Explanation of the diffraction fine-structure in overexposed thick holograms

The far-field diffraction patterns of overexposed thick holographic gratings often contain circular or arcuate structures. The geometrical properties of these patterns were explained earlier. This letter explains the fine-structure of the rings using an extension of the kinematical theory of X-ray diffraction.     

Blazed vector gratings fabricated using photosensitive polymer liquid crystals and control of polarization diffraction

The blazed vector grating possessing antisymmetric distributions of the birefringence were fabricated by exposing the line-focused linearly polarized ultraviolet light on the photosensitive polymer liquid crystals. The polarization states of the diffraction beams can be highly and widely controlled by designing the blazed structures, and the diffraction properties were well-explained by Jones calculus.     

Diffraction management

By using the diffraction properties of waveguide arrays, we propose a scheme to produce structures with designed diffraction. We fabricated arrays with reduced, canceled, and even reversed diffraction. Results of experiments with such waveguides are presented and compared with the predictions made by coupled-mode theory.     

Diffraction effects in guided photonic band gap structure

The concept of the photonic band gap (PBG) structures stems from ideas of Yablonovitch. The idea is to design components so that they affect the properties of photons, in much the same way that ordinary semiconductor crystals affect the properties of electrons. In fact, the PBG structures forbid propagation of photons for a particular range of energy. They can be used to realise optical filters with large stop band and sharp transmission resonance. In the guided PBG structures, the existence of diffractive effects in the vertical dimension could limit the quality factor of such filters. In this paper, we have investigated the origin of diffraction losses in one-dimensional guided PGB structures using 2D and 3D numerical tools. We propose an analytical approach based on Bragg diffraction relation to explain these losses phenomenon. From this approach, the influence of some design parameters on the electromagnetic behaviour and the spectral response of PBG resonators will be explained.     

Nb/SnO2复合薄膜的制备、结构及光电性能

用溶胶-凝胶旋涂法在玻璃基底上制备出Nb/SnO₂复合透明导电薄膜,利用XRD,SEM,紫外—可见分光光度计,四探针电阻仪等测试方法对Nb/SnO₂复合薄膜的结构和物性进行了研究.结果表明: 当Nb含量小于0.99at%时,Nb/SnO₂复合薄膜为较纯的四方金红石结构;复合薄膜中晶粒分布均匀,平均尺寸在5—7 nm.当Nb含量小于0.99at%时,Nb/SnO₂复合薄膜的电阻率先减小后增大,当Nb含量为0.37at%时 关键词： 溶胶-凝胶法 2复合薄膜')" href="#">Nb/SnO₂复合薄膜 结构表征 光电性能     

Strain induced changes in the magnetic phase diagram of metamagnetic heteroepitaxial EuSe/PbSe(1-x)Tex multilayers

Multilayers of strained metamagnetic EuSe intercalated with nonmagnetic PbSe1-xTex were grown by molecular beam epitaxy under conditions optimized by electron diffraction. From detailed structural and magnetic characterization using anomalous synchrotron x-ray diffraction and magnetization measurements, the phase transition temperatures and the magnetic phase diagrams of strained EuSe as a function of the in-plane lattice constant are determined. In this way, it is demonstrated that the magnetic properties of the samples can be significantly changed by applying biaxial strain on EuSe in superlattice structures.     

Light propagation in double-periodic nonlinear photonic lattices in lithium niobate

Single- and double-periodic one-dimensional photonic lattices are formed in lithium niobate using both titanium in-diffusion and holographic grating recording. We investigate linear band structures and diffraction properties of such superlattices and observe a decrease of discrete diffraction with increasing modulation depth of the second superimposed lattice. In weakly modulated superlattices with tailored diffraction properties, our results demonstrate the formation of discrete solitons having a propagation constant inside the extra mini-gap formed inside the Brillouin zone. PACS 42.65.Tg; 42.70.Qs; 42.82.Et     

Transfer functions of nonuniform transmission photonic structures in polymer-dispersed liquid-crystal materials

The transfer functions of nonuniform transmission photonic structures holographically formed in polymer-dispersed liquid-crystal materials are calculated. The diffraction properties of these structures are numerically calculated for different types of inhomogeneity, coupling parameters, and light beam divergences.     

