Regions Of Low Reflectivity Research Articles

Allochroic effect of praseodymium doped neodymium molybdate pigment synthesized by solid-state reaction

This paper was to synthesize praseodymium-doped neodymium molybdate allochroic pigments with different molar ratios of molybdenum (Mo), neodymium (Nd) and praseodymium (Pr) through solid-state reactions. The pigments synthesized were investigated by X-ray diffraction (XRD), selected area electron diffraction (SAED), thermogravimetry and differential thermal analysis (TG-DSC), diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS) and colorimeter. The results show that a pure neodymium molybdate (i.e., Nd2MoO6) can be formed after sintering at 1000 °C for 3 h, and praseodymium-doped neodymium molybdate can be synthesized after sintering at 1000 °C for 3 h due to praseodymium ions entering in the lattice of Nd2MoO6. The crystal structure in the pigment synthesized can transform from cubic to tetragonal depending on the molar ratio of neodymium/molybdenum and sintering procedure. The color properties of pigments were measured in color space CIEL*a*b* under different standard illuminants (i.e., solar-light (D65), incandescent light (A), cool white fluorescent lamp (F2), and three-band fluorescent lamp (F11)). The results show that the colors of the pigments vary under different illuminants. According to the color difference (ΔE) analysis, neodymium molybdate pigment exhibits a maximum color difference of 10.69, and the maximum color difference of praseodymium-doped neodymium molybdate pigment with doping content of 0.2 increases to 17.63 due to the Pr3+ ions substitution for Nd3+ ions in the lattice. In addition, the color difference of the pigments with allochroic effect was discussed based on the colorimetry. The color difference of the pigments is due to a high reflectance region at 540–550 nm and a low reflectance region at 380–500 nm.

Investigation of a supercell merger leading to the EF4 tornado in the Czech Republic on June 24, 2021 using radar data and numerical model outputs

An unprecedented deadly and destructive EF4 tornado struck the Czech Republic across Břeclav and Hodonín districts on June 24, 2021. On this day, several supercells developed in Central Europe, however, in Austria and the Czech Republic region only one cell produced a tornado. For this reason, in addition to the macrosynoptic setup, it is also worth exploring the small-scale cell interactions that can lead to the formation of a devastating EF4 tornado. We use ECMWF analysis and forecast fields, sounding profiles, and radar measurements to examine the synoptic weather situation and convective processes. Moreover, to investigate the evolution and structure of convection, two Weather Research and Forecasting (WRF) model simulations were carried out at 1.5 km grid spacing with onemoment and two-moment microphysical parameterizations. WRF captures the overall spatial distribution and supercellular nature of thunderstorms, although discrepancies exist in the magnitude and spatial location of individual cells. The low-reflectivity region accompanying the thunderstorms is better represented by the one-moment microphysics scheme.

Proterozoic tectonothermal processes imaged with magnetotellurics and seismic reflection in southern Australia

Over the last two decades, co-located seismic and magnetotelluric (MT) profiles provided fundamental geophysical data sets to image the Australian crust. Despite their complimentary nature, the data are processed and often interpreted separately without common processes in mind. We here qualitatively compare 2D resistivity inversion models derived from MT and seismic reflection profiles across a region of Archean–Proterozoic Australia to address the causes of variations in seismic response and anomalous conductivity in the crust. We find that there exists a spatial association between regions of low reflectivity in seismic sections and low resistivity in co-located 2D MT modelled sections. These relationships elucidate possible signatures of past magmatic and fluid-related events. Depending on their diffuse or discrete character, we hypothesize these signatures signify fossil melting of the crust due to mafic underplating, magma movement or hydrothermal fluid flow through the crust. The approach discussed herein is a process-oriented approach to interpretation of geophysical images and a significant extension to traditional geophysical methods which are primarily sensitive to a singular bulk rock property or state.

Robust 3D surface recovery by applying a focus criterion in white light scanning interference microscopy.

White light scanning interference (WLSI) microscopes provide an accurate surface topography of engineered surfaces. However, the measurement accuracy is substantially reduced in surfaces with low-reflectivity regions or high roughness, like a surface affected by corrosion. An alternative technique called shape from focus (SFF) takes advantage of the surface texture to recover the 3D surface by using a focus metric through a vertical scan. In this work, we propose a technique called SFF-WLSI, which consists of recovering the 3D surface of an object by applying the Tenegrad Variance (TENV) focus metric to WLSI images. Extensive simulation results show that the proposed technique yields accurate measurements under different surface roughness and surface reflectivity, outperforming the conventional WLSI and the SFF techniques. We validated the simulation results on two real objects with a Mirau-type microscope. The first was a flat lapping specimen with Ra=0.05 μm for which we measured an average value of Ra=0.055 μm and standard deviation σ=0.008 μm. The second was a metallic sphere with corrosion, which we reconstructed with WLSI versus the proposed SFF-WLSI technique, producing a better 3D reconstruction with less undefined depth values.

