Optical Properties Reflectance Research Articles

Enhanced optical reflectivity and electrical properties in perovskite functional ceramics by inhibiting oxygen vacancy formation

Perovskite functional ceramics have attracted considerable interest owing to their promising catalytic, optical and electrical performance. To understand the relationships between oxygen vacancy defects and the properties of perovskite materials through Nb5+ doping, new perovskite composite La0.9Sr0.1Ti0.75Nb0.25O3+δ (LSTN0.25) ceramics are proposed. These are synthesized by Nb2O5-doped La0.9Sr0.1TiO3+δ and exhibit layered perovskite (La, Sr)2(Ti, Nb)2O7 and scheelite LaNbO4. Their microstructure, reflectivity, band structure, and dielectric properties were investigated through experiments and first-principles calculations. The energy calculations and electron paramagnetic resonance signals performed on oxygen vacancy suggested that oxygen vacancy defects are difficult to form and at a lower level in LSTN0.25, which significantly improved the near-infrared reflectivity. The dielectric properties and band structure demonstrated that LSTN0.25 sintered at 1400 °C exhibited higher reflectivity than those of the La0.9Sr0.1TiO3+δ ceramics. These findings shed light on the role of Nb5+ in inhibiting oxygen vacancy formation in perovskite and provide an approach for fabricating novel, full-band, high reflectivity perovskite ceramics through high valence element doping.

Concentration, anisotropic and apparent colour effects on optical reflectance properties of virgin and ocean-harvested plastics

We present reflectance measurements collected from virgin and ocean-harvested plastics. Virgin plastics included high and low density polyethylene (HDPE, LDPE), polypropylene (PP) as well as polystyrene (PS). Ocean-harvested plastics were ropes, sheets, foam, pellets and fragmented items previously trawled from the North Pacific Garbage Patch. Nadir viewing angles and plastic pixel coverage were varied to advance our understanding of how reflectance shape and magnitude can be influenced by these parameters. We also investigated the effect of apparent colour of plastics on the measured reflectance from the ultraviolet (UV - 350 nm), visible, near to shortwave infrared (NIR, SWIR - 2500 nm). Statistical analyses indicated that the spectral reflectance of the plastics was significantly correlated to the percentage pixel coverage. There was no clear relationship between the reflectance observed and the viewing nadir angle but dampened materials seemed to be more isotropic (near-Lambertian) than their dry counterparts. A loss in reflectance was also determined between dry and wet plastics. Location of absorption features was not affected by the apparent colour of objects. In general, ocean-harvested plastics shared more identical absorption features (~960, 1215, 1440, 1732, 1920 nm) and had lower reflectance intensity compared to the virgin plastics (~980 nm). Prospects for satellite retrieval of plastic type and pixel plastic coverage are discussed based on Top-of-Atmosphere (TOA) signal simulated through radiative transfer computation using the documented plastic reflectances. Non-linear relationships between TOA reflectance and plastic coverage were observed depending on wavelength and plastic type. Most of the plastics analysed impact significantly the TOA signal but two plastic types did not produce strong signal at TOA (hard fragments, LDPE). Nevertheless, all plastic types produced detectable signals when observations were simulated within the sunglint direction. The measurements collected in this study are an extension to available high quality spectral reference libraries and can support further research in developing remote sensing algorithms for marine litter.

BRDF characterization of Al-coated thermoplastic polymer surfaces

In this paper, we present a combined morphological and optical characterization of aluminum-coated thermoplastic polymer surfaces. Flat plastic substrates, obtained by means of an injection molding process starting from plastic granules, were coated with ultra-thin aluminum films evaporated in vacuo, on top of which a silicon-based protective layer was plasma deposited in order to prevent oxidation of the metal reflective surface. Different sample treatments were studied to unravel the influence of substrate chemistry, substrate thickness, aluminum and protective layer thickness, and surface roughness on the actual optical reflectance properties. Bidirectional reflectance distribution function measurements, corroborated by surface morphological information obtained by means of atomic force microscopy, correlate reflectance characteristics with the root-mean-square surface roughness, providing evidence for the role of the substrate and the thin films’ morphology. The results unravel information of interest within many applicative fields involving metal coating processes of plastic substrates as an example in the case of automotive lighting.

