Measured Reflectance Spectra Research Articles

An Ultra-Portable Vis-NIR Spectrometer with an Integrated Light Source for Chemometric Applications

On-site material inspection and quality analysis of food and agricultural produce require portable sensing systems. We report the development of a miniaturized spectrometer with an integrated light source operating in the visible and near-infrared range, for chemometrics based material-sensing applications. The proposed system uses off-the-shelf light source and detector. The electronic circuit is designed, developed, and tested in-house. To validate the system’s usability, a set of classification experiments are carried out with measured spectra from culinary white powders and medicinal pills. Several classification algorithms are used to build predictive models and the best-suited ones give prediction accuracies of 80% and 92.6% respectively. A regression model built to estimate the curcumin content in turmeric shows a coefficient-of-determination of 0.97 for prediction. With more than 90% repeatability in the measured reflectance spectra, robustness of the device is demonstrated. Realization of a portable spectrometer, along with a framework for building appropriate prediction models, is expected to spur the development of point-of-use material sensing in the Vis-NIR range.

Photoluminescence evolutions and physical mechanism discussions of Sr1.95-(Ca/Ba) MgSi2O7:Eu2+ phosphors

Abstract With Ca and Ba substitutions for Sr, the photoluminescence evolution of Sr2MgSi2O7:0.05Eu2+ has been investigated and discussed. With spectra and crystal structure analysis, the photoluminescence intensity variation, the emission band shift, the chromaticity coordinate evolution and the thermal stability changes have been revealed. The corresponding physical mechanisms have been discussed. With Ca/Ba substitution, the emission spectra show entire red/blue shift; while the photoluminescence intensity are both quenched. The crystal structure and lattice parameter variations were refined. Based on the reflection spectra measurements, the forbidden band gap of the series of Sr1.95-x(Ca/Ba)xMgSi2O7:0.05Eu2+ phosphors were calculated. Both for Ca and Ba substitution, the forbidden band gap is elevated. The regular emission band red/blue shifts are ascribed to the increased/decreased crystal splitting of the Eu2+ 5d electron. Based on the estimation of the crystal splitting intensity and the calculation of the forbidden band gap, the Eu2+ 5d electron transition process and the Ca/Ba substitution tuning process are illustrated. The thermal stability of Sr2MgSi2O7: Eu2+ is promoted with Ca/Ba substitution. The thermal quenching mechanism of this phosphor can be confirmed to self-ionization. This work proves cation substitution is a suitable route to tune the photoluminescence property of the phosphors. The revealed physical mechanisms are in favor of understanding the tuning process of the phosphors.

Ground-based visible spectroscopy of asteroids to support the development of an unsupervisedGaiaasteroid taxonomy

Context.TheGaiamission of the European Space Agency is measuring reflectance spectra of a number to the order of 105small Solar System objects. A first sample will be published in theGaiaData Release scheduled for 2021.Aims.The aim of our work was to test the procedure developed to obtain taxonomic classifications for asteroids based only onGaiaspectroscopic data.Methods.We used asteroid spectra obtained using the DOLORES (Device Optimised for the LOw RESolution) instrument, a low-resolution spectrograph and camera installed at the Nasmyth B focus of the Telescopio NazionaleGalileo. Because these spectra have a higher spectral resolution than that typical of theGaiaspectra, we resampled them to more closely match the expectedGaiaspectral resolution. We then developed a cloning algorithm to build a database of asteroid spectra belonging to a variety of taxonomic classes, starting from a set of 33 prototypes chosen from the 50 asteroids in our observing campaign. We used them to generate a simulated population of 10 000 representative asteroid spectra and employed them as the input to the algorithm for taxonomic classification developed to analyzeGaiaasteroid spectra.Results.Using the simulated population of 10 000 representative asteroid spectra in the algorithm to be used to produce theGaiaasteroid taxonomy at the end of the mission, we found 12 distinct taxonomic classes. Two of them, with 53% of the sample, are dominant. At the other extreme are three classes each with <1% of the sample, and these consist of the previously known rare classes A, D/Ld, and V; 99.1% of the simulated population fall into a single class.Conclusions.We demonstrated the robustness of our algorithm for taxonomic classification by using a sample of simulated asteroid spectra fully representative of what is expected to be in theGaiaspectroscopic data catalogue for asteroids. Increasingly larger data sets will become available as soon as they are published in the futureGaiadata releases, with the next one coming in 2021. This will be exploited to develop a correspondingly improved taxonomy, likely with minor tweaks to the algorithm described here, as suggested by the results of this preliminary analysis.

