Wide Angular Aperture Research Articles

Characterizing the seasonal cycle and vertical structure of ozone in Paris, France using four years of ground based LIDAR measurements in the lowermost troposphere

Systematic ozone LIDAR measurements were completed during a 4 year period (2011–2014) in Paris, France to study the seasonal variability of the vertical structure of ozone in the urban boundary layer. In addition, we use in-situ measurements from the surface air quality network that is located in Paris (AIRPARIF). Specifically, we use ozone and NO2 measurements made at two urban stations: Paris13 (60 m ASL) and the Eiffel Tower (310 m ASL) to validate and interpret the LIDAR profiles. Remote sensed tropospheric NO2 integrated columns from the SAOZ instrument located in Paris are also used to interpret ozone measurements. Comparison between ozone LIDAR measurements averaged from 250 m to 500 m and the Eiffel Tower in-situ measurements shows that the accuracy of the LIDAR (originally ±14 μg·m−3) is significantly improved (±7 μg·m−3) when a small telescope with a wide angular aperture is used. Results for the seasonal cycle of the ozone vertical gradient are found to be similar using two methods: (1) measured differences between AIRPARIF stations with measurements at 60 m ASL and 310 m ASL and (2) using LIDAR profiles from 300 m to the top of the Planetary Boundary Layer (PBL). Ozone concentrations measured by the LIDAR increase with altitude within the PBL, with a steeper gradient in winter (60 μg·m−3·km−1) and a less strong gradient in summer (20 μg·m−3·km−1). Results show that in winter, there is a sharp positive gradient of ozone at the surface, which is explained by ozone titration by NO combined with increased atmospheric stability in winter. In the afternoon during summer, photochemistry and vertical mixing are large enough to compensate for ozone titration near the surface, where NOx is emitted, and there is no gradient in ozone observed. In contrast, in the summer during the morning, ozone has a sharper positive vertical gradient similar to the winter values. Comparison of the vertically averaged ozone concentrations up to (0–3 km) and urban layer (0–310 m) ozone concentrations shows that the ratio between these two quantities is the largest in summer (86%) and the lowest in winter (49%). We conclude that satellite measurements that represent the 0–3 km integrated ozone column are not necessarily a good proxy for surface ozone and may lead to incorrect conclusions about the surface ozone seasonal variability. The ratio between the urban layer NO2 average concentration and the boundary layer NO2 average concentration obtained from SAOZ NO2 tropospheric columns is always less than 50%, meaning NO2 does not decrease linearly in the PBL, but with a sharper decrease close to the surface.

A universal design to realize a tunable perfect absorber from infrared to microwaves.

We propose a design for an universal absorber, characterized by a resonance frequency that can be tuned from visible to microwave frequencies independently of the choice of the metal and the dielectrics involved. An almost perfect absorption up to 99.8% is demonstrated at resonance for all polarization states of light and for a very wide angular aperture. These properties originate from a magnetic Fabry-Perot mode that is confined in a dielectric spacer of λ/100 thickness by a metamaterial layer and a mirror. An extraordinary large funneling through nano-slits explains how light can be trapped in the structure. Simple scaling laws can be used as a recipe to design ultra-thin perfect absorbers whatever the materials and the desired resonance wavelength, making our design truly universal.

Acousto-Optic Devices Based on Potassium Rare-Earth Tungstates Laser Crystals

We present a family of original acousto-optical devices. They are based on new acousto-optical materials KRE(WO4)2, where RE = Y, Gd, Yb, Lu. A combination of features such as radiation resistance, optical biaxiality and appreciable acousto-optical gure of merit, makes possible creation of polarization-insensitive modulator with almost 100% of di raction e ciency. The de ector with wide angular aperture and broad range of de ection, and collinear tunable lter are also suggested.

Filter performance of reduced sized defect photonic crystals based on single‐negative materials

Single‐ and multiple‐channeled filters based on single‐negative (SNG) photonic crystals with inversion defects have been demonstrated theoretically. Omnidirectional filters with wide angular apertures and multiple‐channeled filters at normal incidence have been reported. Sharp, high transmission peaks with frequencies lying within the SNG range are obtained. By variation of the structural parameters these structures can also be made to function as tunable or polarizing filters, though the frequencies of the transmission modes in these two cases will lie outside the SNG range. A significant feature of all the filter types discussed is the drastic reduction in size of the proposed structures in comparison to those reported earlier. This implies easier fabrication methods and lower cost. Improved performance of devices has been achieved by proper placement of defects.