Microemulsion-directed synthesis of different CuS nanocrystals

Through a microemulsion-directed route, CuS nanoparticles, nanorods, nanowires and tube-like structures were prepared while different quantities of surfactant were used. The arrangement of the microemulsion vesicles directed the growth behavior of the nanocrystals. Transmission electron microscopy (TEM) was used to investigate the different morphology of the as-synthesized products. X-ray powder diffraction (XRD) and electron diffraction (ED) were applied to characterize the properties of the products obtained. The mechanism of the nanocrystals with different shapes is discussed.     

Influence of parameters on light propagation dynamics in optically induced planar waveguide arrays

The diffraction and refraction of light beam in optical periodic structures can be determined by the photonic band-gap structures of spatial frequency. In this paper, by employing the equation governing the nonlinear light propagations in photorefractive crystals, we study the photonic band-gap structures, Bloch modes, and light transmission properties of optically induced planar waveguide arrays. The relationship between the photonic band-gap structures and the light diffraction characteristics is discussed in detail. Then the influence of the parameters of planar waveguide arrays on the band-gaps structures, Bloch modes, and linear light transmissions is analyzed. It is revealed that the linear light transmission properties of waveguide arrays are tightly related to the diffraction relationships determined by band-gap structures. And the Bloch modes corresponding to different transmission bands can be excited by different excitation schemes. Both the increases of the intensity and the period of the array writing beam will lead to the broadening of the forbidden gaps and the concentration of the energy of the Bloch modes to the high-index regions. Furthermore, the broadening of the forbidden gaps will lead to separation and transition between the Bloch modes of neighboring bands around the Bragg angle. Additionally, with the increase of the intensity of the array writing beams, the influences from light intensity will tend to be steady due to the saturation of the photorefractive effect. Supported by the Youth for Northwestern Polytechnical University (NPU) Teachers Scientific and Technological Innovation Foundation, the NPU Foundation for Fundamental Research, and the Doctorate Foundation of NPU (Grant No. CX200514)     

Special features of the diffraction of light on optical Fibonacci gratings

Diffraction gratings designed on the basis of the geometrical properties of 1D quasicrystalline structures are considered. It is shown that the diffraction peak position is highly stable against small perturbations of the grating profile when the periodic pattern of the alternating elements comprising the profile remains unchanged. The fractal character of the diffraction patterns is also preserved during perturbations, with the fractal geometry obeying the principle of the Golden ratio.     

Comparative analysis of the spectral and scaling characteristics of optical aperiodic-structure elements

Comparative analysis is given for the properties of deterministic aperiodic diffraction gratings and multilayer structures based on one-dimensional quasicrystal models. Quantitative relation is established between the self-similarity of their structures and the scaling of the spectral characteristics. Some applications of aperiodic devices are discussed.     

ZnO micro-nano composite hydrophobic film prepared bythree-step method

<正>The hydrophobicity of the lotus leaf is mainly due to its surface micro—nano composite structure.In order to mimic the lotus structure,ZnO micro-nano composite hydrophobic films were prepared via the three-step method.On thin buffer films of SiO₂,which were first fabricated on glass substrates by the sol—gel dip-coating method,a ZnO seed layer was deposited via RF magnetron sputtering.Then two different ZnO films,micro-nano and micro-only flowerlike structures,were grown by the hydrothermal method.The prepared films have different hydrophobic properties after surface modification.The structures of the obtained ZnO films were characterized using x-ray diffraction and field-emission scanning electron microscopy.A conclusion that a micro-nano composite structure is more beneficial to hydrophobicity than a micro-only structure was obtained through research into the effect of structure on hydrophobic properties.     

Nonlinear sharpening of holographically processed sub-microstructures

Wavelength dispersion of a computer-generated hologram causes spatial broadening of the focal spot in holographic femtosecond laser processing. From the paraxial approximation, we theoretically derived that the spatial broadening is proportional to only the diffraction position, defined as the distance from the optical axis. We performed experiments under the large-dispersion condition to analyze the influence of the diffraction position on the processed structure. In the processing experiment, a high-numerical-aperture lens and a laser energy near the threshold energy were used to fabricate sub-microstructures. We found a nonlinear dependence of the broadening of the processed structures on the diffraction position, and the degree of broadening was much smaller than the predicted value because of the nonlinear properties of the laser processing.     

