Aperture Surface Research Articles

Off-solar X-ray observations and a new detector concept with a concentrator optics

Two off-solar coronal X-ray observations made by XSM (X-ray Solar Monitor) and the respective spectral analysis are presented. The observed peculiar line spectra are also compared with a fluorescence line laboratory spectrum obtained from the XSM flight spare measurements. There are at least three emission lines in common in the off-solar and laboratory spectra: Cr K α at 5.41 keV, Cr K β at 5.9 keV and Fe K α at 6.4 keV. In order to remove this fluorescence line contamination, a graded shielding method should be applied in future XSM generic detectors. This means that the inner aperture surface must be coated, e.g. with a thin layer of gold, to inhibit the possible fluorescence line emission entering the detector area. A technical description is also given of a new small size FoV (Field of View) X-ray spectrometer equipped with a single reflection double concentrator. This new instrument is dedicated solely for off-solar coronal observations.

The Radiotransparent Windows Formed of Waveguides with Complex Cross Sections

Radiotransparent windows with plane surfaces that are periodic structures are considered. The periodic structures are based on dual-polarization and wideband waveguides of a complex section with minimal intercenter distances between waveguides. The aperture e-ciency for such windows is simulated. The use of radiotransparent waveguide windows (frequency-selective surfaces) is a promising line in the development of highly durable and vibrostable centimeter- and millimeter-wave antennas. One of the main requirements of radiotransparent waveguide windows is good matching of their aperture surfaces with free space. Re∞ections from a window are determined to a considerable extent by re∞ections from the apertures of its waveguide elements. Available structures of waveguide windows contain square, circular, and rectangular waveguides whose open ends are poorly matched with free space. In addition, windows based on square and circular waveguides cannot be applied in a wide frequency band when frequency-coverage coe-cient Kf equal to the ratio of the upper and lower frequencies of a waveguide window exceeds » 1:2:::1:3. The use of waveguides with a complex shape of the cross section can improve the characteristics of radiotransparent windows (1{4). A change in the parameters of sections can govern the internal and external characteristics of radiators and radiotransparent windows based on such waveguides. In our investigations, we used the mathematical model of an inflnite array antenna (AA) formed of waveguides with an arbitrary section. The model involves solution of the exterior problem by means of the fleld-joining projection method and solution of the interior problem by means of the flnite element method (5). Propagation of electromagnetic waves through both surfaces of a window is taken into account. The walls of waveguides are assumed inflnitely thin.

RADIATION FROM CAVITY-BACKED FRACTAL APERTURE ANTENNAS

This paper investigates the properties of probe fed cavity- backed fractal aperture antennas. The problem is formulated using the flnite element-boundary integral (FE-BI) method in which the fleld inside the cavity is formulated using the flnite element method, and the mesh is truncated at cavity aperture surface using the boundary integral method. Several dual-band cavity-backed fractal aperture antennas based on Sierpinski gasket, Sierpinski carpet, plus shape fractal and Minkowski fractal are investigated. The numerical results obtained from the FE-BI code have been validated with simulations on HFSS.

High-frequency response of subwavelength-structured metals in the petahertz domain

Electromagnetic plane waves, incident on and reflecting from a dielectric-conductor interface, set up a standing wave in the dielectric with the B-field adjacent to the conductor. It is shown here how the harmonic time variation of this B-field induces an E-field and a conduction current J (c) within the skin depth of a real metal; and that at frequencies in the visible and near-infrared range, the imaginary term sigmai of the complex conductivity sigma = sigma(r) + isigma(i) dominates the optical response. Continuity conditions of the E-field through the surface together with the in-quadrature response of the conductivity determine the phase relation between the incident E-M field and J(c). If slits or grooves are milled into the metal surface, a displacement current in the dielectric gap and oscillating charge dipoles at the structure edges are established in quadrature phase with incident field. These dipoles radiate into the aperture and launch surface waves from the edges. They are the principle source of light transmission through the apertures.

Measurement of High-NA Axisymmetric Aspherical Surface with Continuous Interference Method

The measurement principle where in a high-NA (Numerical Aperture) surface, for which the degree of the angle of surface inclination exceeds π/3 radians, could be evaluated with high precision and high speed is proposed. This is based on the stitching method, where aspherical surface measurement becomes possible by dividing the surface of the sample into a range so that measurements can be made with an interferometer and finally combined. We examine the method of applying an interferometer to the condition in which the sample is rotated on an air spindle at a constant speed. It is not necessary in this method to make the sample static. Therefore, the vibration of the servo motor and any location errors can be eliminated. Moreover, the measurement time does not depend on the number of divided areas which are necessary for the stitching method, allowing for high-speed measurement. The principle behind this technique is expanded first, and an experiment system based on it was constructed. The principle proposed was evaluated, and its effectiveness was confirmed.

