Reflectance Spectra Research Articles

Estimating retinal blood oxygenation from diffuse reflectance spectra of semi-infinite tissue using principal component analysis

Estimating retinal blood oxygenation from diffuse reflectance spectra of semi-infinite tissue using principal component analysis

Advancing nitrogen nutrition index estimation in summer maize using continuous wavelet transform

Rapid and non-destructive diagnosis of plant nitrogen (N) status is crucial to optimize N management during the growth of summer maize. This study aimed to evaluate the effectiveness of continuous wavelet analysis (CWA) in estimating the nitrogen nutrition index (NNI), to determine the most suitable wavelet analysis method, and to identify the most sensitive wavelet features across the visible to near-infrared spectrum (325–1,025 nm) for accurate NNI estimation. Field experiments were conducted across two sites (Kaifeng and Weishi) during the 2022 and 2023 growing seasons using four summer maize cultivars (XD20, ZD958, DH661, and DH605) under varying N application rates (0, 80, 160, 240, and 320 kg N ha-1). Canopy reflectance spectra and plant samples were collected from the V6 to V12 growth stages. The wavelet features for each spectral band were calculated across different scales using the CWA method, and their relationships with NNI, plant dry matter (PDM), and plant N concentration (PNC) were analyzed using four regression models. The results showed that NNI varied from 0.61 to 1.19 across different N treatments during the V6 to V12 stages, and the Mexican Hat wavelet was identified as the most suitable mother wavelet, achieving an R² value of 0.73 for NNI assessment. The wavelet features derived from the Mexican Hat wavelet were effective in estimating NNI, PDM, and PNC under varying N treatments, with the most sensitive wavelet features identified as 745 nm at scale 7 for NNI, 819 nm at scale 5 for PDM, and 581 nm at scale 6 for PNC using linear regression models. The direct and indirect methods for NNI estimation were compared using independent field data sets. Both methods proved valid to predict NNI in summer maize, with relative root mean square errors of 10.8% for the direct method and 13.4% for the indirect method. The wavelet feature at 745 nm, scale 7, from the direct method (NNI = 0.14 WF (745 nm, 7) + 0.3) was found to be simpler and more accurate for NNI calculation. These findings provide new insights into the application of the CWA method for precise spectral estimation of plant N status in summer maize.

Investigation of Charge Transfer Mechanism and Dielectric Relaxation in CsCdCl3 Perovskite

ABSTRACTThe exceptional optoelectronic capabilities of all‐inorganic metal halide perovskite semiconductor components open up a multitude of possible applications. Among all the metal halides, the chloride of cesium cadmium has not been investigated in great detail. Here, we describe a straightforward method for creating CsCdCl3 perovskite single crystals using the slow evaporation solution growth approach. These were investigated by utilizing X‐ray powder diffraction, and optical and impedance spectroscopies. The creation of a single‐phase with a hexagonal‐type structure was verified by the X‐ray powder diffraction (XRPD) data. The compound's semiconductor characteristics were verified by the optical measurement, indicating a direct band‐gap value of about 3.16 eV. The absorption and reflectance spectrum was also used to calculate and explain the optical extinction coefficient, Urbach energy, and skin depth as functions of the input photon's wavelength. Besides, the impedance spectroscopy technique was employed to investigate the characteristics of this component, across a frequency range of 10−1 Hz to 106 Hz and at temperatures ranging from 313 K to 453 K. The frequency behavior of the AC conductivity, σac, was analyzed using the universal Jonscher law. The outcomes of the charge transport investigation on CsCdCl3 imply that the perovskite material possessed a quantum mechanical tunneling (QMT) model for T < 363 K and a large polaron tunneling (OLPT) paradigm for T > 363 K. A correlation between the ionic conductivity and the crystal structure was established and discussed. Ultimately, the low dielectric loss and high dielectric constant of CsCdCl3 make it a promising material for energy harvesting devices.

