Drop In Reflectivity Research Articles

Comparative Assessment of the Durability of Glass and Polymer Mirrors for Concentrated Solar Thermal Technologies

This paper describes experimental work that has been carried out to assess the durability of glass and polymer mirrors against erosion and soiling phenomena. Both outdoor and indoor tests have been performed and the results show that polymer mirrors are quickly degraded in comparison to glass. Soiling deposition has been simulated using a soiling test rig developed for laboratory use. Results show that polymer mirrors, due to their high roughness and their surface energy properties, can accumulate more soiling on their surface generating a higher drop in specular reflectance compared to glass mirrors. Contact cleaning processes have also been simulated on polymer mirrors and the results show that using a hard brush, even at low speed with rotational motion, causes significantly more degradation than a soft brush.

Validation of GPM DPR Rainfall and Drop Size Distributions Using Disdrometer Observations in the Western Mediterranean

Dual-frequency precipitation radar (DPR) on the Core GPM satellite provides spaceborne three-dimensional observations of precipitation fields and surface rainfall rate with quasi-global coverage. The present study evaluates the behavior of liquid precipitation intensity, radar reflectivity factor (ZKu and ZKa) and drop size distribution (DSD) parameters (weighted mean diameter Dm and intercept parameter Nw) of the GPM DPR-derived products, version 07, from 2014 to 2023. Observations from seven Parsivel disdrometers located in different topographic zones in the Western Mediterranean are taken as ground references. Four matching techniques between satellite estimates and ground level observations were tested, and the best results were found for the so-called optimal comparison approach. Overall, GPM DPR products captured the variability of the observed DSD well at different rainfall intensities. However, overestimation of the mean Dm and underestimation of the mean Nw were observed, being much more sensitive to errors in drop diameters larger than 1.5 mm. Moreover, the lowest errors were found for radar reflectivity factor and Dm, and the highest for Nw and rainfall rate. In addition, the GPM DPR convective and stratiform classification was tested, and a substantial overestimation of stratiform cases compared to disdrometer observations were found.

Magneto-Optic and thermoelectric response of doped SrZrO3 for energy storage applications

Using first-principles calculations, we have investigated the electronic, optical, magnetic and thermoelectric properties of the pristine and doped SrZrO3 (SZO) with Mn, Co, and Fe atoms at Sr-site. Our calculations shows that the Mn. Co, and Fe atoms at Sr-site can be easily incorporated in SZO due to their lower formation energies. The pristine SZO exhibit a non-magnetic semiconductor nature with a band gap of 4.06 eV. Interestingly, the dopants Mn, Co, and Fe atoms alters the semiconductor nature of SZO to magnetic one with a band gap values of 1.38(↑)/1.74(↓), 2.98(↑)/0.0(↓), and 0.0(↑)/1.87(↓) for Mn-, Co–, and Fe-doped SZO. This 100 % spin-polarization near the fermi-level, confirming the half-metallic nature of the X-SZO (X = Mn, Co, Fe). Importantly, the magnetic anisotropy energy (MAE) of 1.27 meV, 1.45 meV, and 0.67 meV is estimated for Mn-, Co–, and Fe-doped SZO, respectively. Thus, the realization of MAE could pave a way to utilize Mn-, Co–, and Fe-doped SZO for applications in magnetic semiconductor devices. Optical absorption of Mn-SZO spectra exhibits blueshift upon introducing impurities and materials exhibits good absorption in the visible region. A drop in reflectivity and rise in conductivity in the high energy Ultraviolet region make these materials as potential candidate for fabrication of devices in the field of photodetectors, optoelectronic, power electronics, solar cells, photonic and photovoltaics. The ZT value is recorded 0.35, 0.45, 1.12 and 0.55 for pristine, Mn-SrZrO3, Co-SZO and Fe-SZO respectively at different temperature values. It would be ideal to perform more experimental research on the effective application of these systems in the domains of magnetic semiconductor devices and photo-electronics.

