveguide. Using the temporal coupled-mode theory and two-dimensional nonlinear finite-difference time-domain method, the performance of the proposed device is investigated and the simulation results show the validity of the proposed design.     

树脂基稀土-铁系超磁致伸缩复合材料的磁致伸缩及其高频磁性能

以粉末粘结、模压成型方法,研制了0-3型的E-44环氧树脂基稀土-铁系超磁致伸缩复合材料。采用电阻应变片技术与Agilent 4294A型动态阻抗分析仪,研究了超磁致伸缩合金/环氧树脂复合材料的磁致伸缩性能及高频磁性能,并对所制备的磁致伸缩复合材料的磁导率、截止使用频率等随频率和树脂体积分数的变化规律进行了系统研究。结果表明:树脂的添加,不仅可以提高复合材料的截止频率和高频磁性能,使其具有良好的高频响应特性,其截止频率达30 MHz以上;而且通过适当选择树脂的体积分数,复合材料仍能保持良好的磁致伸缩性能,当树脂体积分数分别为20%、30%时,磁致伸缩系数分别达808×10-6、821×10-6,而当复合材料中树脂的体积分数为50%时,其磁致伸缩系数仍高达592×10-6。探讨了树脂/磁致伸缩复合材料的磁电耦合机制。      

Symmetry reduction for maximization of higher-order stop-bands in two-dimensional photonic crystals

A large absolute higher-order stop-band is achieved in two-dimensional (2D) photonic crystals of square lattice. A genetic algorithm is used to search through a large number of possible structures. In this algorithm, the unit cell is divided into a grid of square pixels and a 2D binary chromosome is assigned to each filling pattern of the pixels. An initial structure with a small higher-order stop-band is included in the initial population to accelerate the search procedure. This initial structure is formed by breaking the symmetry of the supercell of a photonic crystal having a square lattice of square dielectric rods in air. In the optimization process, the effect of reducing the symmetry of the unit cell on the photonic band-gap is investigated. A structure showing an absolute higher-order band-gap as large as 0.1522(2πc/a) is obtained, which is larger than the values reported so far for photonic stop-bands.     

Computing equifrequency contours of two-dimensional photonic crystals by Dirichlet-to-Neumann maps

Equifrequency contours provide important information for designing special photonic crystal devices. In this paper, we present an efficient method to compute equifrequency contours for two-dimensional photonic crystals with triangular and honeycomb lattices. Our method is based on the Dirichlet-to-Neumann (DtN) operator of a unit cell in the photonic crystal. The DtN operator maps the wave field on the boundary of the unit cell to its normal derivatives. For photonic crystals with a triangular or honeycomb lattice, a small linear eigenvalue problem is formulated to calculate the dispersion relation. The formulation is based on the DtN map of the unit cell for a given angular frequency, and the eigenvalue is related to the wave vector. Our method is especially suitable for calculating the equifrequency contours, if a relatively small number of frequencies are involved.     

Omnidirectional reflection in several frequency ranges of one-dimensional photonic crystals

We investigate omnidirectional reflection from higher-order gaps in one-dimensional photonic crystals. Moreover, we present a designing criterion for omnidirectional reflection from several distinct gaps simultaneously, using only a single photonic crystal with a constant period. We show that for practical values of photonic crystals parameters, several relatively large omnidirectional gaps may be obtained. As an example, we demonstrate an omnidirectional reflector that exhibits two distinct wide omnidirectional ranges at near-infrared wavelengths. This omnidirectional reflector that operates in several ranges of wavelengths may have various potential applications.     

Mie resonances and Bragg-like multiple scattering in opacity of two-dimensional photonic crystals

The lowest (main) and high-order Mie resonances and the Bragg-like multiple scattering of electromagnetic (EM) waves are determined as three mechanisms of formation and frequency position of two opaque bands, with narrow peaks in one of the bands in the transmission spectra of 2D photonic crystals composed of dielectric cylinders arranged parallel to the EM wave's electric vector in the square lattice. The main Mie resonance in a single cylinder defines the frequency position of the main gap whose formation results from the Bragg-like scattering. An additional gap with narrow transmission peaks opens in the spectrum of a cylinder layer and becomes pronounced with the number of layers. It is argued that higher-order Mie resonances are responsible for the transmission peaks within the additional band of a perfect crystal. It is shown that 2D photonic crystals with a filling factor ranging from 3% to 20% at a fixed crystal period may be a good zero approximation to study wave transmission through a localizing 2D dense random medium slab.     

Optical properties of graphene based annular photonic crystals

The optical properties of a graphene based annular photonic crystal (APC) are theoretically investigated. The proposed structure is a hollow core cylindrical shell consists of the alternate dielectric layer and graphene monolayer immersed in free space. In order to study the photonic band structures of the APC, we obtained the optical spectra of the graphene based APC by employing the transfer matrix method in the cylindrical waves for both TE and TM polarizations. In this work we study the effect of different geometrical and optical parameters of the structure on the low loss high reflectance graphene induced band gap. It is found that the graphene induced band gap which appeared in the frequency below 10 THz is polarization independent and remains almost invariant with the change in the period number, the radius of the inner core region and the refractive indices of the inner core region and the surrounding medium. However, its width increases by increasing the azimuthal mode number and the chemical potential of the graphene monolayers and decreases by increasing the refractive index and the thickness of the dielectric layers.     