Level splitting induced by collective interaction in a waveguide QED system

In waveguide quantum electrodynamics (QED) systems, noninteracting quantum entities can be coupled through the vacuum field of the waveguide, leading to both coherent and dissipative interactions. We systematically demonstrate that the reflection spectrum is clear and direct in representing the collective interaction using the mirror reflection properties of semi-infinite waveguides. Due to the destructive interference with its mirror image, the emitter is decoupled from the waveguide, suppressing the decay so that the reflection spectrum shows an anticrossing phenomenon induced by coherent interaction. Moreover, the reflection spectrum of an anti-parity-time symmetric Hamiltonian caused by dissipative coupling exhibits an unconventional splitting characterized by level attraction transitioning at exceptional points. Our findings highlight the significance of exploring non-Hermitian effects in waveguide quantum systems, offering potential applications in quantum networks reliant on long-distance interactions. Published by the American Physical Society 2024

Exciton dynamics in CsPbBr3 single crystal: LT splitting energy, exciton-polariton dispersion, and biexciton binding energy.

Metal halide perovskite materials (MHPs) are promising for several applications due to their exceptional properties. Understanding excitonic properties is essential for exploiting these materials. For this purpose, we focus on CsPbBr3 single crystals, which have higher crystal quality, are more stable, and have no Rashba effect at low temperatures compared to other 3D MHPs. We have estimated exciton energy positions, longitudinal-transverse splitting energy, and damping energy using low-temperature reflection spectra. Under high excitation intensity, two biexciton emissions (M-emission) and exciton-exciton scattering emission (P-emission) were observed. We assign the two M-emissions to the emission to the states of longitudinal and transverse excitons, i.e., ML and MT emissions. From the energy position of the MT emission, the biexciton binding energy has been estimated to be ∼2meV. By analyzing P-emission obtained from the back side of the sample, we have estimated the exciton binding energy to be 17.8-23.7meV. This estimation minimizes the influence of the wavenumber distribution in the scattering process. In addition, time-resolved transmittance measurements using pulsed white light have revealed the group velocity dispersion. Comparing experimental results with theoretical calculations using the Lorentz model clarifies that exciton dynamics in CsPbBr3 can be described with a simple Lorentz model. These insights enhance the understanding of exciton behavior and support the development of exciton-based devices using MHPs.

Controllability of optical, electrical, and magnetic properties of synthesized Cd-doped MgFe2O4 nanostructure

This study explores the synthesis of cadmium-magnesium co-doped magnesium ferrite (Mg1+xCdxFe2–2xO4, 0.00 ≤ x ≤ 0.30, Δx = 0.05) via a citric acid-assisted sol-gel auto-combustion method. X-ray diffraction (XRD) analysis revealed crystal structure changes due to Cd and Mg co-doping, with the 0.25 Cd sample showing the smallest crystallite size and highest lattice strain. High-resolution transmission electron microscopy (HRTEM) confirmed the nanostructure and largest surface area for this sample. Diffuse reflectance spectra, analyzed using the Kubelka-Munk relation, indicated that the 0.25 Cd sample had the lowest energy gap (2.08 eV), supported by band position calculations. Electrical conductivity and charge transport mechanisms were examined at various temperatures. Magnetization studies showed reduced saturation magnetization at x = 0.25, ascribed to cation distribution changes and spin canting, with coercivity and anisotropy decreasing with higher Cd content. Overall, the 0.25 Cd sample exhibited optimized optical, electrical, and magnetic properties due to its distinct microstructure.

Curing monitoring of adhesive layers between metal adherends by ultrasonic resonance technique

Abstract Layer resonance induced by ultrasonic wave incidence is applied to monitor the viscoelastic properties of a curing adhesive layer between metal plates. The theoretical analysis shows that when the adhesive layer is modeled as a linear viscoelastic material, the ultrasonic reflection spectrum takes local minima at the layer resonance frequencies depending on the wave velocity of the adhesive. On the other hand, the local minima of the reflection spectrum can be expressed as a function of the loss factor of the adhesive. Based on these results, a characterization technique for the wave velocity and the loss factor of a curing adhesive layer is proposed. This technique enables the evaluation of the viscoelastic properties even if the reflected waves from both faces of a bond layer cannot be separated. The proposed method is employed to investigate the curing behavior of bi-component epoxy adhesives. As a result, it is shown that the bonding condition affects the variation of the wave velocity and loss factor. The estimated wave velocity increases as the curing proceeds, while the loss factor sometimes takes a local maximum depending on the bonding condition and the frequency range. When the mixing ratio of the main and curing agents is imbalanced, the variation of the wave velocity becomes gradual. Furthermore, the increase in the curing temperature leads to fast changes in the wave velocity and loss factor. The proposed technique has the potential to provide insight into the curing behavior of the adhesive layer by incorporating it into other measurement methods and theories.