The crustal geophysical signature of a world-class magmatic mineral system

World-class magmatic mineral systems are characterised by fluid/melt originating in the deep crust and mantle. However, processes that entrain and focus fluids from a deep-source region to a kilometre-scale deposit through the crust are unclear. A magnetotelluric (MT) and reflection seismic program across the margin of the Gawler Craton, Australia yield a distinct signature for a 1590 Ma event associated with emplacement of iron-oxide copper gold uranium (IOCG-U) deposits. Two- and three-dimensional MT modelling images a 50 km wide lower-crustal region of resistivity <10 Ωm along an accreted Proterozoic belt. The least resistive (~1 Ωm) part terminates at the brittle-ductile transition at ~15 km, directly beneath a rifted sedimentary basin. Above the brittle-ductile transition, three narrow low-resistivity zones (~100 Ωm) branch to the surface. The least resistive zone is remarkably aligned with the world-class IOCG-U Olympic Dam deposit and the other two with significant known IOCG-U mineral occurrences. These zones are spatially correlated with narrow regions of low seismic reflectivity in the upper crust, and the deeper lower-crust conductor is almost seismically transparent. We argue this whole-of-crust imaging encapsulates deep mineral system and maps pathways of metalliferous fluids from crust and mantle sources to emplacement at discrete locations.

Optical Properties of Chemical Vapour Etching Based Porous Silicon for Multicrystalline Solar Cells

In this paper, we report a study on the possibility of fabricating porous silicon by exposing a multicrystalline silicon surface to gaseous etchants. The structural and optical properties of porous silicon (PS) layers prepared by vapour –etching (VE) are investigated. FTIR analysis confirms the existence of hydrogen incorporation bonding to the silicon atoms. Photoluminescence measurements reveal an efficient emission around 640 nm. The optical behaviour in the 350-1000 nm wavelength range was determined before and after PS formation, resulting in a notable reduction of reflectance and an enlargement of low reflectance region into short wavelengths and near IR region after PS formation. A significant increase of the quantum efficiency particularly in the short wavelength region is observed. The results make the use of such thin film very promising for multicrystalline silicon solar cell application.

The Generation of Near-Cloud Turbulence in Idealized Simulations

Abstract This study explores the generation of turbulence in the upper outflow regions of simulated idealized mesoscale convective systems. The simulated storms are shown to generate parameterized turbulence that occurs significant distances (&amp;gt;100 km) from the main convective regions, in both the clear air surrounding the convection and low simulated reflectivity regions with cloud ice but negligible amounts of graupel and snow. The source of the turbulence is related to Kelvin–Helmholtz instabilities that occur in the shear zones above and below the storm-induced upper-level outflow jet that is centered near the tropopause; the model produces resolved-scale billows within regions of low gradient Richardson number. Short-scale gravity waves are also coincident with the regions of turbulence, become trapped within the jet core, and appear to be generated by the shear instability. Additional experiments with different initial upper-level wind shear show similar mechanisms to those simulations with no initial upper-level shear. These results help elucidate the dynamics of turbulence generation near convection, which has important implications for the aviation industry and the fundamental understanding of how convective clouds interact with their environment.

Analytical Localization Lengths in an One-Dimensional Disordered Electron System

Abstract Analytical approximations of the Lyapunov exponent are derived for a random displacement model with equal potential barriers and random positions of the scatterers. Two asymptotic regions are considered corresponding to high and low reflectivity of the single scattering potential. The analytical results are in terms of a distribution function W for certain phases of the transfer matrices. A functional equation for W is derived and numerically solved. This serves to validate the analytical asymptotic formulas which turn out to be accurate in the high and low reflectivity regions with dimensionless wave number K &lt; 2 and K &gt; 6, respectively. The high wave number asymptotics allows for an analytical examination of the sufficient conditions for Anderson localization

Relationship of the Optical Coherence Tomography Signal to Underlying Retinal Histology in the Tree Shrew (Tupaia belangeri)