Accurate Design of Solar Selective Absorber Based on Measured Optical Constants of Nano-thin Cr Film

Solar selective absorbers have significant applications in various photothermal conversion systems. In this work, a global optimization method based on genetic algorithm was developed by directly optimizing the solar photothermal conversion efficiency of a nano-chromium (Cr) film-based solar selective absorber aiming to work at the specified working temperature and solar concentration. In consideration of the semi-transparent metal absorption layer employed in multilayered solar selective absorbers, the optical constants of ultrathin Cr film were measured by spectroscopic ellipsometer and introduced into the optimization process. The ultrathin Cr film-based solar selective absorber was successfully designed and fabricated by the magnetron sputtering method for the working temperature at 600 K and a solar concentration of 1 Sun. The measured reflectance spectra of the sample show a good agreement with the numerical simulations based on measured optical constants of ultrathin Cr film. In comparison, the simulated results by using the optical constants of bulk Cr film or literature data exhibit a large discrepancy with the experimental results. It demonstrates the significance of considering the actual optical constants for the semi-transparent metal absorption layer in the design of nano-metal film-based solar selective absorber.

Metal–organic frameworks derived C/TiO2 for visible light photocatalysis: Simple synthesis and contribution of carbon species

A series of in-situ carbon-doped TiO2 (Cx/TiO2) composites with a porous and crystalline structure were successfully synthesized via one-step and low-temperature calcination of titanium metal–organic framework (MOF), MIL-125(Ti). The resultant materials were comprehensively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption measurements, UV–vis diffuse reflectance spectrum (DRS), photoluminescence (PL) spectra and photoelectrochemical measurements, and their photocatalytic activities for bisphenol A (BPA) degradation were assessed. Compared with the benchmark TiO2 photocatalyst (P25), the Cx/TiO2 composite material with high specific surface, lower band gap, and reduced photogenerated electron hole ratio exhibited outstanding photodegradation activity and durability for BPA, which could be attributed to the combined effect of co-doping of multiple carbon species (substituent carbon and carbonate) and porous structure. During BPA degradation, the holes and superoxide radicals were the primary role oxidative species in the reaction process. Therefore, this new efficient photocatalyst is promising candidate for photodegradation of organic pollutants.

Second Harmonic Generation from Phase-Engineered Metasurfaces of Nanoprisms.

Metasurfaces of gold (Au) nanoparticles on a SiO-Si substrate were fabricated for the enhancement of second harmonic generation (SHG) using electron beam lithography and lift-off. Triangular Au nanoprisms which are non-centro-symmetric and support second-order nonlinearity were examined for SHG. The thickness of the SiO spacer is shown to be an effective parameter to tune for maximising SHG. Electrical field enhancement at the fundamental wavelength was shown to define the SHG intensity. Numerical modeling of light enhancement was verified by experimental measurements of SHG and reflectivity spectra at the normal incidence. At the plasmonic resonance, SHG is enhanced up to ∼3.5 × 10 times for the optimised conditions.