The preparation and properties of TiO2 nano-porous layers for perovskite solar cells

In this paper, we investigated different TiO2 nanoporous layers on the performance of perovskite solar cells. The structure and morphology of TiO2 nanoporous layers were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. It indicates that the order of crystalline rutile TiO2 nanorod porous layers is better than that of anatase TiO2 nanotubes and anatase TiO2 nanoflowers. The optical reflectance properties of FTO/TiO2 nanoporous layers and the optical absorption properties of the FTO/TiO2 nanoporous layers/CH3NH3PbI3 structure were analyzed by ultraviolet visible absorption spectroscopy. The performance of perovskite solar cells with different TiO2 nanoporous layers was tested by using a current-voltage curve.

Optical reflection properties for the interface associated with Chern insulator and chiral metamaterial

Based on the topological nature of Chern insulator and magnetoelectric coupling of chiral metamaterial, we investigate the electrodynamics for the interface associated with the two media. The Fresnel coefficients of the interface between Chern insulator and chiral metamaterial, as well as the corresponding polarization rotation angles, are derived. The reflection characteristics of the linearly polarized incident wave at the interface, such as complete polarization conversion and change of polarization state, are discussed. Under the combined influence of Chern insulator and chiral metamaterial, the partial polarization conversion may be enhanced to the complete polarization conversion, and the chiral metamaterial may compensate for the suppression effect of longitudinal conductivity of Chern insulator on the polarization conversion.

Diffuse reflectance spectroscopic analysis of barium sulfate as a reflection standard within 173–2500 nm: From pure to sintered form

The use of barium sulfate (BaSO4) as a low-cost reflectance standard has been documented for some time. In its pure salt form, the optical characteristics do have advantages compared to laboratory-grade sintered polytetrafluoroethylene (PTFE). However, its practical use has little advantage against the stability of PTFE. In this paper, a process of producing pelletized BaSO4, and characterization of its optical reflectance properties is reported. In its sintered form, data-driven analysis shows that BaSO4 is a commendable low-cost, high-reflection and a high-consistency material. The current sintered forms, though crude, registers a relatively stable texture to withstand minor mechanical stress, while having up to an average 92% reflectivity across the UV-VIS-NIR range (173–2500 nm) compared to a PTFE reflection standard.

Fatigue Performance of Type I Fibre Bragg Grating Strain Sensors.

Although fibre Bragg gratings (FBGs) offer obvious potential for use in high-density, high-strain sensing applications, the adoption of this technology in the historically conservative aerospace industry has been slow. There are several contributing factors, one of which is variability in the reported performance of these sensors in harsh and fatigue prone environments. This paper reports on a comparative evaluation of the fatigue performance of FBG sensors written according to the same specifications using three different grating manufacturing processes: sensors written in stripped and re-coated fibres, sensors written during the fibre draw process and sensors written through fibre coating. Fatigue cycling of the fibres is provided by a customized electro-dynamically actuated loading assembly designed to provide high frequency and amplitude loading. Pre- and post-fatigue microscopic analysis and high-resolution transmission and reflection spectra scanning are conducted to investigate the fatigue performance of FBGs, the failure regions of fibres as well as any fatigue-related effects on the spectral profiles. It was found that because of the unique fabrication method, the sensors written through the fibre coating, also known as trans-jacket FBGs, show better fatigue performance than stripped and re-coated FBGs with greater control possible to tailor the optical reflection properties compared to gratings written in the draw tower. This emerging method for inscription of Type I gratings opens up the potential for mass production of higher reflectivity, apodised sensors with dense or complex array architectures which can be adopted as sensors for harsh environments such as in defence and aerospace industries.

Multicolored Photonic Crystal Carbon Fiber Yarns and Fabrics with Mechanical Robustness for Thermal Management.

Multicolored photonic crystal carbon fiber (CF) yarns and fabrics with mechanical robustness in a full spectrum are reported. By facilely controlling the thickness of the periodic layer, a series of photonic CF yarns and fabrics with vivid structural colors ranging from purple, green, yellow, orange, to red are obtained. Interestingly, the prepared multicolored CF yarns show anisotropic optical reflection properties because of their unique axisymmetric geometry, while the plain-woven fabrics exhibit vivid colors even under ambient scattering light. Most importantly, they can withstand cyclical mechanical rubbing, laundering, and accelerated light aging, indicating great potential for practical uses. Finally, considering such impressive characteristics as well as reflection in the visible and near-infrared regions, the above photonic crystal microstructure is further used as a new material for the application of outdoor reflective cooling of the textile surface, demonstrating a superior temperature reduction up to ∼12 °C with respect to the control sample.

