Complex Apertures Research Articles

New analytical diffraction expressions for the Fresnel–Fraunhofer transition regime

We propose a method to obtain improved expressions for the diffraction of circular apertures in the boundary between near and far field. The method is based on an analysis of the center value of the diffraction pattern derived from the Fresnel expression. The method is shown to be applied to both amplitude, phase, and complex apertures, and is compared to Fresnel propagation in simulations. We show how these new expressions allow for the use of the Fraunhofer approximation at Fresnel numbers at least twice as large as the conventional expressions. This facilitates analytical Fraunhofer propagation to be used for much larger apertures, at shorter propagation distances, and with more complex aperture transmission functions.

An Analytical Evaluation of the Shielding Effectiveness of Enclosures Containing Complex Apertures

Analytical methods are significantly useful tools for engineers in computational modeling. These methods provide fast simulations, while preserving physical meaning. In this paper, a full analytical formulation is developed based on the circuit approach, also known as Intermediate Level Circuit Model (ILCM), to evaluate the shielding effectiveness (SE) of metallic structures. The proposal focuses on rectangular enclosures containing apertures of complex geometrical shape, illuminated by an external plane wave. The enclosure is taken as a short-circuited waveguide. The aperture is treated as a transmission line with an intrinsic impedance that depends on the aperture polarizability, according to Bethe’s theory of small holes. The formulation easily takes into account high-order resonant modes and different apertures, such as circular, elliptical, round-ended and cross-shaped apertures. The ILCM technique reveals good agreement with a numerical full-wave method for various configurations of enclosures, covering a large range of frequencies from 0.5 GHz to 2.5 GHz, thus offering perspectives for parametric and/or optimization SE studies.

Removing image artefacts in wire array metamaterials.

Hyperlenses and hyperbolic media endoscopes can overcome the diffraction limit by supporting propagating high spatial frequency extraordinary waves. While hyperlenses can resolve subwavelength details far below the diffraction limit, images obtained from them are not perfect: resonant high spatial frequency slab modes as well as diffracting ordinary waves cause image distortion and artefacts. In order to use hyperlenses as broad-band subwavelength imaging devices, it is thus necessary to avoid or correct such unwanted artefacts. Here we introduce three methods, namely convolution, field averaging, and power averaging, to remove imaging artefacts over wide frequency bands, and numerically demonstrate their effectiveness based on simulations of a wire medium endoscope. We also define a projection in spatial Fourier space to effectively filter out all ordinary waves, leading to considerable reduction in image distortion. These methods are outlined and demonstrated for simple and complex apertures.

Late Paleocene‐middle Eocene benthic foraminifera on a Pacific seamount (Allison Guyot, ODP Site 865): Greenhouse climate and superimposed hyperthermal events

AbstractWe investigated the response of late Paleocene‐middle Eocene (~60–37.5 Ma) benthic foraminiferal assemblages to long‐term climate change and hyperthermal events including the Paleocene‐Eocene Thermal Maximum (PETM) at Ocean Drilling Program (ODP) Site 865 on Allison Guyot, a seamount in the Mid‐Pacific Mountains. Seamounts are isolated deep‐sea environments where enhanced current systems interrupt bentho‐pelagic coupling, and fossil assemblages from such settings have been little evaluated. Assemblages at Site 865 are diverse and dominated by cylindrical calcareous taxa with complex apertures, an extinct group which probably lived infaunally. Dominance of an infaunal morphogroup is unexpected in a highly oligotrophic setting, but these forms may have been shallow infaunal suspension feeders, which were ecologically successful on the current‐swept seamount. The magnitude of the PETM extinction at Site 865 was similar to other sites globally, but lower diversity postextinction faunas at this location were affected by ocean acidification as well as changes in current regime, which might have led to increased nutrient supply through trophic focusing. A minor hyperthermal saw less severe effects of changes in current regime, with no evidence for carbonate dissolution. Although the relative abundance of infaunal benthic foraminifera has been used as a proxy for surface productivity through bentho‐pelagic coupling, we argue that this proxy can be used only in the absence of changes in carbonate saturation and current‐driven biophysical linking.

