Refractive Index Spectra Research Articles

Study on Terahertz Time-domain Spectral Signatures of Wheat from Different Years

Wheat freshness is an important factor affecting wheat quality. In the current research, a total of 375 wheat samples from five different years (2016, 2017, 2018, 2019 and 2020) were identified by using the terahertz time-domain spectroscopy (THz-TDS) technology. The THz spectrum information of wheat was acquired, and the THz-TDS signals of wheat were obtained after Savitzky-Golay pretreatment. Then the data of frequency-domain spectra, refractive index spectra and absorption spectra of wheat within the range of 0.2∼1.4 THz were obtained after calculating the optical parameters. According to the experiment results, there were differences in the refractive indexes and absorption coefficients of wheat samples from different years. The refractive indexes of wheat samples from 2020 were significantly lower than that of wheat samples from other years, and the absorption coefficients of all five kinds of samples increased as the frequency increased. The study results showed that THz-TDS was of great potential for the effective determination of wheat freshness and it provided a basis for determination of wheat quality, bearing great practical significance.

Characterization and discrimination of human colorectal cancer cells using terahertz spectroscopy

Terahertz technology has been widely used in biomedical research. Herein, terahertz time-domain attenuated total reflection (THz TD-ATR) spectroscopy was employed to characterize and discriminate human cancer cell lines (DLD-1 and HT-29). Terahertz responses of the cell lines were measured and Savitzky-Golay algorithm was applied to smooth the spectra of refractive index, absorption coefficient and dielectric loss tangent in terahertz regime. Principal component analysis (PCA) was then adopted for feature extraction and cell characterization. Based on the processed data, cancer cell lines were discriminated by applying random forests (RF) method to analyze three characteristic parameters separately and the results from them were compared. Results indicate that absorption coefficient was the most sensitive parameter for cancer cell discrimination. Our study suggests great potential for human cancer cell recognition and provides experimental basis for liquid biopsy.

Ex vivo sensing and imaging of corneal scar tissues using terahertz time domain spectroscopy

Pulsed terahertz (THz) broadband spectroscopy and imaging were applied to investigate ex vivo rabbit corneal scar tissues. Scars with different depths and densities were created by laser ablation on four corneal samples while two corneal samples untouched were used as control. The THz time-domain spectroscopy and reconstructed images illustrated the scar tissue density and gave out depth variation distribution profiles. Calculated absorption coefficient spectra displayed extra absorption peaks compared with control corneas and refractive index spectra showed the optical properties changed at the corneal scar locations. THz spectroscopy and imaging demonstrated adequate contrast in reviewing the scar densities and locations and showed potential applications in depicting spatial distribution and composition changes of corneal scars.

A comparative study of structural, thermal, and optoelectronic properties between zircon and scheelite type structures in SrMoO4 compound: An ab-initio study

First principles calculations were carried out on the SrMoO4 compound which has been of interest owing to its technologically important physical properties. The structural, electronic, optical and thermal properties of this compound have been investigated under low pressure (LP) through the full potential linearized augmented plane wave method (FP-LAPW) within the framework of density functional theory. It is found that the phase transition from zircon to scheelite type structure occurs at negative pressure. The calculated ground state properties in scheelite structure are found to be in good accord with previously published data. Using the recently developed Tran Blaha-modified Becke Johnson approach, we have also studied the electronic band structure of this compound which shows the semiconducting behavior with a direct band gap of 4.30 eV in the scheelite phase, whereas the band gap is found to be 2.18 eV in the zircon phase. Various thermodynamic properties including the thermal volume expansion coefficient and heat capacities at constant volumes and pressures were calculated via the quasi-harmonic Debye model at different temperatures (0–1000 K). Furthermore, optical properties such as complex dielectric function, refractive index, and reflectivity spectra of the titled compound were studied for incident electromagnetic waves in an energy range up to 16 eV. The contributions to various transitions peaks in the optical spectra are analyzed and discussed with the help from the energy dependent imaginary part of the dielectric function.

