Design of Narrowband Optical Filters Using Binary Number Sequence Photonic Crystals

A theory to design narrow band optical filters by using a new photonic crystal structure is presented. This new photonic crystal structure is composed of low index layers and high index layers arranged in mod. 4 up and down binary number sequence. The new structure exhibits narrow transmission peaks in the forbidden frequency gap region with high optical transmission (greater than 99.98%) at C.W.D.M. (Coarse Wavelength Division Multiplexing) center wavelengths. The proposed filters use only 8 layers. These new binary number sequence photonic crystal narrowband optical filters are much smaller in size, lower in cost and easier to fabricate as compared to narrowband photonic crystal optical filters based on defect Fractal Cantor multilayers, suggested recently by a group of researchers.     

多个单负材料缺陷一维光子晶体的孪生缺陷模

分析了含有多个单负材料缺陷层的一维光子晶体中缺陷模的性质。在两种单负(负介电常量或负磁导率)材料交替堆叠形成的一维光子晶体中,掺入了多个周期排列的单负材料缺陷层,得到在该光子晶体的零有效相位(zero-effective phase)带隙内存在孪生缺陷模。通过改变缺陷的数目或缺陷层的厚度,可调节缺陷模的频率间隔,但缺陷模的数目总保持为两个。计算结果显示,该孪生缺陷模的频率对入射角度的依赖较弱;随着入射角度的改变,缺陷模频率的相对改变量总保持在0.03以下。此外,对应缺陷模频率的电场在该光子晶体中传播时,将被强烈地局域在缺陷层与周期结构的交界面上。     

含复合缺陷层的光子晶体的光学特性

运用传输矩阵法研究了在一维光子晶体中插入缺陷层的透光特性。在无缺陷层的一维光子晶体中能产生467~510 nm、1 279~1 715 nm两处明显的光子带隙。重点研究了插入缺陷层后,在1 279~1 715 nm的光子带隙中缺陷层厚度和入射角度大小分别与透射光谱变化的关系。研究发现：缺陷模的位置对入射角变化很敏感;出现缺陷模的数量和插入缺陷层的数量相同;一维光子晶体厚度的增大不会改变缺陷模的数量和位置,只改变透射峰的宽度和透射率。     

Independent Modulation of Omnidirectional Defect Modes in Single-Negative Materials Photonic Crystal with Multiple Defects

Single-negative materials based on photonic crystal with multiple defect layers are designed and the free modulation of defect modes is studied. The results show that the multi-defect structure can avoid the interference between the defect states. Therefore, the designed double defect modes in the zero effective-phase gap can be adjusted independently by changing the thickness of different defect layers. In addition, the two tunable defect modes have the omnidirectional characteristics. This multi-defect structure with above-mentioned two advantages has potential applications in modern optical devices such as tunable omnidirectional filters.     

Enhancement of the defect field of one-dimensional photonic crystals containing <Emphasis Type="Italic">ε</Emphasis>-and <Emphasis Type="Italic">μ</Emphasis>-negative layers

Employing the transfer matrix method, we study the electromagnetic field of one-dimensional photonic crystals with a defect inserted by pairs of μ-negative (MNG) and ε-negative (ENG) material layers. The fields within the pairs of layers and the matrix defect are independent of each other, and the whole field is their superposition. The whole defect field can be significantly enhanced by pairs of ε-negative and μ-negative layers. In contrast to the conventional defect modes, the intensity and volume of the defect field with pairs of ε-negative and μ-negative layers can be precisely adjusted.     

Study of Defect-Layers Effect in Ferroelectric Thin Film with TransverseIsing Model

By taking into account the two-spin interaction in the transverse Ising model (TIM), the influence of the defect layers (including J_B andΩ_B on the polarization and Curie temperature are calculated numerically, within the framework of the decoupling approximation under Green's function. The numerical results show that the polarization and Curie temperature will both become large sensitively due to the large values ofJ_B and the small value of Ω_B of the defect layers. Meanwhile, the dependence of the crossover values of the exchange interaction J_A, the transverse field Ω_Aof the bulk material on the exchange
interaction J_B and the transverse fieldΩ_B of the defect layers are shown in 3-Dimensional (3-D) figures for the first time. Moreover, the transition features of the ferroelectric thin film with defect layers are presented.     