Physical and optical studies of the novel non-crystalline CuxGe20-xSe40Te40 bulk glasses and thin films

In this research work, the authors devoted their efforts to study the physical and optical properties of the novel CuxGe20-xSe40Te40, CGST (x = 0,5,10,15,20 at.%) matrix. Physical properties of bulk samples and optical properties of thin films of this matrix have been studied. The bulk glassy samples were prepared using the known melt quenching method, while thin-film samples were deposited by vacuum deposition technique under pressure 10−4 Pa. X-ray diffraction examination showed that thin-film samples were deposited in the amorphous state, where no discrete sharp diffraction peaks were formed. Energy-dispersive X-ray spectroscopy exhibited also that there is a good agreement between the experimental results and the selected elemental composition ratios for all samples. The optical properties were studied using the spectrophotometric measurements of transmittance and reflectance spectra within the spectral range 300 nm–2500 nm. The absorption coefficient, the absorption index, optical energy gap, Urbach energy and others have been investigated and discussed on the view of the chemical band approach model. The optical energy gap decreases from 1.208 eV to 1.045 eV, while Urbach energy increases from 0.186 eV to 0.219 eV as Cu ratio increased from zero to 20%. These CGST samples showed very low absorbance nature in the NIR and IR regions. Therefore, they can be used in the applications of optoelectronics devices, infrared imaging, and optical filters and detectors.

Spectroradiometric detection of competitor diatoms and the grazer Poteriochromonas in algal cultures

To address challenges in early detection of pond pests, we have extended a spectroradiometric monitoring method, initially demonstrated for measurement of pigment optical activity and biomass, to the detection of algal competitors and grazers. The method relies upon measurement and interpretation of pond reflectance spectra spanning from the visible into the near-infrared. Reflectance spectra are acquired every 5 min with a multi-channel, fiber-coupled spectroradiometer, providing monitoring of algal pond conditions with high temporal frequency. The spectra are interpreted via numerical inversion of a reflectance model, in which the above-water reflectance is expressed in terms of the absorption and backscatter coefficients of the cultured species, with additional terms accounting for the pigment fluorescence features and for the water-surface reflection of sunlight and skylight. With this method we demonstrate detection of diatoms and the predator Poteriochromonas in outdoor cultures of Nannochloropsis oceanica and Chlorella vulgaris, respectively. The relative strength of these signatures is compared to microscopy and sequencing analysis. Spectroradiometric detection of diatoms is then further assessed on beaker-contained mixtures of Microchloropsis salina with Phaeodactylum tricornutum, Thalassiosira weissflogii, and Thalassiosira pseudonana, respectively, providing an initial evaluation of the sensitivity and specificity of detecting pond competitors.

Embossed Mie resonator arrays composed of compacted TiO2 nanoparticles for broadband anti-reflection in solar cells

Mie resonator arrays formed by embossing titanium dioxide (TiO2) nanoparticles (NPs) from solution are investigated as optical coatings for anti-reflection applications. Compacted nanoparticle assemblies offer unique possibilities to tailor the effective refractive index (RI). Here, we demonstrate a simple table-top, low pressure, and low temperature method to fabricate structured optical coatings. TiO2 nanostructures in the form of nanodisks support Mie resonances in the visible wavelength spectrum and exhibit strong forward scattering into the high index substrates, making them suitable as broadband anti-reflection coatings for solar cells. TiO2 NP-based nanodisk arrays are designed, fabricated, and characterized regarding their anti-reflection properties on Si, GaAs, and InP substrates and solar cells. Detailed finite-difference time-domain simulations are performed to optimize the TiO2 NP-based Mie resonator arrays for the broadband anti-reflection as well as to explain the measured reflectance spectra. The solar-weighted reflectance is used as a figure of merit (FoM). TiO2 nanodisk arrays on Si show a FoM of ~ 7% in the 400–1,100 nm wavelength spectrum; similar values are obtained for GaAs and InP substrates. TiO2 nanodisk arrays embossed directly on prefabricated planar single-junction Si, GaAs, and InP solar cells result in an appreciable increase (~ 1.3 times) in the short-circuit current densities.

Distributed measurement of Brillouin dynamic grating spectrum localized by an intensity- modulated correlation-domain technique.

Higher spatial resolution of the reflection spectrum measurement of Brillouin dynamic grating (BDG) was achieved by controlling phonon power distribution. We experimentally demonstrate the improvement effect of the light-source intensity-modulation method, proposed recently in a correlation-domain technique, and successfully detected an 8-cm cooled section in a 100-m-long polarization-maintaining fiber. Our method can improve the spatial resolution of BDG measurements, leading to high resolution discriminative and distributed fiber sensing of temperature and strain.

