Vertical Field Profile Research Articles

Spatially resolved x-ray detection with photonic crystal scintillators

We study the self-collimation phenomenon in photonic crystals (PhC) of wide bandgap materials for ultra-fast and high spatial resolution x-ray detection. We work on various heavy inorganic scintillators: BaF2, GaN, ZnO, CsI:Tl, NaI:Tl, LYSO, WO4 compounds, and plastic scintillators. Conventional scintillator detectors do not rely on a direct detection mechanism; hence, they require intricate design and fabrication processes. We offer a PhC design to observe self-collimation phenomena and overcome the ongoing spatial resolution challenges with these types of materials. We investigate the photonic band diagrams and iso-frequency contours. Fourier transforms based on finite-difference time-domain and frequency domain simulations are done for verifying and analyzing the self-collimation with the selected material. Light extraction efficiency at the PhC–air interface, depending on the truncation distance from the excitation point, is measured. Beam divergence values are calculated at 1 mm propagation distance. The vertical field profiles are obtained to observe the confinement. For the spatial resolution analysis, cross-sectional beam profiles have been examined. Gaussian envelopes are fitted to beam profiles for a consistent data analysis, and full-width-at-half-maximum values are considered. As a result, we theoretically prove and demonstrate the spatially resolved x-ray detection at the sub-micrometer level for a wide range of scintillator materials.

Vertical field enhancement of a spot-size converter using a nanopixel waveguide and window structure.

Spot-size converter (SSC) is an essential building block for integrated photonic circuits applied as a mode transformer between optical components. One typical issue for SSC is the difficulty of broadening the vertical field profile. Herein, we propose a nanopixel SSC (1 × 2 μm2) with changing hole size and density. Unlike a typical SSC, this configuration controls both the lateral and vertical field profiles relatively easily by enhancing the nanopixel density. A vertical field expansion of 1.21 μm was obtained by enhancing the nanopixel density. In addition, we designed the optical field in the lateral direction using deep neural network (DNN)-based learning to realize a perfect circular spot for high coupling efficiency that reached -3 dB at λ0 = 1.572 μm when the optical field aspect ratio was adjusted to 1 after training for 200 epochs. Furthermore, the vertical expansion was increased from 1.21 to 4.9 μm and the coupling efficiency from -3 to -0.41 dB by combining it a silicon dioxide window structure (5 × 15 × 10 μm3). The 1-dB operating bandwidth of the designed SSC structure is 100 nm (1.5-1.6 μm), while fabrication tolerance of the nanopixels and window structure length for the designed SSC structure are ±15 nm and ±250 nm when the coupling efficiency drops by 1 dB.

Coupled-wave analysis for photonic-crystal surface-emitting lasers on air holes with arbitrary sidewalls

The coupled-wave theory (CWT) is extended to a photonic crystal structure with arbitrary sidewalls, and a simple, fast, and effective model for the quantitatively analysis of the radiative characteristics of two-dimensional (2D) photonic-crystal surface-emitting lasers (PC-SELs) has been developed. For illustrating complicated coupling effects accurately, sufficient numbers of waves are included in the formulation, by considering their vertical field profiles. The radiation of band-edge modes is analyzed for two in-plane air-hole geometries, in the case of two types of sidewalls: i.e. "tapered case" and "tilted case." The results of CWT analysis agree well with the results of finite-difference time-domain (FDTD) numerical simulation. From the analytical solutions of the CWT, the symmetry properties of the band-edge modes are investigated. In-plane asymmetry of the air holes is crucial for achieving high output power because it causes partial constructive interference. Asymmetric air holes and tilted sidewalls help in inducing in-plane asymmetries. By breaking the symmetries with respect to the two orthogonal symmetric axes of the band-edge modes, the two factors can be tuned independently, so that the radiation power is enhanced while preserving the mode selectivity performance. Finally, top-down reactive ion etching (RIE) approach is suggested for the fabrication of such a structure.

PETOOL: MATLAB-based one-way and two-way split-step parabolic equation tool for radiowave propagation over variable terrain

