Polarization conversion from highly conducting holographic gratings with asymmetric profile

We use a rigorous electromagnetic code to investigate the differences between the zeroth-order cross-polarization reflection coefficients in highly conducting metallic gratings with asymmetric profile. This study results of interest from a practical point of view due to its possible applications as a technique to detect gratings asymmetries. Numerical results in holographic gratings made of gold with asymmetric profiles are presented.     

波长尺寸表面浮雕光栅的严格理论计算和分析

本文采用麦克斯韦(Maxwell)方程微分积分方程对波长尺寸的梯形反射光栅在TE模式(横向电场)偏振状态入射下,进行严格理论的计算和分析.表面电场计算表明,光栅刻槽斜边边界对于金属材料金和铝光栅表面电场分布具有明显效应,对于入射波长为λ=1.0μm附近的红外区域,金材料具有强烈的金属特性,此时表面电场的斜边边界效应更加明显.表面电场及其远场衍射效率伴随刻槽深度变化,而振荡特性进一步阐明了此效应对近,远场的作用.该方程适用于任意形状光栅或单一散射体对任意入射方向,任意偏振态的衍射和散射问题.     

Chaos in numerical analysis of ordinary differential equations

The discretisation of the ordinary nonlinear differential equation $\text{d}\text{y}\text{d}\text{t}\text{= y(1?y)}$ by the entral difference scheme is studied for fixed mesh size. In the usual numerical computation, this method produces some “ghost solution” for the long range calculation. Regarding this discretisation as a dynamical system in R², these pathological behaviors are shown to be a kind of “chaos” in the dynamical system for any mesh size. Moreover, some combination of the central difference scheme and the Euler's scheme is studied for the above equation. It gives some motivation for Hénon's model. The usual discretisation of a second order differential equation are studied also. It gives some chaotic behaviors numerically which is similar to the behavior of the orbits of the system of differential equations proposed by Hénon-Heiles.     

Numerical study of perfectly conducting wire gratings in the resonance domain

Using the Hamiltonian method that we have recently proposed [1, 2] and which is described in the IEEE transactions on antennas and propagation [3], we give some theoretical data related to perfectly conductor wire gratings. Phase-shift and efficiency are given for several diffraction orders, for four shapes of cross-section, and for both fundamental cases of polarization.     

A numerical study of blazed holographic gratings

The spectral performance of blazed holographic gratings is compared with that of triangular groove gratings.     

Diffraction properties of highly birefringent liquid-crystal composite gratings

We have fabricated electrically switchable holographic gratings, using Polaroid Corporation's DMP-128 photopolymer filled with the nematic liquid crystal E7. It is shown that a coupled-wave theory that includes the effects of the birefringence of the liquid crystal must be used to explain the diffraction properties of these anisotropic volume gratings. Furthermore, a detailed comparison of theory and experiment provides information about the alignment of the liquid crystal within the polymer host.     

Universal scaling in highly doped conducting polymer films

Electrical transport of a highly doped disordered conducting polymer, viz. poly-3,4-ethylenedioxythiophene stabilized with poly-4-styrenesulphonic acid, is investigated as a function of bias and temperature. The transport shows universal power-law scaling with both bias and temperature. All measurements constitute a single universal curve, and the complete J(V,T) characteristics are described by a single equation. We relate this scaling to dissipative tunneling processes, such as Coulomb blockade.     

Accuracy analysis of simplified and rigorous numerical methods applied to binary nanopatterning gratings in non-paraxial domain

A set of simplified and rigorous electromagnetic vector theories is used for analyzing the transmittance characteristics of diffraction phase gratings. The scalar diffraction theory and the effective medium theory are validated with the exact results obtained via the rigorous coupled-wave theory and the finite-difference time-domain method. The effects of surface profile parameters and also the angle of incidence is demonstrated to be a limiting factor in the accuracy of these theories. Therefore, the error of both simplified theories is also analyzed in non-paraxial domain with the intention of establishing a specific range of validity for both simplified theories.     

Microwave verification of a numerical optimization of fourier gratings

A recent holographic technique makes it possible to manufacture a new kind of grating, the “Fourier grating”, whose profile is given by a superposition of a fundamental and a finite number of harmonics. A local theoretical optimization shows that this type of grating can have an efficiency better, than that of the echelette, for a broad wavelength range. In this paper, we show the validity of this theoretical prediction by an experimental verification in the microwave region.     