To interpret the retinal origin of the optical coherence tomography (OCT) signal by objectively (i.e., minimal investigator bias) aligning in vivo OCT longitudinal reflectivity profiles (LRPs) with corresponding vertical histologic sections. The Zeiss StratusOCT system was used to obtain retinal B-scans in vivo in eyes from adult tree shrews. Subsequently, the retinas were fixed and embedded. Semithin vertical sections through the retina were obtained from the same locations as the LRPs. A statistical correlation procedure that accounted for axial tissue shrinkage determined the best relationship between features in the LRP and sublaminae boundaries in corresponding histology sections. For the optimal relationship, the three regions of high reflectivity in the inner OCT signal corresponded to (1) the nerve fiber and ganglion cell layers, (2) the inner plexiform layer and amacrine cell somas, and (3) the outer plexiform layer. The two regions of low reflectivity in the inner OCT signal corresponded to (1) the somas of Müller, bipolar, and horizontal cells in the inner nuclear layer and (2) the outer nuclear layer. The outer OCT signal had a region of high reflectivity that corresponded to the photoreceptor inner and outer segments, the pigment epithelium, Bruch's membrane, and at least part of the choriocapillaris. These results provide a clear interpretation for the OCT signal in terms of the underlying retinal anatomy. This interpretation can be used in vivo to identify sublaminae affected by retinal disease and has implications for the origin of the inner OCT signal in human retina.

Magnetotelluric evidence for a deep-crustal mineralizing system beneath the Olympic Dam iron oxide copper-gold deposit, southern Australia

The iron oxide copper-gold Olympic Dam deposit, situated along the margin of the Proterozoic Gawler craton, South Australia, is the world's largest uranium deposit and sixth-largest copper deposit; it also contains significant reserves of gold, silver, and rare earth elements. Gaining a better understanding of the mechanisms for genesis of the economic liberalization is fundamental for defining exploration models in similar crustal settings. To delineate crustal structures that may constrain mineral system fluid pathways, coincident deep crustal seismic and magnetotelluric (MT) transects were obtained along a 220 km section that crosses Olympic Dam and the major crustal boundaries. In this paper we present results from 58 long-period (10–104 s) MT sites, with site spacing of 5– 10 km. A two-dimensional inversion of MT data from 33 sites to a depth of 100 km shows four notable features: (1) sedimentary cover sequences with low resistivity ( 1000 Ω·m) Archean crustal core from a more conductive crust and mantle to the north (typically <500 Ω·m); (3) to the north of Olympic Dam, the upper-middle crust to ∼20 km is quite resistive (∼1000 Ω·m), but the lower crust is much more conductive (<100 Ω·m); and (4) beneath Olympic Dam, we image a low-resistivity region (<100 Ω·m) throughout the crust, coincident with a seismically transparent region. We argue that the cause of the low-resistivity and low-reflectivity region beneath Olympic Dam may be due to the upward movement of CO2-bearing volatiles near the time of deposit formation that precipitated conductive graphite liberalization along grain boundaries, simultaneously annihilating acoustic impedance boundaries. The source of the volatiles may be from the mantle degassing or retrograde metamorphism of the lower crust associated with Proterozoic crustal deformation.

Readout Power Dependence of Signal Distribution Observed in Fourier Plane of Focus Spot

We describe the readout-power dependence of the properties optical readout signals, and discuss the super-resolution mechanism in platinum-oxide super-resolution near-field structure (super-RENS) disks. From the Fourier analysis of a focused spot, it is clear that a low-reflectivity region within the beam spot during readout above the threshold readout power is generated in a special-temperature region, which, in turn, is associated with the readout mechanism. These phenomena suggest that such readout is related to a large change in the optical properties of a phase change layer, as proposed in the ferroelectric catastrophe model.

Efficient and individually controllable mechanisms for mode and polarization selection in VCSELs, based on a common, localized, sub-wavelength surface grating