Microfossil evidence for trophic changes during the Eocene–Oligocene transition in the South Atlantic (ODP Site 1263, Walvis Ridge)

Abstract. The biotic response of calcareous nannoplankton to environmental and climatic changes during the Eocene–Oligocene transition was investigated at a high resolution at Ocean Drilling Program (ODP) Site 1263 (Walvis Ridge, southeast Atlantic Ocean) and compared with a lower-resolution benthic foraminiferal record. During this time interval, global climate, which had been warm under high levels of atmospheric CO2 (pCO2) during the Eocene, transitioned into the cooler climate of the Oligocene, at overall lower pCO2. At Site 1263, the absolute nannofossil abundance (coccoliths per gram of sediment; N g−1) and the mean coccolith size decreased distinctly after the E–O boundary (EOB; 33.89 Ma), mainly due to a sharp decline in abundance of large-sized Reticulofenestra and Dictyococcites, occurring within a time span of ~ 47 kyr. Carbonate dissolution did not vary much across the EOB; thus, the decrease in abundance and size of nannofossils may reflect an overall decrease in their export production, which could have led to variations in the food availability for benthic foraminifers. The benthic foraminiferal assemblage data are consistent with a global decline in abundance of rectilinear species with complex apertures in the latest Eocene (~ 34.5 Ma), potentially reflecting changes in the food source, i.e., phytoplankton. This was followed by a transient increased abundance of species indicative of seasonal delivery of food to the sea floor (Epistominella spp.; ~ 33.9–33.4 Ma), with a short peak in overall food delivery at the EOB (buliminid taxa; ~ 33.8 Ma). Increased abundance of Nuttallides umbonifera (at ~ 33.3 Ma) indicates the presence of more corrosive bottom waters and possibly the combined arrival of less food at the sea floor after the second step of cooling (Step 2). The most important changes in the calcareous nannofossil and benthic communities occurred ~ 120 kyr after the EOB. There was no major change in nannofossil abundance or assemblage composition at Site 1263 after Step 2 although benthic foraminifera indicate more corrosive bottom waters during this time. During the onset of latest-Eocene–earliest-Oligocene climate change, marine phytoplankton thus showed high sensitivity to fast-changing conditions as well as to a possibly enhanced, pulsed nutrient supply and to the crossing of a climatic threshold (e.g., pCO2 decline, high-latitude cooling and changes in ocean circulation).

A SIMPLE TECHNIQUE FOR OPTIMIZING THE IMPLEMENTATION OF THE APERTURE THEOREM BASED ON EQUIVALENCE PRINCIPLE

The electromagnetic characteristic of the aperture located on a PEC (Perfect Electric Conductor) cavity is an important and challenging research in CEM (Computational Electromagnetics) and practical applications. Researches have been done well when the apertures located on a large ∞at surface. But the complex slots and apertures are still di-cult to analyze, such as a thin long slot. The thin long slots present on difierent kinds of structures, such as missiles, aircrafts, handset equipments, and computers. In addition, most of the surfaces are non-∞at. Furthermore, the multiscale characteristic of the structure makes the modeling very di-cult in such cases. It becomes an increasing interesting research recently. A better result can be obtained by generating much more denser meshes. Because of the complexity of the algorithm and the ill-posed matrix problem, it is not an optimized option. In order to get a better use of the aperture theorem in the multiscale problems, a separation technique is developed in this paper. By using the readjustment of the equivalent electric and magnetic currents, a simplifled model is proposed. Arbitrary shaped aperture can be very well handled through this method, especially the thin long slots.

BAND-LIMITED CORONAGRAPHS USING A HALFTONE-DOT PROCESS. II. ADVANCES AND LABORATORY RESULTS FOR ARBITRARY TELESCOPE APERTURES

The band-limited coronagraph is a nearly ideal concept that theoretically enables perfect cancellation of all the light of an on-axis source. Over the past years, several prototypes have been developed and tested in the laboratory, and more emphasis is now on developing optimal technologies that can efficiently deliver the expected high-contrast levels of such a concept. Following the development of an early near-IR demonstrator, we present and discuss the results of a second-generation prototype using halftone-dot technology. We report improvement in the accuracy of the control of the local transmission of the manufactured prototype, which was measured to be less than 1%. This advanced H-band band-limited device demonstrated excellent contrast levels in the laboratory, down to 10-6 at farther angular separations than 3 lambda/D over 24% spectral bandwidth. These performances outperform the ones of our former prototype by more than an order of magnitude and confirm the maturity of the manufacturing process. Current and next generation high-contrast instruments can directly benefit from such capabilities. In this context, we experimentally examine the ability of the band-limited coronagraph to withstand various complex telescope apertures.