Electrical and optical characteristics of boron doped nanocrystalline diamond films

Boron doped diamond is a prospective material which can be used as a conductive and optically transparent thin-film electrode in a variety of optoelectronic applications. In this work, we present the results of the temperature resolved electrical conductivity, optical reflection, transmission and absorption of thin boron doped nanocrystalline diamond films grown by a microwave plasma enhanced chemical vapor deposition. Optical transmittance, reflectance and absorptance properties of layers were studied by a photo-thermal deflection spectroscopy analysis. Raman spectroscopy with various excitation wavelengths was employed for the analysis of nanocrystalline diamond layers. The measured position, shift and broadening of the characteristic boron doped diamond Raman lines were used for the determination of the boron concentration. Correlation between the results of the atomic boron concentration estimated via the Raman analysis and measured electrical conductivity values is presented.

Blue shift effect on the reflectance in two-dimensional Si-ZnO photonic structure with patterned coupled optical micro-cavities

This research has revealed the possibility to induce a shift towards short wavelengths on the optical reflectance properties of a two-dimensional photonic crystal which include an embedded array of optical micro-cavities. This ability was attributed to the resonant effect produced by the micro-cavities array embedded in the regular photonic crystal and by the variation on the structural parameters that defined the photonic crystal under research. These results can be useful in future applications of solar cell systems and telecommunications, among others, which can be of great help to increase technological developments.

Radiophotoluminescent organic materials based on photoswitchable fluorescent diarylethene derivatives

Photochromism refers to reversible changes in a material’s optical absorption and reflectance properties which are triggered by the absorption of a photon. This property can be useful in radiation imaging applications if such changes can be induced by ionizing radiation. The X-ray-induced isomerization of a photoswitchable fluorescent diarylethene-based material, 1,2-bis(2-methyl-6-phenyl-1-benzothiophen-1,1-dioxide-3-yl)perfluorocyclopentene (1a), was investigated with the aim of developing tissue equivalent imaging sensor materials. When 1a was irradiated with X-rays, the fluorescence intensity at 520 nm increased, which is consistent with isomerization processes. This work represents the first fabrication of an organic radiophotoluminescence material based on the photochromic molecule 1a. The resulting materials were sensitive to X-rays for doses in the range of 0.1–10 kGy and exhibited a linear, dose-dependent response.

Discovering coherency of specific gene expression and optical reflectance properties of barley genotypes differing for resistance reactions against powdery mildew.

Hyperspectral imaging has proved its potential for evaluating complex plant-pathogen interactions. However, a closer link of the spectral signatures and genotypic characteristics remains elusive. Here, we show relation between gene expression profiles and specific wavebands from reflectance during three barley—powdery mildew interactions. Significant synergistic effects between the hyperspectral signal and the corresponding gene activities has been shown using the linear discriminant analysis (LDA). Combining the data sets of hyperspectral signatures and gene expression profiles allowed a more precise differentiation of the three investigated barley-Bgh interactions independent from the time after inoculation. This shows significant synergistic effects between the hyperspectral signal and the corresponding gene activities. To analyze this coherency between spectral reflectance and seven different gene expression profiles, relevant wavelength bands and reflectance intensities for each gene were computed using the Relief algorithm. Instancing, xylanase activity was indicated by relevant wavelengths around 710 nm, which are characterized by leaf and cell structures. HvRuBisCO activity underlines relevant wavebands in the green and red range, elucidating the coherency of RuBisCO to the photosynthesis apparatus and in the NIR range due to the influence of RuBisCO on barley leaf cell development. These findings provide the first insights to links between gene expression and spectral reflectance that can be used for an efficient non-invasive phenotyping of plant resistance and enables new insights into plant-pathogen interactions.

Film Confinement Induced "Jump-Percolation" Wetting Transition in Amphiphilic Block Copolymer Films.