First-principles theory-based design of highly reflective metals for radiative cooling

Motivated by recent great progress in passive radiative cooling, we performed a first-principles-based material design study of high reflectivity mirror components. From the electronic structure analysis on noble metals, we found that the control of the d-to-sp band transition can be the key to achieve the better reflection performance than pure silver. As concrete examples, we first suggest electron doping into Ag through, e.g., Cd alloying. Our simulation results show that, by 10% Cd alloying, Ag0.9Cd0.1 exhibits the reflectivity drop starting at the higher photon energy than silver. The second is to use alkali metal whose stable treatment has recently been much more feasible. Two alkali metals, namely sodium and lithium, also show the higher energy reflectivity drop which is attributed to the absence of localized d-electron bands below the Fermi energy. Our study provides the useful insight for designing better mirror materials and hopefully stimulates further theoretical and experimental investigations.

Ion flux–energy distributions across grounded grids in an RF plasma source with DC-grounded electrodes

We present an experimental investigation of the ion flux–energy distribution functions (IFEDFs) obtained across grounded grids in an asymmetric capacitively coupled RF source using a helium discharge. The powered electrode in the RF source is DC-grounded via a λ/4 filter, which lifts its DC potential to zero. Grids of different dimensions (hole width, thickness, and geometric transparency) were used to confine the plasma, while the IFEDF of the ion beam departing the grid and reaching the reactor walls was studied using a retarding field energy analyser. The IFEDF obtained was double-peaked, indicating the presence of fast ions arriving from the plasma source, and cold ions generated upon charge exchange collisions between the fast ions and neutrals. The flux, as well as the peak energies of the two ion groups, depended significantly on the process parameters: RF power, He pressure, the distance between grids and walls, and the dimensions of the grids. The results indicate that confining plasma with grids can reduce the ion flux at the walls by over 60%, significantly lowering the wall sputtering rate. This was confirmed with a dedicated long-exposure plasma discharge with a gridded plasma reactor, wherein less than 1 nm of Cu deposition was found on the DC-grounded powered electrode, and the surface reflectivity was preserved to pristine values. In contrast, a similar experiment in a gridless reactor resulted in Cu deposition of 35 nm with a drastic drop in surface reflectivity. These studies are of great importance for the application of similar RF plasma sources with in-situ cleaning of diagnostic mirrors in fusion devices, as well as in a variety of plasma processing applications.

Assessment of Palm Jumeirah Island’s Construction Effects on the Surrounding Water Quality and Surface Temperatures during 2001–2020

Climate change stressors like rising and warmer seas, increased storms and droughts, and acidifying oceans are rapidly threatening coastal zones, which are the world’s most densely inhabited places. This research assesses the effects of Palm Jumeirah Island (PJI) construction on its surrounding water quality and temperature, using Landsat-7 and 8 spectral and thermal bands for the years 2001, 2014, 2016, 2019, and 2020. To aid in this goal, the changes in water spectral reflectance was observed and interpreted, based on previous research and measurements, to discover the correlation between water quality and its spectral reflectance. Then, the sea surface temperature (SST) was calculated for the years under review and changes in water temperature were evaluated. Finally, the Green Normalized Difference Vegetation Index (GNDVI) and the Normalized Difference Turbidity Index (NDTI) were calculated to estimate water chlorophyll levels and water turbidity, respectively, and changes were observed and interpreted for the time period under review. The present study showed that the PJI construction not only increased the water reflectance in the 0.5–0.8 µm of wavelength, which can be considered to be the increase of suspended sediments and chlorophyll but the water temperature also increased by 7.5 °C during the 19 years. In addition, a gradual increase in the values of GNDVI (by 0.097–0.129) and NDTI (by 0.118~0.172) were observed. A drop in chlorophyll and suspended sediment spectral reflectance and GNDVI and NDTI values were also observed in 2020 compared to 2019 which can be attributed to the 63 to 82% decrease in tourists in Dubai in 2020 as a result of the COVID-19 pandemic. This study aims to draw attention to environmental issues by clarifying the effect of creating artificial islands in the sea and our analysis and results are a suitable reference for specialized hydrological and environmental studies based on spectral information and distance measurements, as presented in this paper.