Visualization of patterned alkanethiol monolayers on an antireflective substrate using a reflected-light microscope

Abstract Organic monolayers are extensively used in various applications to modify substrate surfaces. However, their characterization remains challenging owing to their small thickness. Optical visualization techniques using antireflective (AR) substrates are particularly effective for patterned monolayers. In this study, we demonstrate the visualization of patterned alkanethiol monolayers on an AR multilayer substrate produced through microcontact printing methods. The AR substrate was designed to achieve near-zero reflectance at ~540 nm. The formation of the octadecanethiol monolayer caused a shift of approximately 2 nm toward longer wavelengths in the reflection spectrum. This shift was detected as a color difference by the digital camera attached to a reflected-light microscope. The contrast of the imaged pattern was enhanced by over 15% using a narrow band-pass filter at 530 or 550 nm. Consequently, line patterns with a width of approximately 6 μm were easily photographed using a commercially available color camera in a standard setup.

The Formation of Porous Silicon Using Vertical Photoelectrochemical Method with Laser Energy Variation

Due to its notable characteristics, porous silicon (PSi) has become the main focus in silicon upgrades for optoelectronics, medical, and sensor applications. Here, successful vertical photoelectrochemical fabrication of PSi based on crystalline silicon n-type wafers with orientation (100) has been accomplished. Silicon surfaces were anodized for 10 min in a solution of hydrofluoric acid and ethanol at a ratio of 1:3 and a current density of 4 mA/cm2 under laser illumination. Illumination sources were red, green, and purple laser beams with energies of 1.91 eV, 2.33 eV, and 3.06 eV, respectively, conducted to observe the effects on PSi microstructures and optical properties. The pores formed on the silicon surface were characterized via SEM, XRD, FTIR, and UV-Vis DRS Spectrophotometer. SEM analysis showed pore size distributions of 2.130, 3.353, and 1.078 mm for PSi samples with red, green, and purple lasers, respectively. XRD investigation revealed diffraction angles of 33.14° and 69.47° belonging to (211) and (422) planes, respectively, corresponding to the PSi. For samples of silicon and PSi with red, green, and purple lasers, the crystallite size and crystallinity were 168.55 nm and 44.80%, 25.02 nm and 17.12%, 29.19 nm and 23.56%, and 145.05 nm and 35.17%, respectively. FTIR observation confirmed that the PSi surface contained chemical bonds of Si-Si, Si-H, Si-H2, Si-O-Si, and C=O. UV-Vis DRS examination revealed the reflectance spectrum oscillation, indicating that lasers caused pore formation in PSi with bandgap energies of between 1 and 2 eV.

Probing plexciton emission from 2D materials on gold nanotrenches

Probing strongly coupled quasiparticle excitations at their intrinsic length scales offers unique insights into their properties and facilitates the design of devices with novel functionalities. In this work, we investigate the formation and emission characteristics of plexcitons, arising from the interaction between surface plasmons in narrow gold nanotrenches and excitons in monolayer WSe2. We study this strong plasmon–exciton coupling in both the far-field and the near-field. Specifically, we observe a Rabi splitting in the far-field reflection spectra of about 80 meV under ambient conditions, consistent with our theoretical modeling. Using a custom-designed near-field probe, we find that plexciton emission originates predominantly from the lower-frequency branch, which we can directly probe and map its local field distribution. We precisely determine the plexcitonʼs spatial extension, similar to the trench width, with nanometric precision by collecting spectra at controlled probe locations. Our work opens exciting prospects for nanoscale mapping and engineering of plexcitons in complex nanostructures with potential applications in nanophotonic devices, optoelectronics, and quantum electrodynamics in nanoscale cavities.