We have theoretically investigated the combined fundamental-mode and polarization selection in 850-nm oxide-confined vertical-cavity surface-emitting lasers (VCSELs) using a locally etched sub-wavelength surface grating. The physical mechanisms behind the selection are, first, the strongly polarization sensitive effective refractive index of the volume occupied by the grating structure, and second, the dramatic change of the reflectivity of a multi-layer Bragg mirror that can occur by simply changing the refractive index of the outermost layer. For a VCSEL cavity this layer is the surface layer and its refractive index is changed by the introduction of the sub-wavelength grating; in this case the grating leads to a higher reflectivity for the desired polarization. By localizing the surface grating area to a carefully chosen region near the optical axis it is therefore possible to ensure that the fundamental mode experiences a high reflectivity, or low cavity loss, while other modes experience more of the low-reflectance region of the peripheral part of the Bragg mirror and thus suffer higher loss. Cold-cavity calculations on a VCSEL with oxide aperture and grating region diameters of 4.5 microm and 2.5 microm, respectively, indicate that a loss difference of ~20 cm(-1) between the fundamental mode and the first higher order mode can be obtained simultaneously with an orthogonal polarization mode discrimination of >15 cm-1. Based on previous experience, these values should enable robust single-mode operation with only the desired polarization orientation. What is also important, for the lasing mode the introduction of a sub-wavelength grating has no detrimental effect, so its characteristics, such as threshold current, slope efficiency, and far-field profile are unaffected. Moreover, since the effective index is a result of an averaging over several sub-wavelength grating periods, it is fairly insensitive to the detailed shape of the grating grooves, which should relax the fabrication tolerances.

The Origin of Belt/Zone Contrasts in the Atmosphere of Jupiter and Their Correlation with 5-μm Opacity

The commonly held interpretation of the observed banded structure of the jovian atmosphere is that “zones” are regions of dense clouds and thus high reflectivity and low thermal emission, while “belts” are relatively cloud-free regions of low reflectivity but high thermal emission. However, it has not been possible with existing data to identify unambiguously which of a number of possible jovian cloud layers (J. S. Lewis 1969, Icarus 10, 365–378) is mainly responsible for these variations. New data from the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft have been analyzed which have good spectral coverage at both near-IR and 5-μm wavelengths over wide areas. These new data provide compelling evidence that the main source of the near-IR reflectivity variability, observed to be anticorrelated with 5-μm brightness, is due to opacity changes of cloud layers lying at pressures between 1 and 2 bar, and not to variability of the higher altitude ammonia cloud, the other main likely candidate. However, whether this main variable cloud layer is composed of ammonium hydrosulfide, the upper levels of a deeper water ice cloud, or something else is unknown. We also find that most of the particles in the upper troposphere must be rather small, in order that their reflectivity drops rapidly with wavelength, but we deduce that there must also be a component of larger sized particles, probably composed of ammonia ice, to account for the correlation between the 5- and 45-μm emission observed by Voyager. However, although a significant opacity of large ammonia particles does provide a good fit to the observed cloud absorption feature near 3-μm, these particles introduce a number of unobserved features at other wavelengths; thus we conclude that their opacity must be small.

Single shot laser damage in ultraviolet mirrors with a stepwise reflectivity profile

The photoacoustic and photothermal beam deflection techniques and microscopic inspection have been used to measure the laser damage threshold of mirrors with a stepwise reflectivity profile designed for KrF excimer lasers (248 nm). Laser damage resistance studies on the low and high reflectivity regions of the mirrors, as well as on the border between the high and low reflectivity regions, have been performed with a KrF laser delivering pulses of 25 ns duration. It is shown that the presence of a sharp step in the coating thickness, and consequently, in the mirror reflectivity profile does not significantly affect the damage threshold. Indeed, the damage threshold fluence of 1.8 J/cm2 in the high reflectivity area reduces to 1.1 J/cm2 in the low reflectivity region as it would for two independent high and low reflectivity coatings. The damage threshold in the border region is 1.4 J/cm2.

Earth surface reflectivity climatology at 340–380 nm from TOMS data

The 340–380 nm (UV) Lambertian equivalent reflectivities (LER) of the Earth's surface, between the latitudes ±70°, are constructed from 14.5 years of radiances measured by Nimbus‐7/total ozone mapping spectrometer (November 1978 to May 1993). The surface LER values are obtained from the minimum reflectivity values for each 1°×1.25° (latitude × longitude) pixel with statistically improbable outlier values removed. The resulting LER climatology shows low surface reflectivity values over the entire globe at 340 and 380 nm that are nearly independent of wavelength (the difference is less than 0.2%). In general, the LER is lower over the land (2–4%) than over the oceans (5–8%), though both land and water have features outside of these ranges. Monthly maps of LER are derived that include seasonally persistent cloud features, as well as showing seasonal surface variations. There are low reflectivity regions in the ocean coastal waters that appear to be indicative of chlorophyll or silt from wave action or from rivers. There also are mid‐ocean areas of comparatively high reflectivity with a seasonal variation that is not solar zenith angle dependent. These areas of high ocean reflectivity correlate with coastal zone color scanner observations that were associated with the absence of phytoplankton (clear water).