The effect of pressure on the electronic and optical properties of $$\hbox {Sr}_{2}{\hbox {CeO}}_{4}$$: a first principles study

To better understand its physical properties, the effect of pressure on the structural, electronic, and optical properties of $$\hbox {Sr}_{\mathrm {2}}\hbox {CeO}_{\mathrm {4}}$$ were investigated through a first principles calculation approach based on density functional theory. The optimized unit cells are in good agreement with experimental data. The mechanical stability of $$\hbox {Sr}_{\mathrm {2}}\hbox {CeO}_{\mathrm {4}}$$ has been confirmed by calculation of the elastic constants. Additionally, the band structure, density of states and the partial density of states were obtained. It was shown that the $$\hbox {Sr}_{\mathrm {2}}\hbox {CeO}_{\mathrm {4}}$$ crystals belong to the semiconductor with a direct band gap of about 2.4 eV using LDA functional. The densities of states verify the observations made in the band structure and exhibit that the conduction bands are dominated by the Ce 4f and O 2p orbitals. The optical properties including the dielectric spectra, absorption coefficient spectra, reflectivity, and the real part of the refractive index spectra are also presented.

Impact of Zn2+ introduced into the central cavity of meso-tetra(4-pyridyl)porphine on its spectroscopic features

Abstract The effect of metal ion (Zn2+) introduced into the center cavity of macrocyclic meso-tetra(4-pyridyl)porphine, H2TPyP, on its spectroscopic feature is explored. Thin films of H2TPyP and ZnTPyP are successfully prepared using the thermal evaporation technique. The linear and nonlinear spectroscopic characterizations of H2TPyP and ZnTPyP films are investigated as comparative study. Our results indicate that some perturbations in the absorption spectroscopy of the H2TPyP due to Zn complexation to form ZnTPyP led to reduce four quasi degenerated Q-bands to two Q-bands and in addition to the Soret (B) band splits into two sub-bands (Bx & By) called Davydov splitting. The indirect allowed transition dominated the electronic absorption bands for both H2TPyP and ZnTPyP films and the values of optical band gap enhanced due to metalation of H2TPyP into ZnTPyP. A single oscillator model is applied to normal dispersion behavior of refractive index spectrum for H2TPyP and ZnTPyP films and from which, the dispersion parameters are calculated and exhibited an increased in their values due to insertion of Zn atom into H2TPyP. The nonlinear optical parameters such as third order susceptibility (χ(3)), nonlinear refractive index (n2) and nonlinear absorption coefficient (βc) are derived from dispersion parameters and showed a decrease in their values for ZnTPyP films with respect to H2TPyP films due to the introduction of Zn atom in central cavity of porphyrin macrocycle.

Comparison of equations used to estimate soot agglomerate absorption efficiency with the Rayleigh-Debye-Gans approximation

The contribution of soot aerosols to radiative forcing and climate change depends on their optical properties. Since these aerosols are agglomerates composed of quasi-spherical primary particles (usually small with respect to the radiation wavelength), their properties can be calculated following the Rayleigh-Debye-Gans approximation, which considers primary particles as independent absorbers and scatterers, with corrections made to account for multiple scattering and absorption within the agglomerate. A variety of equations can be used for particles small compared to the wavelength. In this study, results of the Rayleigh, Bohren and Huffman, simplified Bohren and Huffman, and Dobbins and Megaridis equations have been compared to the results from the exact Mie equations for the absorption efficiency. This comparison has been made for individual particles with different diameters and for an ensemble of primary particles composing an agglomerate. It has been shown that some of the largest primary particles of a typical agglomerate may not behave as Rayleigh absorbers at short wavelengths, contributing to errors in the estimation of the absorption efficiency. Both Bohren and Huffman and Dobbins and Megaridis equations are useful to extend the application range of the Rayleigh equation, thus reducing the accumulated error in the estimation of the agglomerate absorption efficiency. This analysis has been performed for several complex refractive index spectra. To choose the best equation for a particular application, the wavelength spectrum of the radiation and the range of primary particle sizes should be considered, aiming for a compromise between simplicity and accuracy.

Optical Characteristics of Annealed Barium Titanate Films Formed by the Sol-Gel Method

Sol-gel barium titanate films on quartz substrates annealed at 450, 600, and 700°C were studied using spectral ellipsometry and spectrophotometry in the region of intrinsic absorption bands to determine the thicknesses of surface layers, films, and film–air and fi lm–substrate transition layers. Decreases of the band gap, porosity, and thickness of the films and increases of their refractive indices and absorption with increasing annealing temperature were established. Spectra of refractive indices and absorption of the films were interpolated by dispersion formulas for new amorphous materials.