A spectral–structural characterization of European temperate, hemiboreal, and boreal forests

Abstract. Radiative transfer models of vegetation play a crucial role in the development of remote sensing methods by providing a theoretical framework to explain how electromagnetic radiation interacts with vegetation in different spectral regions. A limiting factor in model development has been the lack of sufficiently detailed ground reference data on both the structural and spectral characteristics of forests needed for testing and validating the models. In this data description paper, we present a dataset on the structural and spectral properties of 58 stands in temperate, hemiboreal, and boreal European forests. It is specifically designed for the development and validation of radiative transfer models for forests but can also be utilized in other remote sensing studies. It comprises detailed data on forest structure based on forest inventory measurements, terrestrial and airborne laser scanning, and digital hemispherical photography. Furthermore, the data include spectral properties of the same forests at multiple scales: reflectance spectra of tree leaves and needles (based on laboratory measurements), the forest floor (based on in situ measurements), and entire stands (based on airborne measurements), as well as transmittance spectra of tree leaves and needles and entire tree canopies (based on laboratory and in situ measurements, respectively). We anticipate that these data will have wide use in testing and validating radiative transfer models for forests and in the development of remote sensing methods for vegetation. The data can be accessed at Hovi et al. (2024a, https://doi.org/10.23729/9a8d90cd-73e2-438d-9230-94e10e61adc9) (for laboratory and field data) and Hovi et al. (2024b, https://doi.org/10.23729/c6da63dd-f527-4ec9-8401-57c14f77d19f) (for airborne data).

(In,Ga)N-GaN resonant Bragg structures with single and double quantum wells in the unit supercell

We study the formation of a superradiant optical mode in the room temperature reflection spectra from resonant Bragg structures (RBSs) composed of single and double (In,Ga)N quantum wells (QWs) in the unit cell. The appearance of the mode manifests itself by a significant increase in the resonant optical reflectivity due to the electromagnetic coupling of quasi-two-dimensional excitons in the QWs. The implementation of the supercells with double (In,Ga)N QWs results in an increase in the oscillator strength of the quasi-2D excitons and corresponding rise of the radiative broadening parameter to the value as high as 0.3 ± 0.02 meV. We also show that the supercells with double QWs are preferable for RBS with large number of periods due to better tolerance to deviations from the exact periodicity.

Optical and surface properties of Schiff base ligands and Cu(II) and Co(II) complexes

Objectives. To study the transition of electrons in 1,2-phenyl(4’-carboxy)benzylidene Schiff base ligand and transition metal ions, optical properties, as well as the surface chemistry of supported transition metals using diffuse reflectance spectroscopy (DRS); to study the roughness and morphology of the Schiff base ligand and its complexes using atomic force microscopy (AFM).Methods. DRS, AFM, and Fourier-transform infrared spectroscopy instruments were used to identify electron transitions, optical properties, and surface morphology in Schiff base ligands and their complexes.Results. The DRS revealed the d–d transitions and charge transfer shifts of all compounds, and helped identify the structure of the ligand. One of the optical properties studied was the energy gap calculation of the ligand and its complexes. The copper complex exhibited more semiconducting behavior with surface morphology properties such as surface roughness parameters lower than those of the ligand and the cobalt complex. This can be attributed to the smaller size of the copper atom, as well as lower electron transitions compared to the cobalt complex and the square planar bonding shape.Conclusions. In Schiff base ligands, the reflectance spectrum bands reveal three electron transitions: n→π*, π→π*, and σ→σ* transitions. In cobalt complexes, four transitions are indicated: 4A2(F)→4T1(F), 4A2(F)→4T1(P), charge transfer bands, and tetrahedral geometry. Copper complexes exhibit three transitions: 2B1g→2A1g, 2B1g→2Eg, and charge transfer bands, with a square planar geometry for their structure. The energy gap calculations were 2.42, 2.29, and 2.30 eV, respectively. In the case of the SH ligands, copper complexes, and cobalt complexes, all compounds exhibited semiconductor properties. However, the complexes displayed increased conductivity due to the influence of the metal and coordination structure.