A spectral analysis of common boreal ground lichen species

Lichens dominate a significant part of the Earth's land surface, and are valuable bioindicators of various environmental changes. In the northern hemisphere, the largest lichen biomass is in the woodlands and heathlands of the boreal zone and in tundra. Despite the global coverage of lichens, there has been only limited research on their spectral properties in the context of remote sensing of the environment. In this paper, we report spectral properties of 12 common boreal lichen species. Measurements of reflectance spectra were made in laboratory conditions with a standard spectrometer (350–2500 nm) and a novel mobile hyperspectral camera (400–1000 nm) which was used in a multiangular setting. Our results show that interspecific differences in reflectance spectra were the most pronounced in the ultraviolet and visible spectral range, and that dry samples always had higher reflectance than fresh (moist) samples in the shortwave infrared region. All study species had higher reflectance in the backward scattering direction compared to nadir or forward scattering directions. Our results also reveal, for the first time, that there is large intraspecific variation in reflectance of lichen species. This emphasizes the importance of measuring several replicates of each species when analyzing lichen spectra. In addition, we used the data in a spectral clustering analysis to study the spectral similarity between samples and species, and how these similarities could be linked to different physical traits or phylogenetic closeness of the species. Overall, our results suggest that spectra of some lichen species with large ground coverage can be used for species identification from high spatial resolution remote sensing imagery. On the other hand, for lichen species growing as small assemblages, mobile hyperspectral cameras may offer a solution for in-situ species identification. The spectral library collected in this study is available in the SPECCHIO Spectral Information System.

IR characterization of plant leaves, endemic to semi-tropical regions, in two senescent states.

We are developing a robust and economic electro-optical remote sensing methodology to monitor the state of health and hydration of trees, endemic to subtropical regions. We measured reflectance spectra with Fourier transform infrared (FTIR) of three samples of two different oak trees. We find that spectral bands suitable for monitoring the state of the health and senescence of the oak include intervals around 0.9 µm and 1.8 µm. The easiest and the most cost-effective strategy would be to implement an electro-optical remote sensing radiometric system featuring a commercial camera incorporating a traditional charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) detectors and a wideband transmission filter, from about 0.8 to 1 µm.

A wide band porous silicon omnidirectional mirror for the near infrared range

We report the design, fabrication, and characterization of a porous silicon-based omnidirectional mirror for the near infrared range. The structure consists of 300 porous silicon chirped dielectric layers, optimized to have omnidirectional reflectivity response from 1000 to 2000 nm wavelength range. Measurements of reflectivity spectra are presented for non-polarized light at several incident angles (range 8°–65°) with a reflectivity >95% covering a 1μm band-width. Transfer matrix method calculations were carried out to show the complete angular range for both TM and TE polarizations including a simple model to illustrate the interface scattering effects.

Retraction Note to: Fabrication and characterization of Mg3Y2Ga3O12:Bi3+/TiO2-based UV fluorescent composition for improving the heavy oil viscosity reduction

Luminescent materials have been extensively explored in the past years for use in white LEDs, considering that previous works have paid little attention to it for other promising applications such as catalysts and water purification. Here we prepared a type of UV luminescent Mg3Y2Ga3O12:Bi3+ phosphor. To characterize the photoluminescence (PL) and structural properties, the Mg3Y2Ga3O12:Bi3+ samples have been recorded by the measurements of XRD, SEM, diffuse reflectance and PL spectra. The XRD and SEM results show that the Mg3Y2Ga3O12:Bi3+ samples belong to an inverse garnet cubic structure with a space group of Ia3d (No. 230) and distribute the particle size at about 30 μm. The PL spectra reveal that the Mg3Y2Ga3O12:Bi3+ samples can only show one emission band at around 350 nm, which can precisely cover over the whole UV spectral region from 290 nm to 410 nm. In view of this PL behavior and the TiO2 absorption band at around 387 nm, we design a type of ceramic substrate, i.e., Mg3Y2Ga3O12:Bi3+/TiO2-based composition, by mixing Mg3Y2Ga3O12:Bi3+ and TiO2. As a result, the photocatalytic properties of Mg3Y2Ga3O12:Bi3+/TiO2-based composition under 254 nm UV irradiation is found to enhance the heavy oil viscosity reduction about ~ 4 times as compared to that of TiO2 and bulk heavy oil.