Исследование спектров комплексного показателя преломления пленок мононуклеотидов на кремнии в терагерцевом диапазоне

The possibility of obtaining spectra of the complex refractive index of mononucleotide films deposited on a polished silicon substrate in the terahertz (THz) range has been shown for the first time. The transmission of biopolymer samples with a THz spectrometer was measured. The natural absorption frequencies and refractive index spectrum of adenosine in the THz range were defined.

Structural, elastic, electronic and optical properties of the half-Heusler ScPtSb and YPtSb compounds under pressure

First-principles calculations using the plane-wave pseudopotential method within the generalized gradient approximation method were performed to study the pressure dependence of the structural, elastic, electronic and optical properties for the half-Heusler compounds ScPtSb and YPtSb in a cubic MgAgAs-type structure. The calculations were performed with the inclusion of spin-orbit coupling. The calculated equilibrium lattice parameters are in good agreement with the available experimental and theoretical values. The crystal rigidity and mechanical stability were discussed using the elastic constants and related parameters, namely bulk modulus, shear modulus, Debye temperature, Poisson's coefficient, Young's modulus and isotropic sound velocities. The calculated electronic band structures show that ScPtSb has an indirect gap of Γ-X type, whereas YPtSb has a direct band gap of Γ-Γ type. Furthermore, the effect of pressure on the optical properties, namely the dielectric function, absorption spectrum, refractive index, extinction coefficient, reflectivity and energy-loss spectrum is investigated for both compounds ScPtSb and YPtSb.

Study on the Optical Properties of High Refractive Index TeO2‐PbO‐ZnO‐BaF2 Glass System

The tellurite 60TeO2‐20PbO‐(20‐x)ZnO‐xBaF2 (x = 1, 3, 5, 6, 7, 9, and 11) series glass samples were prepared. The samples’ SEM, XRD, DSC, density, refractive index, and absorption spectra were tested. A self‐made optical platform was used to measure and calibrate the refractive index. The relationship between glass composition and crystallinity was discussed. The effects of density, molar refractive index, and metal standard value on the refractive index of glass samples were studied. The results showed that TPZBF glass has a high refractive index, and the refractive index increases with the increase in BaF2 content.

Thermal annealing induced changes in optical spectroscopic properties of tetrapyridyl-porphyrin thin films for energy applications

Optical properties of uniform and homogeneous thermally evaporated thin films of 5,10,15,20-tetra(4-pyridyl)-21H, 23H-porphyrin (TPyP) have been demonstrated in this paper. X-ray diffraction analysis indicated the presence of a nanocrystalline structure of TPyP films which enhanced by increasing the annealing temperature up to 473K. Optical studies were investigated using spectrophotometric measurements of the transmittance and the reflectance of pristine and annealed TPyP films and from which, optical constants and related parameters were calculated. The absorption spectra of TPyP films characterize different electronic transitions bands locate in the ultra-violet and visible spectral regions which are high intense Soret (B) band, four quasi-electronic states (Q-bands) and three weak bands called N, L and C bands. The edges of absorption bands were analyzed to estimate the type of transition bands, the optical energy gap and the Urbach energy tail of TPyP thin films. A single oscillator model was applied to a normal dispersion district of refractive index spectrum of TPyP films to determine many spectral dispersion parameters. Moreover, the surface and volume energy loss functions of TPyP films were determined. The annealing temperature-induced significant changes in the structural formation and optical spectroscopic features of TPyP thin films. The results of this study show that TPyP thin films could be used in energy conversion devices as a promising absorbing medium. • Uniform thin films of tetra(4-pyridyl)porphyrin (TPyP) were prepared. • Structural characterization of pristine and annealed TPyP films was investigated. • Optical constants and dispersion parameters of TPyP films were determined. • Surface and volume energy loss functions of TPyP films were studied.