Exploring optical properties in cylindrical polymeric-chalcogenides photonic materials

Theoretical formulations for E-polarization electromagnetic wave propagation in a chalcogenide (As2Se3)-polymeric (PEI) photonic material (CPCPM) are thoroughly presented in the present work. The link between two polarization modes, E and H-polarization, is also discussed. We have shown numerical findings for two model structures: the single cylindrical interface and the single cylindrical slab for both chalcogenides (High refractive index) and polymeric (low refractive index) photonic material designed at a visible wavelength window of 632.8 nm by using the Transfer Matrix Technique (TMT) and Henkel Formalism (HF) in cylindrical coordinates. It is discovered that there is always a Brewster initial radius at which a minimal reflectance in E-polarization is achieved for chalcogenides polymeric, both photonic materials. For fundamental mode (m = 0), the minimum reflectance is observed at the polymeric interface as compared to chalcogenides. Even for higher-order modes, a quick drop in reflectance from unity is observed for the polymeric interface. For the cylindrical unit slab of CPCPM, it is found that the reflectance response with the wavelength for both materials has oscillating and non-oscillating parts, which shows the initial radius has a strong effect on the reflectance at fixed slab thickness. As the initial radius goes increasing, the oscillatory reflectance gets squizzing. Further, reflectance is again the oscillatory function of slab thickness for lower mode number, and it gets maximum and flat for (m = 4). These findings open the way for various applications in the optoelectronic devices and sensors domain.

Laser ablation of silicon with THz bursts of femtosecond pulses

In this work, we performed an experimental investigation supported by a theoretical analysis, to improve knowledge on the laser ablation of silicon with THz bursts of femtosecond laser pulses. Laser ablated craters have been created using 200 fs pulses at a wavelength of 1030 nm on silicon samples systematically varying the burst features and comparing to the normal pulse mode (NPM). Using bursts in general allowed reducing the thermal load to the material, however, at the expense of the ablation rate. The higher the number of pulses in the bursts and the lower the intra-burst frequency, the lower is the specific ablation rate. However, bursts at 2 THz led to a higher specific ablation rate compared to NPM, in a narrow window of parameters. Theoretical investigations based on the numerical solution of the density-dependent two temperature model revealed that lower lattice temperatures are reached with more pulses and lower intra-burst frequencies, thus supporting the experimental evidence of the lower thermal load in burst mode (BM). This is ascribed to the weaker transient drop of reflectivity, which suggests that with bursts less energy is transferred from the laser to the material. This also explains the trends of the specific ablation rates. Moreover, we found that two-photon absorption plays a fundamental role during BM processing in the THz frequency range.

Multi-Radar Analysis of the 20 May 2013 Moore, Oklahoma Supercell through Tornadogenesis and Intensification

A multi-radar analysis of the 20 May 2013 Moore, Oklahoma, U.S. supercell is presented using three Weather Surveillance Radars 1988 Doppler (WSR-88Ds) and PX-1000, a rapid-scan, polarimetric, X-band radar, with a focus on the period between 1930 and 2008 UTC, encompassing supercell maturation through rapid tornado intensification. Owing to the 20-s temporal resolution of PX-1000, a detailed radar analysis of the hook echo is performed on (1) the microphysical characteristics through a hydrometeor classification algorithm (HCA)—inter-compared between X- and S-band for performance evaluation—including a hail and debris class and (2) kinematic properties of the low-level mesocyclone (LLM) assessed through ΔVr analyses. Four transient intensifications in ΔVr prior to tornadogenesis are documented and found to be associated with two prevalent internal rear-flank downdraft (RFD) momentum surges, the latter surge coincident with tornadogenesis. The momentum surges are marked by a rapidly advancing reflectivity (ZH) gradient traversing around the LLM, descending reflectivity cores (DRCs), a drop in differential reflectivity (ZDR) due to the advection of smaller drops into the hook echo, a decrease in correlation coefficient (ρhv), and the detection of debris from the HCA. Additionally, volumetric analyses of ZDR and specific differential phase (KDP) signatures show general diffusivity of the ZDR arc even after tornadogenesis in contrast with explosive deepening of the KDP foot downshear of the updraft. Similarly, while the vertical extent of the ZDR and KDP columns decrease leading up to tornadogenesis, the phasing of these signatures are offset after tornadogenesis, with the ZDR column deepening the lagging of KDP.