We report a first-order like sharp surface wettability transition with varying film thickness dependent morphology in cast films of an amphiphilic triblock copolymer. Films composed of poly(2-(N-ethylperfluorooctanesulfonamido) ethyl methyl acrylate), poly(FOSM), and poly(N,N'-dimethyl acrylamide), poly(DMA), with thickness (h) in the transition-range, 200 < h < 300 nm, exhibited an abrupt hydrophobic to hydrophilic dynamic water contact angle transition. After an induction time, ti ≈ 40 to 180 s, water contact angle varied as θc ≈ 116° to 40° with an ultrafast contact angle decay time constant, [Formula: see text] ≈ -18°/s. This behavior is a result of competing heterogeneous and antagonistic effects of bumpy poly(DMA) wetting domains against a nonwetting planar poly(FOSM) background, with a "jump percolation" wetting transition when the poly(DMA) domain density reaches unity. Outside of this film thickness range, relatively shallow decreasing water contact angle gradients were observed with a monotonically increasing poly(DMA) domain area coverage with increasing film thickness in the overall range of 40 nm (hydrophobic, θc ≈ 118°) < h < 500 nm (hydrophilic, θc ≈ 8°). The optical diffuse reflectance properties of these rough surfaces exhibit an onset of diffuse reflectance maxima correlated to the transition morphology film thickness.

Micropatterning of Multiple Photonic Colloidal Crystal Gels for Flexible Structural Color Films.

We herein report the micropatterning of flexible multiple photonic colloidal crystal gels (PCCGs) using single-layered microchannels. These patterned PCCGs exhibit structural colors that can be tuned by adjustment of the diameter and concentration of the colloidal particles in precursor solutions of N-isopropylacrylamide (NIPAM) or polyethylene glycol diacrylate (PEGDA). The precursor solutions containing dispersed colloidal particles were selectively injected into single-layered microchannels where they polymerized rapidly. The shape, density, and height of the patterned PCCG pixels were determined by the microchannels, which in turn determined the optical properties of the PCCG arrays. Furthermore, the preparation of three different types of PCCGs exhibiting three different structural colors at a high pixel density was demonstrated successfully using the single-layered polydimethylsiloxane (PDMS) microchannels. Finally, the optical reflective properties and the mechanical flexibility of the patterned multiple PCCG arrays were evaluated. We expect that our method for the preparation of such patterned PCCG arrays will contribute to the development of flexible optical devices.

Physical Properties of Spray Deposited Fe:ZnOThin Films

Iron doped zinc oxide (Fe:ZnO) thin films were prepared on glass substrates using wet chemical process, spray pyrolysis by varying the substrate temperature (TS) in the range 300 - 450°C at a constant Fe-doping concentration of 2 at. %. The physical properties of as prepared Fe:ZnO films were investigated using various techniques. The XRD studies indicated polycrystalline nature with the (101) plane as dominant orientation and the layers exhibited the hexagonal wurtzite crystal structure. The SEM micrographs revealed uniform growth of spherical grains over the substrate surface and the EDS profiles confirm the presence of Zn, O and Fe elements in all the films. The FTIR spectra revealed that the existence of Zn-O stretching vibrational mode at 531cm-1 as a dominant mode in the films. Further the optical transmittance and reflectance properties were studied by UV-Vis-NIR spectrophotometer. The optical band gaps are computed from the differential reflectance (dR/dλ) spectra. The band gap values were varied with the change of substrate temperature in the range, 3.20 – 3.70 eV.

Correlation between structural properties and iridescent colors of cellulose nanocrystalline films

We investigate the effect of shear flow applied during the drying of aqueous suspension of cellulose nanocrystals on optical reflective properties and structural characteristics of the resulting solidified films. Shear flow can significantly improve internal structural homogeneity of the films, while its effect on optical reflective properties is relatively minor. The measured width of the selective reflection peak is an order of magnitude larger than expected for an ideal helically modulated structure, which reflects a distribution of pitch values and possibly also of regimes of distorted helical modulation. We attribute these imperfections to the broad size distribution of the cellulose nanocrystals.