Optical coating without phase dispersion for a Fabry–Perot interferometer

Phase dispersion induced by coatings can be a critical phenomenon in interferometry. We are interested in special mirrors intended for a Fabry-Perot interferometer with a high reflectance region and a low reflectance region in which phase dispersion on reflection must be avoided. We describe how a classical approach that uses the concepts of admittance and symmetrical multilayers allows the design of simple solutions.

Achieving broad-area laser diodes with high output power and single-lobed far-field patterns in the lateral direction by loading a modal reflector

A modal reflector, which consists of a high-reflectivity region surrounded by low-reflectivity regions, is added to the front facet of two types of broad-area laser diodes (LD's) to control the lateral modes. One type of LD is the self-aligned laser with a bent active layer (SBA LD) that has a real index-guiding mechanism. The other is a planar-stripe LD that consists of a Zn-diffused region to confine the current flow and has a gain-guiding mechanism. For the SBA LD's with a modal reflector, stable single-lobed far-field patterns (FFP's) are obtained at up to 0.3 and 0.4 W output powers in continuous wave (CW) operation and pulsed operation, respectively. In addition, for planar-stripe LD's with a modal reflector, stable single-lobed FFP's are obtained at up to 0.4 W in CW operation. The lateral modes inside the cavity are analyzed by utilizing a slit model and FFP's are calculated. Good agreement is found between experimental and calculated FFP's for a large Fresnel number. >

Kinematic, Thermodynamic, and Visual Structure of Low-Reflectivity Microbursts

Abstract On 9 July 1987, a series of low-reflectivity microbursts were studied over Colorado using dual-Doppler analyses, cloud photogrammetry, and in situ measurements collected by aircraft. These types of wind-shear events are particularly hazardous to the aviation community since the parent cloud and pendant virga shafts appear innocuous. The microburst downdrafts are shown to develop at the location where the virga shafts are, visually, the lowest and opaque. As the downdraft intensifies, sublimation and evaporation (to a smaller extent) rapidly deplete the hydrometeors and result in a shift of the axis of maximum negative vertical velocities into a relatively low reflectivity and transparent region of the virga shafts. Comparisons with weak downdrafts or null null cases reveal that the maximum radar reflectivities within the parent clouds for the two cases are comparable; however, the microburst storm consistently exhibits a larger horizontal area encompassed by the 10-dBZ contour at midlevels prior ...

Seismic structure of oceanic crust in the western North Atlantic

We examine the seismic structure of Cretaceous‐aged ocean crust north of the Blake Spur fracture zone in the western North Atlantic by using a combination of multichannel seismic and wide‐angle reflection/refraction data. Although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure, it displays large variations in crustal reflectivity on both ridge‐normal and ridge‐parallel profiles. The upper crust is highly reflective and contains both subhorizontal (or shallow dipping) events and more steeply dipping reflectors. Subhorizontal reflectors are typically present between 1.5 and 2 km depth and are, in some cases, laterally continuous for distances of 15 to 20 km. Generally, these events do not correlate with the depth of the seismic layer 2/layer 3 boundary determined from refraction data, and their origin is still poorly understood. The steeply dipping events are generally confined to the upper 2.5 km of the crust. We interpret these reflectors as the subsurface expression of ridge‐parallel extensional faults commonly mapped at mid‐ocean ridges. The lower crust in this area exhibits alternating regions of high and low reflectivity. The highly reflective zones are made up of packages of linear or concave‐upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust. Some of these dipping reflectors appear to cut through the whole crustal section, although most fade out in the acoustically transparent midcrust. On ridge‐normal profiles the majority of these events dip to the east, toward the paleo‐spreading center, whereas on ridge‐parallel profiles the events typically dip south, toward the Blake Spur fracture zone. The base of the crust is generally not associated with a strong, discrete Moho reflection but is a comparatively indistinct boundary usually associated with a 1‐ to 2‐km‐thick band of diffuse high reflectivity. We interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8–10 km. With the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge‐normal and ridge‐parallel profiles. We may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults. Alternatively, these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere.

Magneto-optic spectra of EuS-Gd and EuS-Tb films

Doping EuS films with Tb or Gd increases the Curie point because of strong exchange through the free carriers and increases the coercivity because of strong crystal fields associated with vacant sulfur sites. The two main features of the reflectivity spectra of the doped samples are a strong free carrier contribution below 1.5 eV and a region of low reflectance between 1.5 and 4 eV. The Kerr peak in EuS at 2.4 eV is greatly enhanced in some of the doped compositions, probably as a consequence of low values of the optical functions n( omega ) and k( omega ) in this low-reflectance region.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>