Real-time monitoring of order-disorder transformation of FePt thin films by light scattering

ABSTRACT An optical method is proposed to real-time monitor the order transformation of FePt thin films. By measuring the surface reflection amplitude, onset temperature can be determined correctly. In a heating process, order transformation drastically increases the surface roughness due to recrystallization and grain growth, resulting in rapid drop of surface reflectivity. By tracking this drop, the kinetic ordering temperature can be determined. Experimentally measured kinetic ordering temperature for Fe50.4Pt49.6, Fe46.6Pt53.4, and Fe52.7Pt47.3 thin films of 60 nm in thickness are 396°C, 437°C, and 368°C, respectively. They are in good agreement with the published DSC data.

Soiling loss of solar glass and mirror samples in the region with arid climate

Solar energy market is still growing in Morocco and in other countries around the world. The development and deployment of any solar project require a prefeasibility study before the establishment of the plant. The most important parameters influencing the site selection or the project feasibility are solar radiation and the climatic conditions. Soiling is a limiting efficiency factor that drastically affect the optical and the electrical performances of the solar plants. The objective of the current study is to evaluate and compare the soiling impact, from an optical point of view, on both PV and CSP technologies by investigating the reflectance and transmittance drop of glass and mirror samples exposed at Green Energy Park research facility (Morocco) during the dry period of the year. The results show that the effect of soiling is very important in the dry period of the year, it can reach up to 35% of soiling loss after only one week for the mirror samples. For the glass samples, the drop can reach up to 12%. Besides, it has been found that the CSP mirrors are three times more affected by soiling than PV glass samples.

Cavity-resonator-integrated guided-mode resonance mirror with high-confinement channel structure for improvement of reflectance

A cavity-resonator-integrated guided-mode resonance mirror (CRIGM) consists of a grating coupler integrated in a waveguide cavity formed by a pair of distributed Bragg reflectors (DBRs) on a high-reflectance substrate. It is theoretically predicted that a CRIGM can give interesting characteristics of not only a narrow band-stop reflection spectrum but also flat high reflectance with steep reflection-phase spectrum according to its structure, whereas only a CRIGM with a band-stop reflection spectrum has been confirmed experimentally so far. This time, the cause of undesirable reflectance drop in a CRIGM for flat high reflectance spectrum is investigated theoretically and experimentally. It is theoretically shown that the CRIGM reflectance at the resonance wavelength is affected considerably by a slight reduction of the DBR reflectance. A CRIGM with a rib-structured channel for high lateral confinement is designed and fabricated to keep the DBR reflectance high by suppressing a propagation loss of a guided wave. The improvement of the CRIGM reflection spectrum is confirmed experimentally by measurement of the reflection spectra of the fabricated devices.

Numerical and experimental investigations of dust effect on CSP performance under United Arab Emirates weather conditions

Dust is one of the main challenges in implementing concentrating solar power (CSP) systems in arid and semi-arid zones. The characterization of dust particles and the measure of the soiling loss effects under real operating conditions can be useful to select the appropriate cleaning methods and frequency. In this study, experimental and numerical investigations were carried out to study the characteristics of dust particles and their effects on the CSP performance under UAE weather conditions. The results showed that the soiling rate is correlated to the wind speed and direction. The monthly soiling effect was evaluated for 5 months of exposure with the highest decrease recorded in March. The results of cumulative dust experiment showed a drop in specular reflectivity by about 63% after 3 months of exposure. Based on the experimental data, the power soiling loss and thermal performance were determined for a parabolic trough solar collector. The predicted results were in consistence with the dust accumulation leading to a 36% decrease in thermal efficiency. In addition, the power soiling loss was compared with that of PV technology, and it was found that soiling effect is more pronounced in CSP systems by 3–5 times drop in performance.

Effect of the surface wind flow and topography on precipitating cloud systems over the Andes and associated Amazon basin: GPM observations