Light extinction spectroscopy applied to polystyrene colloids: Sensitivity to complex refractive index uncertainties and to noise

The measurement of particle size distribution (PSD) in colloids and nanofluids presents many challenges, especially when it requires to be conducted in-situ and real-time. Our work aims to assess the capabilities of Light Extinction Spectroscopy (LES) technique to determine concentration and volumetric PSD (vPSD) of colloids and nanofluids. Numerical simulations are performed to verify robustness of LES data inversion algorithm and to identify the most relevant uncertainty sources. Experiments are carried out on well-dispersed water based colloids containing Polystyrene particles with median diameters at 120 nm and 300 nm. LES results are compared with supplier’s data, 3D-DLS measurements, and SEM images. Via sensitivity analyses, LES sensitivity to particle complex refractive index spectrum and to system noise level are examined. To improve accuracy, we limit LES data inversion at shorter wavelengths where high SNR is acquired. The noise is experimentally characterized and embedded into numerical simulations to better mimic the real conditions. We show that while noise level affects mostly vPSDs’ peak height, small inaccuracies in complex refractive index spectrum deteriorate the stability of the inversion, especially for smaller vPSDs. To overcome the latter, we carry out a preliminary in-situ calibration on particle complex refractive index spectrum. Accordingly, results are substantially improved with maximal discrepancies smaller than 3% for median and volume mean diameters, and lower than 13% for number concentrations. Besides, the retrieval of bi-modal distribution is promising. In conclusion, our methodology can be considered as a guideline to evaluate the applicability and accuracy of the LES technique in colloids.

The off-axis multi-Gaussian Schell-model hollow vortex beams propagation in free space and turbulent ocean

A new theoretical model for off-axis multi-Gaussian Schell-model hollow vortex (MGSMHV) beam generated by multi-Gaussian Schell-model (MGSM) source is introduced. The evolution properties (average intensity and beam widths) of the off-axis MGSMHV beams propagating in free space and turbulent ocean are analyzed. It is seen that the flatness of off-axis MGSMHV beam at the longer propagation distance can be controlled by choosing the source parameters total number F and coherence length σ. It is also seen that the larger effective beam sizes can be obtained by choosing the source parameters (larger total number F, or smaller coherence length σ) or propagating in turbulent ocean with smaller rate of dissipation of kinetic energy per unit mass ε, larger dissipation of mean-squared temperature xT or ratio of temperature and salinity contributions to the refractive index spectrum ζ.

Doping non-uniformity influence on the electrical and optical properties of chromium doped zinc selenide

Extended first-principles calculations have been performed to give a microscopic description of the local structure and their related electrical and optical properties for chromium doped zinc selenide (Cr2+: ZnSe) with different doping non-uniformity. Cr2+: ZnSe models with different Cr locations and distributions were built. The electronic band structure, density of state and complex dielectric constant of each model were calculated. It is proved that the distance between the two dopants Cr2+ ions can affect the local crystal structure of ZnSe, which makes the crystal changes from isotropic to anisotropic. The refractive index, extinction coefficient, absorption coefficient and reflectance spectra indicate the optical anisotropy of the non-uniformity Cr2+: ZnSe. The optical anisotropy is essentially caused by the unbalanced interaction between doped Cr2+ ions.

Optical spectra of titanium modificated surfaces with LIPSS and gold nanoparticles

This paper presents the reflection spectra of s - and p-polarized light and the refractive index spectra of anodized titanium surfaces modified by laser-induced periodic structures with gold nanoparticles. It is shown that these surfaces generate surface plasmons in the visible region of the spectrum, which leads to the formation of a negative values region in the refractive index.

Absorption properties and forcing efficiency of light-absorbing water-soluble organic aerosols: Seasonal and spatial variability.

Light-absorbing organic aerosols, also known as brown carbon (BrC), enhance the warming effect of the Earth's atmosphere. The seasonal and spatial variability of BrC absorption properties is poorly constrained and accounted for in the climate models resulting in a substantial underestimation of their radiative forcing estimates. This study reports seasonal and spatial variability of absorption properties and simple forcing efficiency of light-absorbing water-soluble organic carbon (WSOC, SFEWSOC) by utilizing current and previous field-based measurements reported mostly from Asia along with a few observations from Europe, the USA, and the Amazon rainforest. The absorption coefficient of WSOC at 365nm (babs-365) and the concentrations of carbonaceous species at Kanpur were about an order of magnitude higher during winter than in the monsoon season owing to differences in the boundary layer height, active sources and their strengths, and amount of seasonal wet precipitation. The WSOC aerosols during winter exhibited ∼1.6 times higher light absorption capacity than in the monsoon season at Kanpur site. The assessment of spatial variability of the imaginary component of the refractive index spectrum (kλ) across South Asia has revealed that it varies from ∼1 to 2 orders of magnitude and light absorption capacity of WSOC ranges from 3 to 21W/g. The light absorption capacity of WSOC aerosols exhibited less spatial variability across East Asia (5-13W/g) when compared to that in the South Asia. The photochemical aging of WSOC aerosols, indicated by the enhancement in WSOC/OC ratio, was linked to degradation in their light absorption capacity, whereas the absorption Ångström exponent (AAE) remained unaffected. This study recommends the adoption of refined climate models where sampling regime specific absorption properties are calculated separately, such that these inputs can better constrain the model estimates of the global effects of BrC.