Reflection chromaticity of cholesteric liquid crystals with sandwiched periodical isotropic defect layers

We study one-dimensional photonic crystals made of cholesteric liquid crystals with sandwiched isotropic defect layers. Based on the Berreman Fast 4 × 4 matrix method, the dispersion relation of one-dimensional photonic crystals is calculated and the corresponding reflection chromaticity is obtained. It is found that the color shift could be controlled by adjusting the thickness and refractive index of the isotropic defect layers. Compared with conventional structures, the reflection chromaticity of this structure is insensitive to the incident angle, if the thickness ratio of the cholesteric liquid crystals to that of the isotropic defect layers and the refractive index of periodical isotropic defect layers are properly set. Furthermore, the common forbidden bands for both left and right circular polarizations can be obtained, and we also take the wavelength-dependent refractive indices into consideration and obtain the reflected light chromaticity with the incident angle increasing. The proposed device can be used as a reflective color filter in the display industry.     

The effect of random variations of structure parameters on photonic band gaps of one-dimensional plasma photonic crystals

The electromagnetic properties of one-dimensional plasma photonic crystals influenced by random perturbation of structure parameters are studied through transfer matrix method in the paper. The random perturbation of thicknesses of layers as well as plasma frequency of layers can influence the electromagnetic properties of one-dimensional plasma photonic crystal. The random perturbation of thicknesses of layers tremendously impacts the band structure of one-dimensional plasma photonic crystals in high frequency domain. The random perturbation of plasma frequency, however, contributes to the moving of a cutoff frequency in the low frequency domain. The position of the defect mode of one-dimensional plasma photonic crystal randomly moves due to the random perturbations. The effect of random perturbation of plasma frequency is more significant than that of random perturbation of thickness of layers, which alters not only the transmittance but also the position of the defect mode. An improved quality factor and a shortened moving distance of the defect mode are both acquired by increasing the period number. And then this work may provide a crude but useful instrument in analyzing disordered 1DPPCs.     

Magnetic structure and magnetic properties of non-stoichiometric Fe1−xO

The magnetic structure of non-stoichiometric Fe_1?xO was investigated by neutron scattering measurements on a quenched single crystal of Fe_0.91O below the Néel temperature. In each defect cluster, there exist ferromagnetic moments and they are arranged antiferromagnetically with the periodicity of twice that of the atomic superstructure. Relation between the defect structure and the magnetic properties for a wide range of compositions was discussed.     

Dispersion-flattened polarization-maintaining photonic crystal fiber for nonlinear applications

In this paper, we propose a novel photonic crystal fiber design to achieve high birefringence, high nonlinearity with a flattened dispersion profile. The air holes which are embedded in silica are arranged in triangular lattice configuration. In the core region, an air hole is introduced as the central defect to flatten the dispersion curve. The shape of the defect air hole is deliberately designed elliptical so that high birefringence in the PCF is obtained for polarization-maintaining purpose. The structural parameters are carefully selected to optimize the optical properties.     

Torque on birefringence in one-dimensional finite photonic band-gap structure with defects

We theoretically study the optical torque on the layers in one-dimensional finite photonic crystal with birefringent defect layers. The fields in each layer are deduced by Berreman's 4 × 4-matrix method. We found out that the strong electromagnetic fields enhance both torque and force on the layers at defect mode. These investigations suggest an efficient method to construct the micro-motor driven by electromagnetic wave.     

Superradiance of the system comprised of two parallel resonant thin layers

A cavity-like regime of superradiance for a double-layer medium populated by atoms with the A-scheme of transitions is considered in a semiclassical approximation under the condition that the layers are arranged symmetrically with respect to initial characteristics of the field and atomic system populating the layers. An analytical law for the linear stage of the superradiance process is found that agrees well with numerical solution of the differential equations of the model under consideration for any initial conditions nearly up to the peak of the appropriate characteristic of the field intensity module. For the case of degenerate lower doublet, the resonant character of the dependence of the superradiance pulse time delay on the distance between the layers is studied.     

双缺陷模一维光子晶体的双光子吸收增强研究

采用真空镀膜工艺制备了具有762 nm和800 nm双缺陷模的含两个CdS缺陷层的TiO2/SiO2一维光子晶体,运用抽运探测技术测量了其双光子吸收。对于两个缺陷模,双光子吸收均得到很大的增强,其中缺陷模为800nm时的双光子吸收系数307 cm/GW要大于缺陷模为762 nm时的116 cm/GW,分别为单层CdS薄膜的48倍和18倍。这种双光子吸收的增强是由于光局域化导致一维光子晶体缺陷层内的电场强度增大而形成的。通过传输矩阵法计算了一维光子晶体的内部场强,发现800 nm波长光入射时缺陷层内的电场强度要大于762 nm波长光入射时的电场强度值。     

Transmission properties of photonic quantum well composed of dispersive materials

The transmission properties of one-dimensional photonic quantum well structure produced by inserting defect layers into dispersive photonic crystals have been investigated. These photonic crystals are stacked alternatively with two kinds of dispersive material layers. The inserted layers structure is another kind of dispersive photonic crystal. It has been turned out that the confined states are quantized.     