In situ monitoring of surface postprocessing in large-aperture fused silica optics with optical coherence tomography

Optical coherence tomography (OCT) is explored as a method to image laser-damage sites located on the surface of large aperture fused silica optics during postprocessing via CO2 laser ablation. The signal analysis for image acquisition was adapted to meet the sensitivity requirements for this application. A long-working-distance geometry was employed to allow imaging through the opposite surface of the 5 cm thick optic. The experimental results demonstrate the potential of OCT for remote monitoring of transparent material processing applications.

Modelization of acoustic waves radiation from sources of complex geometry aperture

The Rayleigh integral giving the impulse response for the acoustic velocity potential cannot always be determined analytically for all types of transducer apertures. The shape of the transducer surface and the spatial distribution of the excitation on its surface can complicate the calculation. This makes using numeric methods indispensable. One of these methods consists in discritizing the aperture surface in polygonal shape elements. Our work makes use of both methods of Jensen and Faure [IEEE UFFC 39-2(1992)262, Acta Acustica 2(1994)257]. In the former, the transducer surface is subdivided in triangles. The latter one uses additional virtual triangles one side of which is that of the physical element and its summit is the field point projection on the plane containing the considered physical element. Additionally, a rotation of the virtual triangle around the field point projection is performed. In this work, the orientations of those virtual triangles are considered according to Jensen method and their contributions to the impulse response are calculated according to the two situations described by Faure. The combination of the two methods is, then, generalized to the case of nonplanar complex surfaces such as concave phased arrays.

Correlation between pressure gradient and phase saturation for oil‐water flow in smooth‐ and rough‐walled parallel‐plate models

The present study is aimed at an investigation of the pressure gradient/phase saturation correlation in the flow of an oil‐water mixture through a parallel‐plate (single fracture) model. It was found that the correlation depends on the interaction between the two flowing fluids, or flow pattern, which is in turn governed by the injection method, water flow rate, viscosity ratio, fracture aperture, and fracture surface roughness. Three distinct flow patterns were identified, namely, channel, dispersed, and mixed flows. Measurements of pressure gradient and phase saturation suggest that the widely used Romm's relative permeability relationship is applicable to channel flow in which oil and water phases are continuous in the fracture. For dispersed or mixed flow in which either the oil or water phase is discontinuous, a Lockhart‐Martinelli type correlation developed for gas‐liquid flow in pipes should be valid at low viscosity ratio but not at high viscosity ratio and water flow rate.

Fluorescence correlation spectroscopy at 100 nM concentrations using near‐field scanning optical microscopic (NSOM) geometries and highly diffracting force sensing fiber probes

AbstractA tapered cantilevered optical fiber probe is introduced for fluorescence correlation spectroscopy (FCS). The probe has an aperture that is coated with metal and its dimension can be very well defined. Its design is based on cantilevered near‐field optical fiber probes that allow for on‐line atomic force feedback. This permits unprecedented stability in FCS measurements. Even such probes that have large diameters are below the cutoff wavelength of light propagation in optical fibers. Such large diameter probes produce bright spots of light that are highly diffracting. Therefore, only the fluence of light very close to the surface of the glass probe aperture is sufficient to exhibit a high probability of fluorescence excitation. From the results presented in this paper the z extent of such probes seems comparable to what can be obtained from prism‐based evanescent wave methods, but with much more flexibility and including force sensing capability. In agreement with theoretical results on prism evanescent field‐based excitation, it is shown that even with large xy dimensions for the fiber probes in this paper, fluorescence correlation spectra from 100 nM fluorophore concentrations can be recorded. This is 10 times larger than the concentrations generally required for conventional confocal FCS. The results suggest that such an approach has considerable potential for applications of FCS in membrane, near‐membrane, solution, and even within‐cell environments. The latter is achieved through atomic force feedback controlled penetration of tapered cantilevered metal‐coated glass fibers into cells, and this is also reported in this paper.

Interference of surface waves in a metallic nanoslit

We investigate the interference of the surface plasmon polariton (SPP) with an incident beam on a metallic slit using the FDTD. We find that the bulk waves radiated at the slit edge by scattering of the SPP leak into the slit and induce accumulated charges within the skin depth, which excite new SPPs on the slit side-walls. The SPP on the top surface of aperture is coupled into the slit and induces the 2D asymmetric field distributions, including the horizontal and vertical Fabry-Perot multi-reflection resonator modes. We show that the addition of these modes with the slit waveguide modes induced by a normally incident beam is the interference between the SPP and the incident beam, which enhances or suppresses the slit transmission, depending on their relative phase.