Temperature-dependent optical and vibrational properties of PtSe2 thin films

PtSe2 has received substantial research attention because of its intriguing physical properties and potential practical applications. In this paper, we investigated the optical properties of bilayer and multilayer PtSe2 thin films through spectroscopic ellipsometry over a spectral range of 0.73–6.42 eV and at temperatures between 4.5 and 500 K. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 1000 nm and approached a constant value in the near-infrared frequency range. The thermo-optic coefficients of bilayer and multilayer PtSe2 thin films were (4.31 ± 0.04) × 10−4/K and (–9.20 ± 0.03) × 10−4/K at a wavelength of 1200 nm. Analysis of the optical absorption spectrum at room temperature confirmed that bilayer PtSe2 thin films had an indirect band gap of approximately 0.75 ± 0.01 eV, whereas multilayer PtSe2 thin films exhibited semimetal behavior. The band gap of bilayer PtSe2 thin films increased to 0.83 ± 0.01 eV at 4.5 K because of the suppression of electron–phonon interactions. Furthermore, the frequency shifts of Raman-active Eg and A1g phonon modes of both thin films in the temperature range between 10 and 500 K accorded with the predictions of the anharmonic model. These results provide basic information for the technological development of PtSe2-based optoelectronic and photonic devices at various temperatures.

Outage probability and channel capacity of an optical spherical wave propagating through anisotropic weak-to-strong oceanic turbulence with Málaga distribution.

The influence of anisotropic weak-to-strong oceanic turbulence on the performance of underwater optical communication (UWOC) systems is investigated in this paper. The Málaga distribution fading model is used to model the statistical distribution of a spherical wave propagating through anisotropic oceanic turbulence, which is a versatile model of weak-to-strong turbulence. First, the scintillation index for a spherical wave propagating in oceanic turbulence is formulated, and closed-form expressions for the outage probability and average channel capacity of the UWOC systems are then proposed in terms of Meijer's G function. The simulation results demonstrate that both the outage probability and the average channel capacity strongly depend on the parameters of oceanic turbulence, such as the ratio of temperature to the contribution of salinity to the refractive index spectrum, the rate of dissipation of kinetic energy per unit mass of fluid, and the rate of dissipation of mean-squared temperature; they are also related to system parameters such as wavelength and aperture diameter. Numerical results are provided to verify the accuracy of our proposed expressions for outage probability and average channel capacity, and perfect agreement is observed.

Temperature-dependent optical constants of monolayer {text {MoS}}_2, {text {MoSe}}_2, {text {WS}}_2, and {text {WSe}}_2: spectroscopic ellipsometry and first-principles calculations

The temperature-dependent (T = 4.5 , hbox {-} , 500 , hbox {K}) optical constants of monolayer {text {MoS}}_2, {text {MoSe}}_2, {text {WS}}_2, and {text {WSe}}_2 were investigated through spectroscopic ellipsometry over the spectral range of 0.73–6.42 eV. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 800 nm and approached a constant value of 3.5–4.0 in the near-infrared frequency range. With a decrease in temperature, the refractive indices decreased monotonically in the near-infrared region due to the temperature-dependent optical band gap. The thermo-optic coefficients at room temperature had values from 6.1 times 10^{-5} to 2.6 times 10^{-4} , hbox {K}^{-1} for monolayer transition metal dichalcogenides at a wavelength of 1200 nm below the optical band gap. The optical band gap increased with a decrease in temperature due to the suppression of electron–phonon interactions. On the basis of first-principles calculations, the observed optical excitations at 4.5 K were appropriately assigned. These results provide basic information for the technological development of monolayer transition metal dichalcogenides-based photonic devices at various temperatures.