含负折射率缺陷的光量子阱的透射特性及理论模拟

利用转移矩阵方法对含负折射率缺陷的一维光子晶体量子阱结构的透射特性进行了研究。结果表明,当缺陷为偶数个时,结构透射谱中有两个透射峰;当缺陷为奇数个时,透射谱中只有中心处一个透射峰,且随着缺陷层数的增加,该处峰的品质因子逐渐提高。因电磁波在缺陷处的强烈局域,所以阱中局域态的透射率对缺陷的折射率有很强的依赖作用。通过控制入射光的强度可使缺陷的折射率发生微小的改变,进而使局域态的透射率发生明显改变,这样就可制作高效光开关。     

FDTD analysis of photonic crystal defect layers filled with liquid crystals

Dielectric and metallic photonic crystals comprising nematic liquid crystal materials as defect layers or elements are investigated by the Finite Difference Time Domain (FDTD) method. Appropriate formulations of the FDTD algorithm, for the simulation of anisotropic and dispersive media as well as periodic geometries, are utilised and combined with the proper absorbing and periodic boundary conditions. The spectral properties of the presented structures are tuned by means of applying static electric fields across the defect layers, thus affecting the molecular orientation of the liquid crystal material. Numerical results show that sufficient tuning ranges are achieved, requiring low operating voltages. Moreover, high and sharp resonance peaks are observed.     

Comparison of photonic crystal narrow filters with metamaterials and dielectric defects

The photonic band-gap of the two kinds of 1D photonic crystal structure which is composed of the form of (AB) ^N1 C ^m (BA) ^N2 , one with a metamaterial defect layer (MDL) and the other one with a dielectric defect layer (DDL) are studied. Our results show that in both cases, where there is only one defect layer, m=1, no defect mode exists, but for two defect layers (m=2) there is a single defect mode which is centered in the middle of the band-gap. The width of the defect mode in DDL is narrower than that in MDL. For a number of defects of more than two (m>2) and even, in both of the MDL and DDL structures, there is only one defect mode. For m>2 and odd, the defect mode in the MDL vanishes, but for DDL there is two defect-mode symmetrically centered in the middle of the band-gap. The effects of the defect layers refractive index value, the periodicity number of the structures and the incident angle on the properties of the defect modes and the transmittance spectrum are discussed.     

Conoscopic patterns in photonic band gap of cholesteric liquid crystal cells with twist defects

We investigate theoretically the effects of the angle of incidence on light transmission through cholesteric liquid crystals. The systems are two-layer sandwich structures with a twist defect created by rotation of the one layer about the helical axis. The conoscopic images and polarization-resolved patterns are obtained for thick layers by computing the intensity and the polarization parameters as a function of the incidence angles. In addition to the defect angle-induced rotation of the pictures as a whole, the rings associated with the defect mode resonances are found to shrink to a central point and disappear, as the defect twist angle varies from zero to its limiting value π/2 and beyond.     

Characterization of LiBC by phase-contrast scanning transmission electron microscopy

LiBC was used as a model compound for probing the applicability of phase-contrast (PC) imaging in an aberration-corrected scanning transmission electron microscope (STEM) to visualize lithium distributions. In the LiBC structure, boron and carbon are arranged to hetero graphite layers between which lithium is incorporated. The crystal structure is reflected in the PC-STEM images recorded perpendicular to the layers. The experimental images and their defocus dependence match with multi-slice simulations calculated utilizing the reciprocity principle. The observation that a part of the Li positions is not occupied is likely an effect of the intense electron beam triggering Li displacement.     

两耦合半无限超晶格中的局域界面声子-极化激元

采用转移矩阵方法，研究了含结构缺陷层的两耦合半无限超晶格(GaAs/AlAs)中的局域界面声子-极化激元模性质. 研究发现，含不同介电特性的缺陷超晶格结构中的局域界面声子-极化激元模在剩余射线区［ω_TO, ω_LO］的分布情况与数量存在不同，而且反对称模表现出不同的特征. 文中着重研究了缺陷层介电常数与角频率无关的缺陷超晶格，发现该结构中的局域界面声子-极化激元模对组分层的排列顺序与厚度、缺陷层的厚度以及横向波数有着不同程度的依赖.