Generalized beams in ABCDGH systems

For a generalized beam at the source plane passing through co-located aperture and a propagation path consisting of an off-axis x–y asymmetric ABCDGH system, the receiver plane irradiance expression is derived using the Collins integral. A collection of source beam profiles that are obtainable from the generalized beam formulation are illustrated. Plots are given for viewing the progress of selected generalized beam types along the propagation axis, containing a single thin lens, co-centric and misaligned in the x-direction. The received power falling onto a finite aperture surface is calculated for various misalignment situations.

Low-loss subwavelength metal C-aperture waveguide

We present a design of a linear optical waveguide that utilizes a C-shaped metallic nano-aperture that efficiently transports light while maintaining a spot size of lambda/10. The performance of a C-aperture waveguide is superior to both a regular ridge waveguide and other surface plasmon based metal nano-optical waveguides. The energy transport mechanisms are explained by the coupling of an aperture surface resonance and the thickness resonances inside the guide channel. Finite-difference time-domain simulations of gold C-aperture waveguides are performed for a 1.5 microm wavelength incident plane wave. The 1/e decay length in power transmission is predicted to be approximately 2.5 microm. The total power throughput is 1.66 for the 2.55 microm long guide, with an intensity 6 times that of the incident wave at a distance 120 nm from the exit plane, having a spot size of 150 nm.

Feed models for coax-driven monopole and dipole antennas

A method is presented for accurately modeling a monopole or dipole antenna fed by a coaxial line. The base of the monopole is attached to a conducting plane through which the coaxial feed line extends to the feed. The feed structures considered are easily adaptable to physically rugged forms and are simple to construct. Equivalent models for the three regions of the structure are devised and coupled integral equations for aperture fields and surface currents are formulated to enforce the boundary conditions. Three variations of the feed configuration are discussed and the reflection coefficient of the antenna feed is determined from the data obtained from the solutions of the coupled integral equations. Computed reflection coefficient values are shown to agree well with values measured on laboratory models

Negative hydrogen ion sources for accelerators

A variety of H/sup -/ ion sources are in use at accelerator laboratories around the world. A list of these ion sources includes surface plasma sources with magnetron, Penning and surface converter geometries as well as magnetic-multipole volume sources with and without cesium. Just as varied is the means of igniting and maintaining magnetically confined plasmas. Hot and cold cathodes, radio frequency, and microwave power are all in use, as well as electron tandem source ignition. The extraction systems of accelerator H/sup o/n sources are highly specialized utilizing magnetic and electric fields in their low energy beam transport systems to produce direct current, as well as pulsed and/or chopped beams with a variety of time structures. Within this paper, specific ion sources utilized at accelerator laboratories shall be reviewed along with the physics of surface and volume H/sup -/ production in regard to source emittance. Current research trends including aperture modeling, thermal modeling, surface conditioning, and laser diagnostics will also be discussed.

A Hybrid FD–MoM Technique for Predicting Shielding Effectiveness of Metallic Enclosures With Apertures

In this paper, a hybrid technique combining the finite-difference (FD) method and the method of moments (MoM) in the frequency domain is proposed to predict the shielding effectiveness of rectangular conducting enclosures with apertures under external illumination. The interior and exterior regions of the enclosure are analyzed separately by employing the field equivalence principle. Internal electromagnetic fields are discretized using the (FD) method, while external fields are formulated by the MoM. Enforcement of continuity of the tangential magnetic field over the aperture surface gives the desired equation to solve for electromagnetic fields everywhere. Numerical results for the shielding effectiveness of a rectangular cavity with apertures calculated by the new hybrid technique are presented and validated by comparing with experimental data.

Matching of equivalent field regions

In aperture problems, integral equations for equivalent currents are often found by enforcing matching of equivalent fields. The enforcement is made in the aperture surface region adjoining the two volumes on each side of the aperture. In the case of an aperture in a planar perfectly conducting screen, having the same homogeneous medium on both sides and an impressed current on one side, an alternative procedure is relevant. We make use of the fact that in the aperture the tangential component of the magnetic field due to the induced currents in the screen is zero. The use of such a procedure shows that equivalent currents can be found by a consideration of only one of the two volumes into which the aperture plane divides the space. Furthermore, from a consideration of an automatic matching at the aperture, additional information about tangential as well as normal field components is obtained. We compare the two procedures in this tutorial article.