The characteristics of the precipitation under the influence of topography and surface wind flow are investigated over South America. Here the precipitating cloud systems (PCSs) are identified using the Global Precipitation Measurement Precipitation Radar (GPM-PR) data, which provides the three dimensional radar reflectivity factor (Ze), rain rate, drop size and droplet concentration. For each PCSs the surface wind properties are estimated using European Center for Medium-Range Weather Forecast Interim data. Based on the direction of surface flow the PCSs are classified into five categories. Over the South America the near surface wind flow transports the moisture from Amazon basin to east flank of Andes and validated here. The directional surface flow decides the occurrences of the PCSs, as upslope consists of the higher and larger PCSs at the peak of Andes compared to downslope flow. The directional flow suggests that northern Andes consists of pronounced bright band characteristics compared to southern Andes, and upslope and easterly flow have higher probability of rain at the eastern slope of Andes compared to westerly and downslope flow in northern Andes. The results show that orography also modulates the precipitation characteristics under different directional flow over and near the Andes. Eastern slope of Andes has higher rain rate compared to western slope of Andes in most of the directional flow. Orographically forced moisture loaded flow, over the eastern slope of Andes causing the higher rain rate, drop radius and droplet concentration in northern Andes. At the low lands, effective drop radius and droplet concentration show the opposite characteristics, and effective drop radius (concentration) is least (highest) except for the downslope flow over the northern Andes. The results of DSD parameters along with rainfall intensity show the microphysical evolution of the precipitation under the complex orography over the Andes mountain. The present study suggests that in future, surface flow must be considered for studying the orographic precipitation in numerical modeling.

Effect of Forest Fire on Snow Reflectance in Uttarakhand Region: An Analysis Using AWiFS and INSAT-3D Data

In the summer of 2016, fire broke out in the forests of Uttarakhand state. The effect of forest fire was thought to have severe impact on nearby glaciated region in terms of faster melting. It is understood that contamination and heat enhance the snowmelt, which reduces the reflectance in different part of EM spectrum. In order to assess the effect, AWiFS and IMAGER data from Resourcesat-1 and INSAT-3D, respectively, for the months of April and May of the years 2012, 2013, 2014 and 2016 were used to compare the reflectance of snow. It was observed that pre- and post-fire data of 2016 show drop in reflectance in comparison with earlier years. The change in reflectance for locations near to forest fire was significantly high in comparison with previous year images, whereas far-away locations did not show much change. This drastic drop may be attributed to deposition of black carbon on nearby locations in snow-covered area. AWS data of nearby Joshimath observatory were also analysed to avoid any anomalous change in temperature for the same duration.

Formation of clays, ferrihydrite, and possible salts in Hydrae Chasma, Mars

We used multiple orbital data sets to analyze the morphology, composition, and stratigraphy of clays and other hydrated deposits within Hydrae Chasma. Fe/Mg-smectites are concentrated along the lower floor of the chasma, adjacent to pit craters. There are at least two distinct smectites that vary in their morphology and spectral properties, with a more Mg-rich smectite associated with a thin brighter upper layered deposit and a more Fe-rich smectite in a lower darker massive deposit. We also identified a younger hydrated unit with spectral features similar to ferrihydrite. The spectra of this unit lack an absorption between 2.28 and 2.31 µm, but display a drop in reflectance near 2.3 µm. The ferrihydrite-type detections correspond to cleaner surfaces of light-toned deposits further north and higher in elevation than the smectites. A few small outcrops in the southern chasma floor appear brighter than the clays and display an additional narrow absorption at 2.4 µm in combination with the smectite features, which may indicate a smectite plus perchlorate mixture. There are no valleys along the plateau that intersect Hydrae Chasma and, consequently, any water that created the Fe/Mg-smectites and ferrihydrite must have been from groundwater in the subsurface or surface water sourced from within the chasma. A terraced fan and associated valley to the north of the clays provide support for melting snow/ice within the chasma and this same water may have also altered materials lower in elevation where the ferrihydrite-bearing unit formed under colder conditions than the smectites. The change in stratigraphy from smectites to ferrihydrite is consistent with a decrease in global temperature during the Hesperian.

In-Process Measurement of Thickness of Cured Resin in Evanescent-Wave-Based Nano-stereolithography Using Critical Angle Reflection