In-situ ionothermal precipitation of well-dispersed ZnO nanoparticles onto 2-dimension neat graphene sheets with excellent photocatalytic activity

Neat graphene (NG) is rarely selected as a support to improve the photocatalytic performance of metal oxides due to its extremely poor processability in aqueous solutions. In this study, an in-situ ionothermal precipitation was developed to fabricate ZnO nanoparticle and few-layered NG sheet nanohybrids (ZnO/NG) in one-pot. Choline chloride/diethylene glycol-based deep eutectic solvent was used as reaction medium to dissolve the zinc salt precursor and disperse graphene. XRD, Raman, XPS and TEM analyses showed that ZnO particles with sizes of 15–25 nm were uniformly dispersed on the graphene sheets and a bond of ZnOC was formed between the interfaces. The UV–vis diffuse reflectance spectra measurement exhibited that the incorporation of graphene slightly narrowed the bandgap of ZnO nanoparticles, but significantly enhanced the light absorption of the nanohybrids in the visible range. Photocatalytic degradation of methylene blue dye evidenced that ZnO/NG offered excellent photocatalytic performance, regardless of UV or visible light irradiation. Under UV light, graphene as an electron transporter effectively promoted the separation of photogenerated carriers, while under visible light graphene was sensitized from the ground-state to the excited-state. The resulting electrons were injected into the conduction band of ZnO to transform wide bandgap ZnO into a photocatalyst driven by visible light. This work provided a green and simple method to prepare ZnO/NG nanohybrids with efficient and stable photocatalytic performance for water pollution remediation.

Synthesis, structure and photophysical properties of a new zinc compound

Under hydrothermal conditions, a new zinc compound, [Zn(2,2’-bpy)3]2S·16H2O·2OH (1) (2,2’-bpy = 2,2’-bipyridine) was synthesized and structurally characterized by using a single-crystal X-ray diffraction method. This compound is characteristic of an isolated (0-D) structure. A solid-state photoluminescence experiment revealed that it shows blue emission, in combination with molecular orbital (MO) calculations, lead us to conclude that the emission originates from the ligand-to-metal charge-transfer (LMCT) transition. Complex 1 possesses CIE chromaticity coordinates of (0.0782, 0.1973). A solid-state UV/Vis diffuse reflectance spectrum measurement unveiled that this compound features a wide optical band gap of 2.03 eV.

Predicting Soil Organic Carbon Content Using Hyperspectral Remote Sensing in a Degraded Mountain Landscape in Lesotho

Soil organic carbon constitutes an important indicator of soil fertility. The purpose of this study was to predict soil organic carbon content in the mountainous terrain of eastern Lesotho, southern Africa, which is an area of high endemic biodiversity as well as an area extensively used for small-scale agriculture. An integrated field and laboratory approach was undertaken, through measurements of reflectance spectra of soil using an Analytical Spectral Device (ASD) FieldSpec® 4 optical sensor. Soil spectra were collected on the land surface under field conditions and then on soil in the laboratory, in order to assess the accuracy of field spectroscopy-based models. The predictive performance of two different statistical models (random forest and partial least square regression) was compared. Results show that random forest regression can most accurately predict the soil organic carbon contents on an independent dataset using the field spectroscopy data. In contrast, the partial least square regression model overfits the calibration dataset. Important wavelengths to predict soil organic contents were localised around the visible range (400–700 nm). This study shows that soil organic carbon can be most accurately estimated using derivative field spectroscopy measurements and random forest regression.

Thermal coagulum formation and hemostasis during repeated multipulse Nd:YAG laser treatment of cutaneous vascular lesions: animal experiment study.