New species of <i>Geissorhiza</i> (Iridaceae: Crocoideae) from the southern African winter rainfall zone, range extensions, taxonomic changes, and notes on pollen morphology and floral ecology

Field work during the past 15 years has resulted in the discovery of 12 new species of the western southern African genus Geissorhiza Ker Gawl. and range extensions for several more. Following a survey of pollen morphology in the genus, we describe new pollen types in one section each of the two subgenera: five species of section Ciliatae Goldblatt and two of section Weihea Eckl. ex Baker have more complex apertures than the plesiomorphic single aperture with a 2-banded oper­culum found in other species. In addition, populations currently referred to the typical white (cream)-flowered G. inflexa (D.Delaroche) Goldblatt with larger, pink, red or purple flowers, have a third pollen type and are recognized as G. erosa (Salisb.) R.C.Foster. The new species are G. altimontana from the high Langeberg near Grootvadersbos; G. helmei from the Piketberg; G. lupidosa from the Du Toits Kloof Mtns; G. monticola from the Swartberg; G. platystigma from Darling, north of Cape Town; G. sufflava from the Piketberg; G. tricolor from Riversdale (all subgenus Weihea (Eckl. ex Baker) Goldblatt); and G. cantharophila from the Klein Roggeveld; G. demissa from the Kamiesberg. Gitberg and Cold Bokkeveld; G. exilis from the Waaihoek Mtns in the Worcester District; G. reclinata from the Swartberg: and G. saxicola from the northern Cedarberg-Pakhuis Mountain complex (all subgenus Geissorhiza). We also report range extensions and provide morphological notes for several species, including G. monanthos Eckl.. new collections o f w hich show that the inclusion of G. leuisiae R.C.Foster in that species was incorrect and we resurrect the species. The addition of 12 new species and recognition of G. erosa and G. lewisiae bring to 99 the number of species in the genus. New identification keys for Geissorhiza are provided that include all new species. We have also accumulated observations of floral ecology in the genus and integrate them with what is known about this aspect of the biology of Geissorhiza. Unusual strategies include deceptive pollination in G. tulhagh- ensis F.Bolus by tabanid flies and in two species, pollination using empid flies in combination w ith halictid bees

Complementary Golay series in multislit dispersion optical spectroscopy

This paper proposes a new application of the complementary Golay series in multislit optical dispersion spectroscopy that differs from traditional dispersion optical spectroscopy in that it uses complex multislit apertures instead of ordinary slits in order to increase the speed. More precisely, based on the given series, new pairs of numerical series have been developed that have a single maximum of the correlation function and that can be used in multislit spectrometers to construct multislit apertures and to search for spectra. The series presented here differ from known analogs by greater simplicity for such a high speed. The possibility of implementing the proposed series is demonstrated using several spectrometer layouts as examples.

Rigorous formulation for electromagnetic plane-wave scattering from a general-shaped groove in a perfectly conducting plane: comment

We show that the problem of scattering of an obliquely incident plane wave by a general-shaped groove engraved on a perfectly conducting plane, which was recently studied by Basha et al. [J. Opt. Soc. Am. A24, 1647 (2007)], was solved 11 years ago using the same formulation. This method was further extended to deal with a finite number of grooves and also with complex apertures including several nonlossy and lossy dielectrics, as well as real metals.

Semi-empirical modeling of apertures for shielding effectiveness simulations

A novel method to include models of complex apertures into the finite-difference time-domain (FDTD) method is presented. Instead of resolving the geometrical details of the aperture, the aperture is treated as a magnetic dipole. The properties of the magnetic dipole moment are determined using the measured transmission cross section of the aperture. This semi-empirical model permits inclusion of complex apertures in FDTD simulations where the smallest dimension of the aperture is only a fraction of the FDTD cell size. Two types of complex apertures positioned in the top panel of a realistic enclosure have been modeled using the semi-empirical model in FDTD. Comparisons between simulated and measured shielding effectiveness of the enclosure are presented, and the results show that complex apertures can indeed be represented by simple magnetic dipoles in shielding effectiveness simulations.