Wide Angular Aperture Circularly Polarized Low-Profile EBG Antenna

This paper describes the design of a compact and wide angular circularly polarized low-profile EBG antenna. Except at 3.7 GHz and forθlower than −25° in the plane equals to 0°, the modelized structure provides an axial ratio lower than 3 dB, over a wide angular aperture of 60° and over a bandwidth of 5.3% ([3.7 GHz–3.9 GHz]). It has a very low height of 11.9 mm (/7 at 3.8 GHz). A prototype has been manufactured, and the measured performances, considering the tolerance of the measurement base (±0.5 dB), are quite similar to the simulated ones.

Structure of sodium above 100 GPa by single-crystal x-ray diffraction

At pressures above a megabar (100 GPa), sodium crystallizes in a number of complex crystal structures with unusually low melting temperatures, reaching as low as 300 K at 118 GPa. We have utilized this unique behavior at extreme pressures to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using high-intensity x-rays from the new Diamond synchrotron source, in combination with a pressure cell with wide angular apertures. We confirm that, at 108 GPa, sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. The results demonstrate the extension of single-crystal structure refinement beyond 100 GPa and raise the prospect of successfully determining the structures of yet more complex phases reported in sodium and other elements at extreme pressures.

Performance of a fuel cell optimized forin situX-ray absorption experiments

A commercial fuel cell has been successfully modified to carry out X-ray absorption spectroscopy (XAS) measurements under optimized in operando conditions. The design is conceived for the performance of XAS experiments in transmission mode over a wide range of X-ray energies above 6 keV, owing to the reduced absorption of the cell. The wide angular aperture allows the collection of XAS in fluorescence mode and of X-ray diffraction patterns when needed. Details of the design of the cell and its performances are given. The quality of the extended X-ray absorption fine-structure spectra under working conditions has been verified at the ESRF and ELETTRA synchrotron radiation facilities, showing that relatively fast and low-noise transmission measurements on electrodes over a wide range of catalyst concentrations and energies are feasible.

Pressure device for resonant magnetic x-ray scattering

A pressure device has been developed and adapted to resonant x-ray magnetic scattering (RXMS) technique. The system is optimized for diffraction studies and allows a wide angular aperture in the scattering plane. Pressure changes are made in situ with pressure determination by the ruby fluorescence method. Since the whole device is compatible with a standard Orange cryostat, an extended (p,T) phase space, with T=1.4–300K and p=0–30kbar, can now be probed by RXMS. As an illustration, first results on Ce(CoxFe1−x)2 single crystal are presented. Such a pressure device can be equally used for other than RXMS techniques as shown with the study at low temperature of SmS under pressure using x-ray fluorescence measurements.

Electro-Static Energy Analyzer with Elliptical Electrode.

The design of electrostatic energy analyzer for charged particles is described in this paper. A program for potential calculation including space charge is used here. The results show that the energy distribution of charged particles can be obtained with good resolution (about 1%) by the combination of elliptical electrodes. This analyzer has a wide angular aperture, thus the high throughput is expected. Also, it can resolve the particleenergy; therefore it can be applied to the short pulse beam measurement like ion scattering spectroscopy (ISS). In conclusion, this analyzer described here will be one of hopeful energy distribution measurements of charged particles like Cylindrical Mirror Analysis (CMA).

Wide angular aperture holograms in photorefractive crystals by the use of orthogonally polarized write and read beams

We demonstrate a method of simultaneous holographic recording and readout in photorefractive crystals that provides high write-read beam isolation and wide angular bandwidth. The method uses orthogonally polarized read and write beams and parallel tangent diffraction geometry near the equal curvature condition to provide spatially separable, orthogonally polarized diffracted output beams with high isolation and wide Bragg-matched angular bandwidth. The available angular bandwidth of this read-write technique is analyzed, simulated, and experimentally investigated. The measured angular bandwidth internal to the crystal is approximately 18° × 6° for our 45°-cut BaTiO(3) crystal, yet the entire hologram still demonstrates high Bragg selectivity. In contrast, traditional nonparallel-tangent geometries yield angular apertures of the order of 1° × 4°.

Head-wave effects on bottom reverberation and scattering strength in shallow water.

For transmitters and receivers having wide vertical angular apertures, there are always at least two paths to and from a rough bottom scattering area: the direct water-borne path and the head-wave path. At long ranges and small grazing angles, contributions to backscatter from these two paths interfere coherently and give rise to complicated reverberent signals that are difficult, if not impossible, to interpret in terms of the conventional scattering strength concept. Numerical simulations using the full-wave two-way PE model in isovelocity shallow-water environments have been used to study reverberation due to bottom roughness when head waves are present. It has been found that backscattering from a rough fluid–fluid interface is not a local phenomenon and cannot properly be described by a cross section per unit area as a function of grazing angle. Only a wave-equation-solving model that combines propagation and scattering can yield correct results. [Work supported by ONR, Code 1125OA.]