Pollen morphology of families Quillajaceae and Surianaceae (Fabales)

The Fabales clade comprises four families: Leguminosae, Polygalaceae, Quillajaceae and Surianaceae. This study presents new information on the pollen morphology of Quillaja, the only genus of Quillajaceae, and Recchia, Guilfoylia, Cadellia, Suriana and Stylobasium, the five genera that comprise Surianaceae. The pollen of 9 of the 11 species currently recognised within the two families was examined using light microscopy (LM), scanning electron microscopy (SEM) and, selectively, with transmission electron microscopy (TEM). Pollen of all taxa is isopolar with tri-zonocolporate apertures, lalongate endoapertures with fastigia adjacent to the endoaperture, and long ectoapertures that are nearly equal to the polar length. Apocolpia are correspondingly small. Quillaja pollen is subprolate to prolate, and striate with a granular aperture surface membrane. Ectexine protrudes over the endoapertures. In thin section the foot layer is thicker in mesocolpial areas and thin to discontinuous around the apertures, where the endexine is thicker. Cadellia pollen is prolate spheroidal, and striate with a granular aperture surface membrane. Exine protrudes over the endoapertures. In thin section the endexine is thicker and lamellate around the endoaperture area, and the foot layer is thicker in mesocolpial regions. Guilfoylia pollen is oblate and gemmate-verrucate, with a granular aperture surface membrane. Columellae are short. Recchia pollen is suboblate to oblate spheroidal, and microreticulate-perforate with a granular aperture surface membrane. Exine protrudes over the endoapertures. The foot layer is thin to discontinuous around aperture margins and thick in mesocolpial regions. Stylobasium pollen is suboblate, and finely rugulate-perforate with a granular aperture surface membrane. Columellae are short, the foot layer is thin or absent. Suriana pollen is suboblate, and finely rugulate-perforate with a granular aperture surface membrane. Pollen of Cadellia and Recchia, and Stylobasium and Suriana are morphologically similar. Verrucate surface ornamentation is only present in Guilfoylia. Quillaja, Cadellia and Recchia share the character of protruding exine over the endoaperture area. Striate ornamentation occurs in Quillaja and Cadellia. The pollen morphology of Quillajaceae has more in common with that of Leguminosae and Surianaceae, and with Cadellia in particular, than with Polygalaceae.

Realization of a filter radiometer-based irradiance scale with high accuracy in the region from 286 nm to 901 nm

A technique is introduced for realizing spectral irradiance in the range between 286 nm and 901 nm. The scale is based on a temperature-controlled filter radiometer constructed from a three-element silicon trap detector, band-pass filters and a precision aperture. Seasoned FEL-type quartz halogen lamps are used as reference standards. In this method, direct traceability of the radiometer to the cryogenic radiometer gives an important advantage in the visible and near-infrared regions. Another advantage of this system is the procedure followed for measurement of the distance between the lamp filament and aperture surface of the filter radiometer. The expanded uncertainty (k = 2) of the spectral irradiance measurement is between 2.3% and 1.5% in the UV region, while the uncertainty is of the order of 0.6% above 400 nm.

Ultrahigh light transmission through a C-shaped nanoaperture.

Optical resolution beyond the diffraction limit can be achieved by use of a metallic nanoaperture in a near-field optical system. Conventional nanoapertures have very low power throughput. Using a numerical finite-difference time domain method, we discovered a unique C-shaped aperture that provides approximately 3 orders of magnitude more power throughput than a conventional square aperture with a similar near-field spot size of approximately 0.1 lambda. Microwave experiments at 6 GHz quantitatively confirmed the simulated transmission enhancement. The high transmission of the C-aperture--or one of the related shapes--is linked to both a propagation mode in the aperture and local surface plasmons.

Influences of the geometrical conditions of die and workpiece on the barreling formation during forging–extrusion process

The main concern of this study is to investigate the material flow characteristics of solid cylinder upsetting with a hollow die subjected to different geometrical conditions such as taper angle and dimension of the aperture surface within the top die, and height-to-diameter ratio of the billet. During the forging–extrusion process, obviously two barrels, namely top barrel and bottom barrel, are successively formed along the lateral free surface of the workpiece, and an extrusion of the top free surface of the billet into the aperture surface within the top die is accompanied. A finite element-based methodology is utilized to examine the radii of the curvature of the barrels and the height of an extrusion of the top free surface into the aperture surface within the top die accordingly.