Fano resonance based on ENZ mode in multilayer film and its optical switch application

This paper proposes a multilayer film structure consisting of an Au film, a Dysprosium-doped cadmium oxide (CdO:Dy) layer, and a SiO2 dielectric layer. The epsilon-near-zero (ENZ) mode is indirectly excited by the surface plasmon polariton (SPP) mode, and Fano resonance can be excited by the coupling of the two modes. The higher the wavelength-sensitivity of the real part of the dielectric constant of the ENZ material at the ENZ wavelength, the narrower the bandwidth of Fano resonance in the reflection spectrum. The indium tin oxide (ITO) is used to replace CdO:Dy as a switching material, and the ENZ wavelength is dynamically controlled by bias voltage. The ON and OFF states of the reflection spectrum can be switched at the wavelength of 1410 nm, achieving an insertion loss of 0.56 dB, an extinction ratio of 13.44 dB, and a figure of merit of 23.48 at this wavelength. This multilayer film structure has potential applications in the field of optical switches.

The Annealing Temperature Effect Enhances the Structural and Optical Parameters of Bi2Te3 Thermally Evaporated Thin Films

The current work investigates the influence of the annealing temperature (373, 473 K) on the structural and optical properties of Bi2Te3 thin films produced by the thermal evaporation technique. The crystal structure of samples is examined using an X-ray diffraction, indicating that Bi2Te3 thin films have a Rhombohedral structure with space group R-3m. The crystallite size (D) increased from 10.85 to 14.46 nm by increasing the annealing temperature. Additionally, increasing the annealing temperature reduces the micro-strain and the dislocation density from (3.33 × 10−3 to 2.5 × 10−3) and (8.49 × 1015 to 4.78 × 1015) Lines / m 2 , respectively. These micro-strain and dislocation density reductions refer to the crystallinity enhancement of sample Bi2Te3 thin films. The transmittance and reflectance spectra were measured using an Fourier transform infrared spectrometer. All samples reveal direct and indirect transition types and both direct and indirect optical energy gap decreased with increased annealing temperature from (0.325 to 0.29 eV) and (0.24 to 0.274 eV), respectively. From transmittance and reflectance data, the refractive index is calculated. These results reveal the refractive index decreasing as temperature annealing increased from (2.45–1.79) at 3250 nm. Other optical parameters such as absorption index, dielectric constant, energy loss function, and optical conductivity were estimated.

Nonlinear optical properties of some nanosized ammonium dihydrogen phosphate crystals incorporated in aluminum oxide nanopores

This study explores the nonlinear optical properties of aluminum oxide (Al2O3) matrices embedded in ammonium dihydrogen phosphate (ADP) nanocrystals using third-harmonic generation (THG) techniques. The successful integration of ADP crystallites within the nanopores of the Al2O3 matrix was confirmed using x-ray structural analysis and electron microscopy. The optical absorption characteristics were examined over a wide wavelength range, revealing that the reflection and transmission spectra were notably affected by pore size and surface scattering effects. THG measurements conducted with a high-intensity infrared laser demonstrated a pronounced third-order nonlinear response, whereas second-harmonic generation was not observed. The absence of SHG can be attributed to phase mismatch and inherent material properties, including centrosymmetry and surface roughness. The comparative and Reintjes models were used to calculate the third-order nonlinear optical susceptibilities. Among these, the Reintjes model provided the most accurate fit for the experimental data. This research underscores the considerable influence of nanopore size on THG efficiency, highlighting the importance of light scattering and phase-matching conditions. These findings contribute to a growing body of knowledge regarding nonlinear optical phenomena in nanocomposite materials and offer valuable insights for future applications in photonic devices. This comprehensive analysis highlights the potential of Al2O3 matrices with embedded ADP nanocrystals to advance the field of nonlinear optics in nanocomposite materials and offers insights for prospective applications in photonic devices.

Chiral polaritons in semiconductor perovskite metasurface enhanced by bound states in the continuum

Abstract The exploration of novel chiral optical platforms holds both fundamental and practical importance, which have shown great promise towards applications in valleytronics, chiral sensing and nanoscopic chiroptics. In this work, we combine two key concepts — chiral bound states in the continuum and exciton polaritons — to showcase a strong chiral response from polaritons. Using the Finite Element Method, we numerically design a CsPbBr3 based metasurface that supports intrinsically chiral bound states in the continuum and verify the chirality by calculating the reflection spectrum and eigen-polarization mapping. We further demonstrate chirality-dependent exciton polariton angular dispersion arising from the strong coupling between the chiral BIC and excitons in CsPbBr3 through simulating the polariton angle-resolved absorption spectrum. Reciprocity analysis reveals that the polariton photoluminescence in different momentum space locations is selectively enhanced by chiral pumping light. Our results suggest a promising first step towards chiral polaritonics.