Anisotropic Bragg diffraction of finite-sized volume holographic grating in photorefractive crystals

Anisotropic diffraction of uniform plane wave by finite-sized volume holographic grating in photorefractive crystals is considered. It is found that the anisotropic diffraction can take place when some special conditions are satisfied. The diffracted image is obtained in experiment for the anisotropic Bragg diffraction in Fe:LiNbO₃ crystals. A coupled wave analysis is presented to study the properties of anisotropic diffraction. An analytical integral solution for the amplitudes of the diffracted beams is submitted. A trade off between high diffraction efficiency and the deterioration of reconstruction fidelity is analyzed. Numerical evaluations also show that the finite-sized anisotropic volume grating exhibits strong angular and wavelength selectivity. All the results are useful for the optimizing design of VHOE based on finite-sized volume grating structures.     

纳米压印制备的光子晶体结构对AlGaN基材料深紫外出光效率的提高

通过利用阳极氧化铝的方法制备高度有序的光子晶体结构作为纳米压印模板,将大面积光子晶体图案转移到了样品表面,解决了国际上小尺寸光子晶体制备困难的问题。采用纳米压印的方法在AlGaN基样品表面上制备了290 nm的周期光子晶体结构,并将表面具有光子晶体结构的AlGaN基样品正面出光强度提高121%。偏振特性的实验结果表明六角排列的孔状光子晶体将原来朝向样品侧面传播的TE偏振光偏折转向正面,从而增加光抽取效率,改变出光偏振度。指出远场角分辨图案的变化归因于光子晶体对出光的衍射和Bragg散射效果。实验中采用的创新性工艺可以用来制备具有高出光效率的深紫外发光二极管。     

氧化锌纳米结构的制备及发光性质研究

采用化学气相沉积方法,在氩气和氧气混合气氛下制备了两种四角结构的纳米氧化锌。初始反应物为纯锌粉,反应过程中没有采用任何触媒。采用X射线衍射、扫描电子显微镜、透射电子显微镜、光致发光光谱研究了纳米产物的结构和光学性质。获得的纳米产物为高纯的纤锌矿结构氧化锌。两种氧化锌纳米产物具有三维立体的四角结构,分别为四角锥-片状结构和四角锥-线状结构,具有较大的长径比,呈典型的微/纳结构。通过对两种氧化锌纳米结构的紫外发射峰和可见发射带的对比研究,探讨了氧化锌纳米产物可见发射带的起源,以及影响其发光性质的主要因素。     

Right handed or left handed? Forbidden x-ray diffraction reveals chirality

Enantiomers, or stereoisomers, have crystal structures that are mirror images of each other and are thus handed, like our right and left hands. The physical properties of enantiomers are identical except for optical activity, which rotates linearly polarized light by equal amounts but in opposite directions. While conventional x-ray Bragg diffraction can determine crystal structures, it does not distinguish between right- and left-handed crystals. We show resonant Bragg diffraction using circularly polarized x rays reveals the handedness of crystals by coupling x-ray helicity to a crystal screw axis. The intensity of resonantly allowed reflection of alpha-quartz is well described by an admixture of a parity-even and a parity-odd process. Our results are of general importance and demonstrate a new method to directly study chiral motifs in structures that include biomaterials, liquid crystals, magnets, multiferroics, etc.     

Investigation of geometrical effects of antireflective subwavelength grating structures for optical device applications

We calculated diffraction efficiencies of subwavelength grating (SWG) structures for various optical device applications by using a rigorous coupled-wave analysis method. The geometrical effects, such as grating shape, height, and period, were investigated in order to obtain better antireflection performance. Cone shaped SWG structures with a taller height provide lower reflectance over a broadband wavelength range compared to that of flat surface and nanorod. It was found that the low reflection regions are quite related with the grating period and the refractive index of substrate materials. From the comparison between external and internal reflection of SWG structures, we also showed that the internal reflection requires shorter grating period than the case of external reflection to acquire broadband antireflection properties.     

Combined Diffraction Grating in the Scheme of Grazing Incidence: Calculation and Parameter Optimization

The method of parameter optimization of combined structures, which consist of a dielectric grating and a metallic or multilayer dielectric mirror, was proposed. It is based on the use of the waveguide properties of the combined diffraction grating. Its main advantage lies in the possibility of determining the optimum parameters of the combined grating without solving the diffraction problem.