A MATLAB-based one-way and two-way split-step parabolic equation software tool (PETOOL) has been developed with a user-friendly graphical user interface (GUI) for the analysis and visualization of radio-wave propagation over variable terrain and through homogeneous and inhomogeneous atmosphere. The tool has a unique feature over existing one-way parabolic equation (PE)-based codes, because it utilizes the two-way split-step parabolic equation (SSPE) approach with wide-angle propagator, which is a recursive forward–backward algorithm to incorporate both forward and backward waves into the solution in the presence of variable terrain. First, the formulation of the classical one-way SSPE and the relatively-novel two-way SSPE is presented, with particular emphasis on their capabilities and the limitations. Next, the structure and the GUI capabilities of the PETOOL software tool are discussed in detail. The calibration of PETOOL is performed and demonstrated via analytical comparisons and/or representative canonical tests performed against the Geometric Optic (GO) + Uniform Theory of Diffraction (UTD). The tool can be used for research and/or educational purposes to investigate the effects of a variety of user-defined terrain and range-dependent refractivity profiles in electromagnetic wave propagation. Program summary Program title: PETOOL (Parabolic Equation Toolbox) Catalogue identifier: AEJS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEJS_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 143 349 No. of bytes in distributed program, including test data, etc.: 23 280 251 Distribution format: tar.gz Programming language: MATLAB (MathWorks Inc.) 2010a. Partial Differential Toolbox and Curve Fitting Toolbox required Computer: PC Operating system: Windows XP and Vista Classification: 10 Nature of problem: Simulation of radio-wave propagation over variable terrain on the Earthʼs surface, and through homogeneous and inhomogeneous atmosphere. Solution method: The program implements one-way and two-way Split-Step Parabolic Equation (SSPE) algorithm, with wide-angle propagator. The SSPE is, in general, an initial-value problem starting from a reference range (typically from an antenna), and marching out in range by obtaining the field along the vertical direction at each range step, through the use of step-by-step Fourier transformations. The two-way algorithm incorporates the backward-propagating waves into the standard one-way SSPE by utilizing an iterative forward–backward scheme for modeling multipath effects over a staircase-approximated terrain. Unusual features: This is the first software package implementing a recursive forward–backward SSPE algorithm to account for the multipath effects during radio-wave propagation, and enabling the user to easily analyze and visualize the results of the two-way propagation with GUI capabilities. Running time: Problem dependent. Typically, it is about 1.5 ms (for conducting ground) and 4 ms (for lossy ground) per range step for a vertical field profile of vector length 1500, on Intel Core 2 Duo 1.6 GHz with 2 GB RAM under Windows Vista.

A novel finite-difference time-domain wave propagator

A novel time-domain wave propagator is introduced. A two-dimensional (2-D) finite-difference time-domain (FDTD) algorithm is used to analyze ground wave propagation characteristics. Assuming an azimuthal symmetry, surface, and/or elevated ducts are represented via transverse and/or longitudinal refractivity and boundary perturbations in 2-D space. The 2-D FDTD space extends from x=0 (bottom) to x/spl rarr//spl infin/ (top), vertically and from z/spl rarr/-/spl infin/ (left) to z/spl rarr//spl infin/ (right), horizontally. Perfectly matched layer (PML) blocks on the left, right, and top terminate the FDTD computation space to simulate a semi-open propagation region. The ground at the bottom is simulated either as a perfectly electrical conductor (PEC) or as a lossy second medium. A desired, initial vertical field profile, which has a pulse character in time, is injected into the FDTD computation space. The PML blocks absorb field components that propagate towards left and top. The ground wave components (i.e., the direct, ground-reflected and surface waves) are traced longitudinally toward the right. The longitudinal propagation region is covered by a finite-sized FDTD computation space as if the space slides from left to right until the pulse propagates to a desired range. Transverse or longitudinal field profiles are obtained by accumulating the time-domain response at each altitude of range and by applying the discrete Fourier transformation (DFT) at various frequencies.

Start-up without preionization in NOVILLO tokamak

Many tokamak devices require a preionization system in order to start up both the discharge cleaning and the main plasma discharge. A stray vertical magnetic field profile of 100 G at the center of the plasma column has been measured on the NOVILLO tokamak by magnetic probes especially made to study the breakdown process. A new vertical field profile, superimposed to the stray field component parallel to the equilibrium field, is created by two additional compensation poloidal coils. Such arrangement of the vertical field profiles, with a 1/R-dependence, has been found optimum to achieve the plasma breakdown without any preionization system.

Edge contrast in ionography

The effect of Mylar foil thickness on the appearance of ionography images has been investigated. Approximate expressions for the depletion width of strips and halfplanes of charge are given with supporting experimental measurements. In microdensitometer traces of charged strips and halfplanes the density is shown to be proportional to the vertical field profile of the charge pattern near the Mylar. This work suggests a simple means of altering edge contrast for ionography by using different thicknesses of Mylar. Ionography images of a hand phantom illustrate this effect.

Guarded Double Field Meter

An instrument is described for absolute measurements of components of electrostatic fields in the free atmosphere near the earth's surface. This apparatus provides a guarding system to minimize field distortion at the measurement site. A method utilizing models in an alternating electric field is used to evaluate the augmentation factor of the instrument as well as the effectiveness of the guarding system. The instrument has been used to measure horizontal electric field and to make vertical field profiles. It is also used simultaneously with a field meter flush with the earth to make an analog record of space charge density.