Depth Mapping in Turbid and Deep Waters Using AVIRIS‐NG Imagery: A Study in Wax Lake Delta, Louisiana, USA

AbstractRemote sensing has been widely applied to investigate fluvial processes, but depth retrievals face significant constraints in deep and turbid conditions. This study evaluates the potential for depth retrievals under such challenging conditions using NASA's Airborne Visible/Infrared Imaging Spectrometer‐Next Generation (AVIRIS‐NG) imagery. We employ interpretable machine learning to construct a hyperspectral regressor for water depth and explore the spectral characteristics of deep and turbid waters in Wax Lake Delta (WLD), Louisiana, USA. The reflectance spectra of WLD show minor effects from depth differences due to turbidity. Nevertheless, a Random Forest with Recursive Feature Elimination (RF‐RFE) effectively generalizes high and low turbidity cases in a single model, achieving a of . Moreover, this model shows a maximum detectable depth of approximately 30 m, outperforming other methods. A spectral analysis using Shapley additive explanations (SHAP) points out the importance of learning various spectral bands and non‐linear relationships between depth and reflectance. Specifically, the short blue and Near‐InfraRed (NIR) bands, with high attenuation coefficients, play a crucial role. This finding highlights attenuation as the key process for deep‐depth retrievals. The depth maps of WLD produced by this model accurately capture the spatial distribution of deep river and shallow delta regions. However, the high dependency on short blue and NIR bands leads to discontinuous areas due to the noise sensitivity of these bands. This result highlights a drawback of remote sensing using empirical models. Future research will focus on correcting such discontinuities by integrating data from multiple remote sensing sources.

Innovative synthesis and practical application of graphitic carbon nitride/α-zirconium phosphate in visible light-activated Knoevenagel condensation

To advance the development of efficient photocatalysts activated by visible light for environmental applications. We employed a simple approach to synthesize g-C3N4/α-ZrP composite in different mass ratios, followed by comprehensive characterization through several techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope coupled with energy dispersive X-ray spectroscopy (SEM/EDS), and UV–Visible diffuse reflectance spectra (DRS). Furthermore, the g-C3N4/α-ZrP with mass ratio 6:1 was demonstrated high performance catalytic activity in Knoevenagel condensation (Yield 88 %–96 %) in a short reaction time (5–20 min) under visible light irradiation. The use of g-C3N4/α-ZrP has transformed the traditional synthesis of carbon–carbon double bonds, offering improved reaction rates and reduced by-products. These next-generation photocatalysts also exhibit remarkable properties that facilitate the activation of substrates under milder conditions, opening up new possibilities for green and sustainable synthesis. This advancement contributes to the development of environmentally friendly and efficient synthetic methodologies.

Near-infrared spectroscopy for analysing livestock diet quality: A systematic review

Near-infrared spectroscopy (NIRS) is a non-invasive and fast technology that has been increasingly used to analyse livestock diet quality. The objective of this study was to conduct a systematic review of the literature to examine the utilisation of NIRS technology for analysing livestock diet quality, with a focus on identifying trends, methodologies, and challenges in recent research. We conducted a systematic search of the literature on five electronic databases and retrieved 718 studies that have been published on the subject. Fifty-four studies were subsequently selected and investigated in depth. These studies were categorised into two groups, namely benchtop and portable, based on the types of NIRS devices utilised, with a majority employing the reflectance spectra mode. Our analysis found that standard normal variate (SNV), detrend (DT), and multiplicative scatter correction (MSC) are the most commonly used spectral data processing methods. The findings indicate that NIRS technology can provide accurate and reliable measurements of key livestock diet quality parameters such as crude protein, fibre, and moisture content. Additionally, we discuss the challenges associated with NIRS technology and provide recommendations for future research directions to further advance the use of NIRS technology in the livestock industry.