Influence of annealing temperature on optical properties of fluoride doped tin oxide films grown by the sol-gel spin-coating method

ABSTRACTFluoride-doped Tin Oxide (FTO) thin films were deposited on glass and silicon substrates by the sol-gel spin-coating method using stannic chloride pentahydrate (SnCl4·5H2O) and ammonium fluoride (NH4F) as starting precursors for Sn and F sources. As-deposited films were annealed at different temperatures ranging from 300–600°C. The effect of annealing temperature on structural and optical properties of FTO thin films including crystallinity, surface morphology, optical transparency, optical reflectivity and optical band gap are investigated by X-ray diffraction, scanning electron microscopy and UV-VIS spectrophotometry. The XRD results suggest that the prominent peak in the (110) orientation can be indexed to the polycrystalline SnO2 structure without any impurity phase caused by fluoride doping. The transmittance spectra of fluoride-doped tin oxide thin films show good transparency in the visible region with noticeable variation in its structural and optical reflectance properties depending on annealing temperature.

Characterization and simulation on antireflective coating of amorphous silicon oxide thin films with gradient refractive index

The optical reflective properties of silicon oxide (SixOy) thin films with gradient refractive index are studied both theoretically and experimentally. The thin films are widely used in photovoltaic as antireflective coatings (ARCs). An effective finite difference time domain (FDTD) model is built to find the optimized reflection spectra corresponding to structure of SixOy ARCs with gradient refractive index. Based on the simulation analysis, it shows the variation of reflection spectra with gradient refractive index distribution. The gradient refractive index of SixOy ARCs can be obtained in adjustment of SiH4 to N2O ratio by plasma-enhanced chemical vapor deposition (PECVD) system. The optimized reflection spectra measured by UV–visible spectroscopy confirms to agree well with that simulated by FDTD method.

Total Internal Reflection Accounts for the Bright Color of the Saharan Silver Ant.

The Saharan silver ant Cataglyphis bombycina is one of the terrestrial living organisms best adapted to tolerate high temperatures. It has recently been shown that the hairs covering the ant’s dorsal body part are responsible for its silvery appearance. The hairs have a triangular cross-section with two corrugated surfaces allowing a high optical reflection in the visible and near-infrared (NIR) range of the spectrum while maximizing heat emissivity in the mid-infrared (MIR). Those two effects account for remarkable thermoregulatory properties, enabling the ant to maintain a lower thermal steady state and to cope with the high temperature of its natural habitat. In this paper, we further investigate how geometrical optical and high reflection properties account for the bright silver color of C. bombycina. Using optical ray-tracing models and attenuated total reflection (ATR) experiments, we show that, for a large range of incidence angles, total internal reflection (TIR) conditions are satisfied on the basal face of each hair for light entering and exiting through its upper faces. The reflection properties of the hairs are further enhanced by the presence of the corrugated surface, giving them an almost total specular reflectance for most incidence angles. We also show that hairs provide an almost 10-fold increase in light reflection, and we confirm experimentally that they are responsible for a lower internal body temperature under incident sunlight. Overall, this study improves our understanding of the optical mechanisms responsible for the silver color of C. bombycina and the remarkable thermoregulatory properties of the hair coat covering the ant’s body.

Near-Infrared Tunable Reflection and Absorption Using Nanostructured Thin Film Structures Employing Phase-Change Material

We present the design of a polarization-dependent tunable nanostructured thin film absorber in the nearinfrared region. Germanium antimonide tellurite (GST) was employed as the phase change material in the designed structure. Our structure is composed of a periodic grating-type array of 150 nm thick Au buried with 50 nm thick GST layer from the top of the Au layer. The period of the gratings is 2 µm and in each period, GST width is 1 µm. GST was selected as the active phase change material because its optical properties undergo a substantial change during a structural transition from amorphous to crystalline phase. The optical absorption and reflection properties of the designed structure with respect to the geometric and material parameters were systematically investigated using the finite dierence time domain computations. It was shown that absorption peak or reflection dip at the resonant wavelengths in the near-infrared region was red shifted from 2039 nm to 2143 nm wavelength by switching the phase change material from its amorphous to crystalline states. The distributions of the electric field and absorbed power at the resonant wavelengths with respect to dierent phases of the GST were investigated to further explain the physical origin of the absorption. Our study provides a path toward the realization of tunable infrared absorbers for applications, such as selective infrared emitters, infrared camouflage, sensors, and photovoltaic devices. DOI: 10.12693/APhysPolA.129.464 PACS/topics: 78.67.Pt