On some optical properties of magnetic photonic crystals

The features of the eigen polarizations of magnetic photonic crystals for large values both of the parameter of magneto-optical activity and the depth of modulation are investigated. The case when the angle between the direction of the external static magnetic field and the normal to the boundary of the layer is zero is considered. The light transmission through the magnetic photonic crystal layer is solved by Ambartsumian's layer addition modified method. The influence of the depth of modulation, the parameter of magneto-optical activity, and the angle of incidence on the eigen polarizations is studied. A simple geometric method for determination of the Bragg frequencies and the widths of forbidden bands is proposed.     

Full photonic band gap properties of plasma photonic crystals with triangular structure

In this study, we analyse full photonic band gap (PBG) properties of two-dimensional plasma photonic crystals (PCs) with triangular lattice, composed of anisotropic tellurium rods with different geometrical shapes immersed in plasma background. Using the finite-difference time-domain method, we discuss the maximization of the full PBG width as a function of tellurium rods parameters with different shapes and orientations. The numerical results show that our proposed structures represent significant large full PBGs in comparison to previously studied plasma PCs.     

Beam splitter and beam bends based on self-collimation effect in two-dimensional photonic crystals

Basing on the self-collimation effect of photonic crystals, one-to-two beam splitter, beam bend and one-to-three beam splitter are, respectively, designed by introducing a different line defect along the same direction. From the equal-frequency contour plot which is calculated by the plane wave expansion method, we obtain the frequency and the propagate direction of the self-collimated beam. The self-collimated beam propagation in photonic crystals with different line defects is simulated by the two-dimensional finite-difference time-domain method with perfectly matched layer absorbing boundary conditions. The simulation results show that one-to-two beam splitter, beam bend and one-to-three beam splitter can be realized by appropriately arranging the line defect along the proper direction. Such devices can greatly enhance photonic crystals for use in high-density optical integrated circuits.     

Critical temperature of a strongly disordered two-dimensional superconductor

Using the Eliashberg equations, we calculated the critical temperature of a two-dimensional superconductor with strong disorder. The retarded Coulomb interaction and the magnetic correlations have been included in the renormalization function Z(). The critical temperature decreases with the concentration of impurities, in agreement with the experiments.     

三维光子晶体的制备技术研究进展

光子晶体是周期性介电结构．它能象周期性原子结构中的电子禁带一样．产生光子禁带。自从1987年Yablonovitch提出光子晶体的概念以来，有关光子晶体的各种研究非常活跃。本文回顾了三维光子晶体的制备技术研究现状，旨在激发不同学科领域研究人员的想象力和创造力．使他们从一些可能的光子晶体制造途径中有所裨益．并将这种可能性转变为现实。     

Genetic optimization of two-dimensional photonic crystals for large absolute band gaps under light line

A binary genetic algorithm with floating crossover and mutation probabilities is used to design two-dimensional photonic crystals for large absolute band gaps under a light line. The unit cell is composed of a small number of round rods and is arranged in a square lattice. The photonic band structure of each chromosome is calculated by the plane-wave expansion method. Starting from randomly generated photonic crystals, the genetic algorithm finally yielded a photonic crystal with an absolute common band gap of 0.0618(2πc/a) at the mid-frequency of 0.4084(2πc/a).     

Investigation of TE-polarized photonic band gap properties in matallodielectric photonic crystals composed of metal coated dielectric cylinders

Using the Dirichlet-to-Neumann map method and generalization of this method, we have been able to calculate the photonic band structure of two-dimensional (2D) metallodielectric photonic crystals composed of metal-coated circular dielectric rods. The rods are embedded in an air background with a square array. We are interested in considering transverse electric (TE) mode of electromagnetic waves. The resulting band structures show the existence of photonic band gaps as well as some flat band regions. We theoretically study the effect of the dielectric constant and radius of the dielectric core on the photonic band structures. There are some interesting results compared to the case of solid metallic rods (without dielectric core) such as appearing the new photonic band gaps and a flat band region with the characteristic of cavity modes.     

Modulation instability of light beam propagation near the supercollimation frequency in nonlinear photonic crystals

The modulation instability (MI) of light beam propagation near the supercollimation frequency in nonlinear photonic crystals was investigated. The role of the nonlinear diffraction effect, merged as a result of nonlinear tuning of the equal-frequency contour curvature, in MI is particularly identified. It is found that the nonlinear diffraction effect tends to stimulate or suppress MI depending on the sign of the product of nonlinear diffraction and Kerr effects. Further analysis reveals that the MI and soliton phenomenon may be steered by the tunable supercollimation frequency in nonlinear photonic crystals.