Synchrotron radiation focusing by a Bragg–Fresnel lens (invited)

Since the discovery of x rays and until the present time the possibilities of their controlling and focusing have been widely discussed. In the hard spectrum region (λ∼1 Å) the main focusing schemes are the following: geometrical focusing based on incoherent interaction of wave packets reflected by different regions of bending crystals and coherent (dynamic) focusing performed at the cost of the effect of refraction index angular dispersion near the exact Bragg angle value −θB. A main disadvantage of geometrical focusing is low spatial resolution (∼0.1 mm) and temperature stability. In the case of coherent focusing a main disadvantage is a narrow angular aperture (∼10 sec. of arc) at spatial resolution (∼1–10 μm). Recently, advances in the development of diffraction physics and microstructuring technology open up possibilities for fabricating effective focusing x-ray optical elements—Bragg–Fresnel lenses (BFL)—with high spatial resolution (∼0.1 μm) at a wide angular aperture and high temperature stability. The present paper describes the main principles of Bragg–Fresnel optics (BFO). It presents the results on the synchrotron experiment and on observation of focusing. In this work the peculiarities of BFL diffraction contrast formation are investigated and image transmission using a BFL is performed. Possibilities of developing x-ray optical schemes of ultrahigh resolution on the basis of BFL elements are also discussed.

Corrected electrostatic lens systems for ion beams

Recent work in our laboratory has introduced a new class of electrostatic focus forming element for beams, the ELCO lens. It compares favourably with the electrostatic and magnetic quadrupole elements conventionally used for microbeam formation. The ELCO lens does however have disadvantages associated with apertures and alignment. We have continued with the development of ion beam lenses and have evolved a further class of lens elements which eliminates aperture and alignment problems. This new element can be combined like optical lenses into an aberration corrected system. Experimental measurement on the basic lens element has confirmed mathematical analysis of ion trajectories through the element. This mathematical analysis predicts that the basic element can be combined into a corrected lens system for, either: 1. (1)high resolution microprobe formation with intrinsic rastering ability, the spot size limited only by the beam properties; or 2. (2)high quality image formation with large magnification/demagnification ratio and wide angular aperture.

Electromagnetic field near the focus of wide-angular lens and mirror systems

The time-averaged electric energy density near the focus of a parabolic mirror of wide-angular aperture is calculated and compared with that of an aplanatic lens system. The distributions exhibit great differences, the mirror showing a greater departure from circular symmetry, a more complicated structure and increased sidelobes. The vectorial integral representation of Wolf is derived from the Stratton-Chu formula.

Improvement of the Resolving Power and Transmission of Electrostatic Spectrometers

A method proposed by Delage for the calculation of electron energy distributions has previously been generalized and applied to various types of electrostatic spectrometers, that is, to toroidal-type and mirror-type spectrometers. Thus, it has been found that spectrometers with wide angular apertures yield an asymmetric energy distribution which may have an important tail on the high-energy side. The analysis of a typical distribution shows that this tail is caused by the angular dispersion of the trajectories; because of the filtering action of slits and electrodes, the positive and negative angular components of the beam contribute unequally to the energy spectrum. By changing the way that the beam is filtered, it is possible to improve the energy distribution profile, without or with little loss of transmitted current. The improvement of the resolution may thus reach 30% or more, as the result of only a small displacement of the exit slit either on the exit equipotential, or toward the beam. Most of these results are new, principally in the case of the spectrometers of the toroidal type where the optimum deflection angle is greater than the focusing angle. Moreover, from Wollnik's conclusions about shielding from fringing field in toroidal spectrometers, supplementary calculations are performed to extend the validity of other results to more practical cases. Finally, results obtained for the cylindrical mirror are found in good agreement with conclusions presented before by Hafner, Kuyatt, and Simpson.

Video-frequency light modulator having a wide angular aperture

This paper discusses the 45° ycut ADP modulator. An investigation of the propagation of light through the crystal shows that there is an inherent limitation in the angular aperture of the simple 2-crystal modulator. The inclusion of a halfwave plate between the crystals to improve the temperature stability also enables large angular apertures to be achieved, provided the crystals are correctly aligned. Results of measurements carried out on experimental modulators are presented and a comparison made with the 45° zcut modulator.