Impulse functions over curves and surfaces and their applications to diffraction

An explicit preferred definition of impulse functions (Dirac delta functions) over lower-dimensional manifolds in RN is given in such a way to assure uniform concentration per geometric unit of the manifold. Some related properties are presented. An application related to diffraction is demonstrated.     

Excitation of eigenmodes at gyroelectromagnetic gratings in conical mounting

We study the diffraction of electromagnetic waves at periodically corrugated isotropic–gyroelectromagnetic surfaces, in the index-matching situation, when the plane of incidence forms an arbitrary angle with the main section of the grating (“conical mounting”). It is shown that, under these conditions, eigenmodes can be excited at certain angles of incidence, which can be calculated. We analyze the variation of the position of the anomalies when the orientations of the plane of incidence and of the optic axis are varied. We also show the dependence of the maximum reflected power as a function of the groove height-to-period ratio.     

Enhanced transmission through nano-aperture on a tapered metallic substrate

The optical transmission and distribution through a subwavelength slit on a tapered metallic substrate was investigated. By using a 45° tapered structure rather than a traditional metallic plate, a 6-fold transmission enhancement could be achieved. This is due to the asymmetrical excitation of surface waves and the matching of propagation constants between the surface waves and slit waveguide. In addition, by patterning surface corrugations in the exit plane, the beam could be focused. By tuning the period of the surface corrugations, we were able to adjust the focal length. For an input wavelength of 0.5 μm, the focal point could be kept within 0.6 μm with a focal length extending from 0.5 μm to 2.5 μm and a grating period ranging from 0.5 μm to 0.6 μm.     

Photoinduced effect in Ga-Ge-S based thin films

Glassy films of Ga₁₀Ge₂₅S₆₅ with 4 μm thickness were deposited on quartz substrates by electron beam evaporation. Photoexpansion (PE) (photoinduced increase in volume) and photobleaching (PB) (blue shift of the bandgap) effects have been examined. The exposed areas have been analyzed using perfilometer and an expansion of 1.7 μm (ΔV/V ≈ 30%) is observed for composition Ga₁₀Ge₂₅S₆₅ exposed during 180 min and 3 mW/cm² power density. The optical absorption edge measured for the film Ge₂₅Ga₁₀S₆₅ above and below the bandgap show that the blue shift of the gap by below bandgap photon illumination is considerable higher (ΔE_g = 440 meV) than ΔE_g induced by above bandgap illumination (ΔE_g = 190 meV). The distribution of the refraction index profile showed a negative change of the refraction index in the irradiated samples (Δn = −0.6). The morphology was examined using a scanning electron microscopy (SEM). The chemical compositions measured using an energy dispersive analyzer (EDX) indicate an increase of the oxygen atoms into the irradiated area. Using a Lloyd's mirror setup for continuous wave holography it was possible to record holographic gratings using the photoinduced effects that occur in them. Diffraction efficiency up to 25% was achieved for the recorded gratings and atomic force microscopy images are presented.     

Configurable coarse wavelength division demultiplexers based on planar reflective gratings

Configurable coarse wavelength division multiplexing filters were realized using a planar reflective grating architecture utilizing small diffraction angles. The novel architecture yielded a 10-fold reduction in the chip size compared to previously reported filter configurations. The proposed design was manufactured using a standard silica-on-silicon process with a waveguide refractive index contrast of 0.82%. Polarization dependent losses of less than 0.2 dB were achieved without any post-processing steps. Fabricated filters showed insertion losses of 2.4 dB and remarkably broad free spectral ranges of more than 500 nm. The combination of single-mode input and multi-mode output guides resulted in box-like passband responses with bandwidth of 14.2 nm. Applications of the filters for integrated coarse wavelength division receivers and monitors are discussed.     

透射式GaAs光电阴极AlGaAs/GaAs外延层在热应力作用下晶格弯曲的倒易点二维图研究

本文介绍了倒易点及其倒易点二维图的概念、倒易点形状及其倒易点二维图同晶格形变的关系,推导了热应力作用下双层膜发生弯曲的理论公式,阐明了晶格弯曲的原因和晶格倒易点二维图展宽的方向,分析了GaAs光电阴极AlGaAs/GaAs外延层的实测倒易点二维图,最后提出了降低AlGaAs/GaAs外延层中的热应力的技术途径.     

可擦写俘精酸酐/PMMA薄膜的全息记录特性研究

对一种新型可擦写有机光致变色材料-吡咯俘精酸酐的光全息记录特性进行了实验研究.用涂布法制成厚度约10 μm 的吡咯俘精酸酐/PMMA聚合物薄膜.样品无色态吸收峰在373 nm处,经紫外光照射发色后,变为呈色态,其吸收峰移动到626 nm处,呈色态在氦氖激光照射下可以重返为无色态.无色态和呈色态在室温下都是稳定的.以呈色态吡咯俘精酸酐/PMMA薄膜作为记录材料,以氦氖激光作为记录光,分别建立了双光束干涉衍射记录装置和四波耦合实验系统.实验测得该薄膜的空间分辨率至少可以达到1680 lines/mm,一级衍射效率大于2%,全息记录中的最佳曝光量约为1 J/cm².实验测量了衍射效率与曝光量的特性曲线,分析了衍射光强度和光斑模式随曝光时间的变化关系.     

The influence of diffraction in microinterferometry and microtomography of optical fibers

Interferometric measurement of two- and three-dimensional refractive index distribution in optical fibers is numerically simulated with the use of wavefront calculation method based on Hyugens–Fresnel principle. The calculation is performed for two models of fibers: homogenous and step-index multi-mode fiber. The simulation is aimed at determination of errors caused by diffraction artifacts due to microdimensions of tested objects and the presence of boundaries of media with different refractive indices. The result of simulation is methodology of increasing the accuracy of measurement in selected areas of measurement field/volume. The simulation outcomes are confirmed by the experiment.