Laser therapy has been widely used to treat port-wine stain (PWS) and other cutaneous vascular lesions via selective photothermolysis. High incident laser fluence is always prohibited in clinic to prevent the thermal damage in normal skin tissue, leading to insufficient energy deposition on the target blood vessel and incomplete clearance of PWS lesion. In this study, repeated multipulse laser (RMPL) irradiation was proposed to induce acute thermal damage to target blood vessels with low incident fluence (40J/cm2 for 1064-nm Nd:YAG laser). The feasibility of the method was investigated using animal models. Repeated multipulse irradiation cycles with 10-min intervals were performed in RMPL. A hamster dorsal skin chamber model with a visualization system was constructed to investigate the instant generation of thermal coagulum and relevant hemostasis by thrombus formation during and after irradiation under 1064nm Nd:YAGsinglemultipulse laser (SMPL) and RMPLirradiation. The diameter of the target blood vessel and the size of thermal coagula were measured before and after laser irradiation. The reflectance spectra of the dorsal skin were measured by a reflectance spectrometer during RMPL. Stasis thermal coagula that clogged the vessel lumen were generated during SMPL irradiation with low incident fluence. However, there was no acute thermal damage of blood vessels. Reflectance spectra measurement showed that the generation of thermal coagula and subsequent thrombus formation increases blood absorption by more than 10% within the first 10min after laser irradiation. Acute vessel thermal damage could be induced in the target blood vessel by RMPL with low incident fluence of 40J/cm2. Compared with our previous SMPL study, nearly 30% reduction in incident laser fluence was achieved by RMPL. Low fluence RMPL may be a promising approach to improve the therapeutic outcome for patients with cutaneous vascular lesions by improving energy deposition on the target blood vessel.

Full spectra coloration and condition-dependent signaling in a skin-based carotenoid sexual ornament

AbstractCarotenoid-based traits commonly act as condition-dependent signals of quality to both males and females. Such colors are typically quantified using summary metrics (e.g., redness) derived by partitioning measured reflectance spectra into blocks. However, perceived coloration is a product of the whole spectrum. Recently, new methods have quantified a range of environmental factors and their impact on reflection data at narrow wavebands across the whole spectrum. Using this approach, we modeled the reflectance of red integumentary eye combs displayed by male black grouse (Lyrurus tetrix) as a function of ornament size and variables related to male quality. We investigated the strength and direction of effect sizes of variables at each waveband. The strongest effect on the spectra came from eye comb size, with a negative effect in the red part of the spectrum and a positive effect in ultraviolet reflectance. Plasma carotenoid concentration and body mass were also related to reflectance variance in differing directions across the entire spectra. Comparisons of yearlings and adults showed that the effects were similar but stronger on adult reflectance spectra. These findings suggest that reflectance in different parts of the spectrum is indicative of differing components of quality. This method also allows a more accurate understanding of how biologically relevant variables may interact to produce perceived coloration and multicomponent signals and where the strongest biological effects are found.

High-performance plasmonic refractive index sensors via synergy between annealed nanoparticles and thin films

Plasmonic nanostructure-based refractive index (RI) sensors are the core component of biosensor systems and play an increasingly important role in the diagnosis of human disease. However, the costs of traditional plasmonic RI sensors are not acceptable to everyone due to their expensive fabrication process. Here, a novel low-cost and high-performance visible-light RI sensor with a particle-on-film configuration was experimentally demonstrated. The sensor was fabricated by transferring annealed Au nanoparticles (NPs) onto a thin gold film with polymethyl methacrylate (PMMA) as a support. RI sensitivities of approximately 209 nm/RIU and 369 nm/RIU were achieved by reflection and transmission spectrum measurements, respectively. The high sensitivity is due to the strong plasmon-mediated energy confinement within the interface between the particles and the film. The possibility of wafer-scale production and high working stability achieved by the transfer process, together with the high sensitivity to the environmental RI, provides an extensive impact on the realization of universal biosensors for biological applications.