Free-to-bound radiative recombination in highly conducting InN epitaxial layers

We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of ‘free-to-bound’ radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7×10¹⁷–6×10¹⁸) cm⁻³ and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between E_g=692 meV for effective mass m_n0=0.042m₀ and E_g=710 meV for m_n0=0.1m₀.     

矩形折射率调制型长周期光纤光栅传输谱的理论分析及数值计算

给出了折射率调制类型为矩形波的长周期光纤光栅的理论分析方法，并利用计算机建模，以FlexcorTM1060光纤为例，对采用振幅掩模法制作的长周期光纤光栅进行了数值模拟计算，计算结果与已有的实验相符合.     

Comparative analysis of diffraction gratings for femtosecond pulse compression systems based on numerical simulation of their parameters

The parameters of diffraction gratings that can be used to compress high-power femtosecond pulses of neodymium glass lasers are analyzed based on numerical calculations. The parameters of gratings of different types are compared to evaluate prospects for their use.     

Investigation of diffraction gratings with the use of a differential heterodyne microscope

A theory of image formation in a scanning differential heterodyne microscope is developed using the results of observations of a metallized periodic structure with a rectangular profile. The phase and amplitude response functions are calculated for TE and TM polarizations. It is demonstrated that the width of grating elements can be measured with a superresolution. The experimental phase and amplitude responses from a metallized grating with a trapezoidal profile are obtained and compared with the results of theoretical calculations.     

Dynamics of pulse propagation in phase-shifted fibre Bragg gratings: a numerical investigation

Fibre Bragg gratings (FBGs) are an integral component of optical fibre based communication systems for signal processing. Uniform FBGs and phase-shifted FBGs find major applications as add-drop filters in dense wavelength division multiplexing (DWDM) systems where these are used to filter out channels and to shape the signal spectrum. We investigate numerically the spatiotemporal deformation of pulses as they propagate through the gratings. In particular, the effect on the pulse dynamics of phase shift or defect present in the gratings, often introduced intentionally in a grating to generate a narrow transmission spike within the stopband, is studied in time domain. This study is of significance since the deformation in the pulse in time limits the maximum bit rate achievable, and hence the pulse duration and shape need to be analysed and controlled accurately. A time domain numerical tool based on the Transmission Line Modelling (TLM) method is developed to study the pulse dynamics. The results obtained give useful information about the signal dynamics in gratings with defects, including the signal oscillations trapped in the defect, to be considered in applications such as while designing spectral filters.     

External removal of endpoint-discontinuity artifacts in the reciprocal-space analysis of spectra

We present a systematic method of removing endpoint-discontinuity artifacts in the Fourier analysis of spectral segments, enabling the more accurate extraction of information. This principal-component-removal approach differs from a previous version by using extrapolated (or extended) data outside rather than inside the spectral range. This not only allows coefficients to be accessed to the white-noise limit with no distortion of the segment, but also generates interpolated coefficients for improved analytic insight. Examples are provided.     

TM-mode perturbation analysis of dielectric gratings

An improved perturbation procedure that was previously presented for TE modes guided by dielectric gratings is extended here to TM modes. In both cases, the fields are described in terms of a transverse transmission-line network, which lends considerable physical insight into the behavior of dielectric gratings. However, TM modes require equivalent sources of both the voltage and current varieties, in contrast to TE modes which involve only current sources. By comparing with an exact solution, the numerical results obtained by the present perturbation procedure are found to be very accurate. This work was supported by the U.S. Office of Naval Research, under Contract No. N 00014-67-A-0438-0014, and by the U.S. Joint Services Electronics Program, under Contract No. F 44620-69-C-0047.     

Improved perturbation analysis of dielectric gratings

An improved perturbation procedure is presented for investigating dielectric gratings of current use in integrated optics. The electromagnetic fields are described in terms of a transverse transmission-line network, which brings considerable physical insight into the overall behavior of the dielectric grating. In addition, the present procedure yields highly accurate results while retaining the simplicity of previous perturbation methods.