Stereolithography is one of the most powerful ways to fabricate complex three-dimensional polymeric-based structures layer-by-layer using optical power. Evanescent-wave-based nano-stereolithography using ultra-thin field distribution of evanescent wave to solidify photosensitive resin can provide a sub-micrometer vertical resolution of each layer. To meet strong demands for in-process thickness measurement of cured resin in evanescent-wave-based nano-stereolithography, a measurement method that utilizes variations of resin’s refractive index after polymerization and a high sensitivity of total internal reflection at the critical angle has been proposed. By launching the a measurement light from a substrate to resin at the critical angle and detecting reflections from the resin, slight change in refractive index and thickness of cured resin that have great influences on reflectivity can be in-process measured. This method has been firstly examined by simulation using the rigorous coupled wave analysis method. Here, we show that an increase in cured resin’s thickness induces a decrease in the reflectivity. In experiments, the largest reflectivity contrast between cured and uncured resins has been proved at the critical angle. In addition, the relationship between reflectivity drop and thickness has been calibrated and a linear relationship within a certain thickness range has been experimentally confirmed. Furthermore, the difference of curing process between continuous and discontinuous exposure has been investigated by using the proposed measurement method. At last, image subtraction and a median filter have been applied in imaging processing to remove influences of uneven illumination and background noises.

Solar Cells Based on Laser-Modified Silicon

The fabrication of solar cells based on laser-modified silicon (LMS) has been experimentally investigated. Laser ablation of single-crystal Si in air results in the formation of a quasi-periodic array of microcones. After removing the oxide layer, this array is coated by a transparent conducting tin oxide layer. A comparison of the spectral dependence of the mirror reflection of laser-modified Si with that of the initial flat Si surface shows a 10 to 15% drop in reflectivity. The light-to-electricity conversion efficiency is shown to exceed that of the initial flat Si surface by more than 20%.

Monitoring of playa evaporites as seen with optical remote sensing sensors: case of Chott El Jerid, Tunisia, from 2003 to present

Chott El Jerid in the Zone of Chotts of Tunisia is one of the largest endorheic basins in the world. During the dry period, from May to August, it is generally covered by continental evaporites which result from the desiccation of the lake formed after a flooding event. This lake comprises runoff water from the surrounding relief and also water resurgence. In order to map and monitor these evaporitic surfaces (mineral composition and evolution in space and time), optical multisource, multispectral, and multidate satellite data have been used. Landsat 4–5 TM, Landsat 8 OLI, SPOT 6, and Landsat Surface Reflectance (LSR) constitute the main data set. The central part of the Chott, north and south of the road crossing the Chott over 70 km from Tozeur to Kebili, has been particularly studied, because it corresponds to the major evaporite accumulation zone. These evaporites precipitated as concentric layers (a relatively rare pattern), mainly north of the road, after several recent flooding events during the last 15 years. Winds can play a significant role in the development of the evaporite layers. Image interpretation associated with field data shows that after the final desiccation of the playa lake, the mineralogy of the salt crust comprised an assemblage dominated by halite south of the road and by gypsum north of the road. Halite and gypsum are the only minerals to be identified using satellite remote sensing data. Sulfates such as gypsum can be identified thanks to a drop in reflectance in the short-wave infrared (SWIR) range caused by vibrations of the SO4 group. Gypsum crusts are more widely distributed than halite crusts. LSR data are particularly suitable for multitemporal comparison because they are calibrated and atmospherically corrected. The classical bull’s eye pattern characterizing evaporitic deposits (from carbonates along the rims to halite, gypsum, and finally potassium-magnesium minerals in the center of the basin) is deeply disturbed by the road crossing Chott El Jerid.

Improvement of Electropolishing of 1100 Al Alloy for Solar Thermal Applications

Aluminum sheets-based mirrors are finding applicability in high-temperature solar concentrating technologies because they are cost-effective, lightweight and have high mechanical properties. Nonetheless, the reflectance percentages obtained by electropolishing are not close to the reflectance values of the currently used evaporated films. Therefore, controlling key factors affecting electropolishing processes became essential in order to achieve highly reflective aluminum surfaces. This study investigated the effect of both the electropolishing process and previous heat treatment on the total reflectance of the AA 1100 aluminum alloy. An acid electrolyte and a modified Brytal process were evaluated. Total reflectance was measured by means of UV–Vis spectrophotometry. Reflectance values higher than 80% at 600 nm were achieved for both electrolytes. Optical microscopy and scanning electron microscopy images showed uneven dissolution for the acid electropolished samples causing a reflectance drop in the 200-450 nm region. The influence of heat treatment, previously to electropolishing, was tested at two different temperatures and various holding times. It was found that reflectance increases around 15% for the heat-treated and electropolished samples versus the non-heat-treated ones. A heat treatment at low temperature combined with a short holding time was enough to improve the sample total reflectance.