Scattering From Thick Frequency Selective Screens

A plane wave impinges on a perfectly conducting thick screen perforated with a periodic array of apertures with arbitrary cross-section. The transmission and reflection coefficients are determined by a mode matching technique, earlier used by other authors for the case where the apertures have the geometry of a classical waveguide. The fields outside the screen are expanded in Floquet modes, while the fields inside the apertures are expanded in waveguide modes, obtained by the finite element method (FEM). Excellent agreement between computed and measured transmission is found for a thick screen at the X-band (8.2-12.4 GHz). Moreover, the present method and the spectral Galerkin method (which assumes that the thickness of the screen is zero) is compared for very thin screens, e.g., for a frequency selective surface (FSS). Investigation of the thickness dependence is also done, and it is found that the bandwidth decreases when the thickness dependence is also done, and it is found that the bandwidth decreases when the thickness of the screen is increased. Finally two complex geometries, hexagonal and tripole apertures, are studied.

Pollen ultrastructure of Panax(the ginseng genus, Araliaceae),an eastern Asian and eastern NorthAmerican disjunct genus

Pollen of ten species of Panax and six species of Aralia was examined in light microscopy and scanning and transmission electron microscopy. Grains of both genera have similar complex apertures, short columellae, and overlapping tectal sculptures, suggesting a close relationship. Most species of Panax have pollen characterized by striato-reticulate tecta, short columellae, thick foot layers, costa ectocolpi, and lalongate endoapertures. The eastern North American P. trifolius, commonly known as the dwarf ginseng, has a distinctive pollen morphology and exine structure, supporting the hypothesis of its phylogenetically isolated position. Pollen of the eastern Asian P. ginseng (ginseng) can be distinguished from the eastern North American P. quinquefolius (American ginseng) by differences in ultrastructure. The monophyly of the three medicinally important species, P. ginseng, P. notoginseng, and P. quinquefolius, suggested by triterpenoid data, is not supported by pollen data. The results of the pollen study are generally congruent with those from the sequences of nuclear ribosomal DNA.

Coded imaging of thermonuclear neutrons (invited)

Imaging of thermonuclear neutrons from inertial confinement fusion pellets is reviewed. The most promising technique, penumbral imaging, is discussed in detail and the design criteria are outlined. Simulations are presented. More advanced aspects are also briefly discussed, such as nonlinear image reconstruction algorithms and more complex apertures.

Finite-difference time-domain (FD-TD) modeling of electromagnetic wave scattering and interaction problems

This article, based upon invited papers at the XXII URSI General Assembly (Tel Avic, August 1987) and the URSI National Radio Science Meeting (Boulder, January 1988), reviews recent applications of the finite-diffrerence time-domain (FD-TD) method for numerical modeling of electromagnetic waves scattering and interaction problems. One of the goals of this article is to demonstrate that recent advances in FD-TD modeling concepts and software implementation, combined with advances in computer technology, have expanded the scope, accuracy, and speed of FD-TD choice for structures that cannot be easily treated by conventional integral equation and asymptotic approaches. As a class, such structure are electrically large and have complex shapes, material compositions, apertures, and interior cavities

Aperture plates for Fraunhofer diffraction

An undergraduate experiment on Fraunhofer diffraction is described. The apertures are made by electron beam lithography in a chromium film on an optical flat, giving a marked improvement of performance over glass photographic plates. Computer control of the electron beam allows complex repetitive apertures to be constructed giving clear demonstrations of the principles of Fraunhofer diffraction. The light source is a 1 mW helium-neon laser. Patterns from simple apertures may be measured to obtain a quantitative test of the theory and more complex apertures illustrate theory such as the use of superposition and the convolution theorem.

Diffraction Plates for Classroom Demonstrations

The diffraction plate, which consists of an array of many identical apertures or obstacles, produces large Fraunhofer diffraction patterns of sufficiently high intensity such that they may be seen easily with the unaided eye when used with conventional sources. If a small, bright source is observed through a diffraction plate held close to the eye, the Fraunhofer pattern of the apertures appears around the image of the source. If the apertures are randomly distributed over the plate, the pattern produced is characteristic of any one of the apertures in the plate. The pattern produced by a regular array has characteristics of both the individual aperture and of the arrangement. The beautifully colored patterns produced by a white light source are easily seen and photographed. This article describes the computer generation of random and regular arrays of apertures and their applications for classroom demonstrations of the Fraunhofer patterns produced by simple and complex apertures, Babinet's principle, resolution according to the Rayleigh criterion, and many other important aspects of diffraction.