Refractive Index Spectra Research Articles

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.

Pressure induced physical variations in the lead free fluoropervoskites XYF3 (X=K, Rb, Ag; Y=Zn, Sr, Mg): Optical materials

Abstract The optical and electronic properties of fluoro-perovskites XYF3 (X = K, Ag, Rb; Y=Zn, Sr, Mg) were investigated under pressure ranging from 0 to 50 GPa, utilizing full potential method based on density functional theory (DFT). The generalized gradient approximation GGA-PBE and GGA-PBEsol functional are used for calculating the structural properties while TB-mBJ functional is employed for the calculation of optical and electronic properties. The lattice constant increases by varying the cation from Zn to Mg to Sr. The volume-ratios as a function of pressure reveal that AgZnF3 has a low compressibility among the whole series of these compounds. At zero pressure, all these compounds exhibit indirect wide bandgap. The band gap of AgMgF3, KMgF3, KZnF3, RbMgF3 and RbZnF3 increases with pressure, while that of AgZnF3 and KSrF3 first increases with rising the pressure from zero to 10 Gpa for AgZnF3 and from zero to 5 Gpa for KSrF3 and then, decreases smoothly with increasing the pressure up to 50 GPa. The band gap type of RbSrF3 and KSrF3 change from indirect to direct at a pressure of 5 GPa and 40 Gpa, respectively. The high pressure shows significant influence on the density of states; it reduces and shift the highest peaks towards negative energy with broader bandwidths. The electron density plots explored a mixed covalent as well as ionic bonding at zero pressure. The covalent bonding becomes stronger under high pressure. The optical conductivity, refractive index and reflectivity spectra reveal a blue shift with the rise in pressure, due to rise in the band gap and the broadening of bands with compression.

Qualitative and Quantitative Analyses of Potassium Sorbate in Milk Powder using Terahertz Spectra

Using the terahertz time-domain spectroscopy system (THz-TDS), the potassium sorbate content in milk powder has been investigated. Firstly, the THz time-domain spectra of the samples were transformed into frequency spectra by Fourier transform, and then, through a set of simplified formulas of the optical parameters, the THz absorption spectra and refractive index spectra were obtained. Based on the characteristic absorption peak of potassium sorbate, a quantitative analysis model of potassium sorbate in milk powder was constructed using ordinary least squares (OLS) linear regression. In the case of low content (<1%) of potassium sorbate, a special THz absorption band (0.887–1.000 THz) was selected to construct a partial least squares (PLS) model that is called Sub-PLS. In addition, for qualitative analysis, we used partial least squares discrimination analysis (PLS-DA). Experimental results show that the proposed Sub-PLS model is superior to the PLS model based on full spectrum in accuracy, and the proposed PLS-DA is superior to other nonlinear regression methods such as support vector machine and neural network.

Non-destructive quantification of fragmentation within tablets after compression from scattering analysis of terahertz transmission measurements.

Material deformation behaviour has a critical impact on tablet formation. Fragmentation is one of the key mechanisms affecting the strength of a final compact, however, quantitative methods for estimating fragmentation are often complex, destructive and time-consuming. The purpose of this study was to investigate the applicability of terahertz time-domain spectroscopy (THz-TDS) to quantify fragmentation upon tableting. Up to five size fractions of microcrystalline cellulose (MCC), dibasic calcium phosphate (DCP), and lactose monohydrate (lactose) in the range of <125µm up to the range of 355-500µm were compressed into tablets and analysed with THz-TDS. The effective refractive index and absorbance spectra of whole tablets were measured in transmission, and the optical properties were clearly affected by fragmentation upon compression. The scattering observed from the absorbance spectra was fitted into a power law equation (y=AνB). It was observed that up to pressures of 50MPa the values of parameter A that were extracted from the power law fit decreased exponentially with increasing compression pressure. For higher compression pressures the value of A remained constant. This observation was more pronounced for DCP, followed by lactose and then MCC and the effect was more pronounced for larger compared to smaller initial particles. The non-destructive measurements correlated with previously obtained results based on particle size distribution measurements of the particles before compression and those obtained from destructive analysis of tablets. The terahertz method can resolve similar differences in fragmentation behaviour upon compression compared to the particle size analysis but requires no sample preparation.

Variable Optical Buffer Using EIT in Three Level System Based on Semiconductor Conical Quantum Dots

A variable semiconductor optical buffer based on the electromagnetically induced transparency (EIT) in a three level conical quantum dot system (CQD) is theoretically investigated. The system is interacting with two (control and signal) laser beams. Signal light with subluminal velocity is possible in such system through the quantum interference effect induced by the control pump field. We investigate the refractive index and absorption spectra of the QD waveguide at different pump levels, which exhibit an optimal pump power for maximum slow-down factor (SDF). The group velocity SDF is theoretically analyzed as a function of the pump intensity at different broadened line widths. The present study is based on the assumption that the medium is homogeneous. In this paper, a SDF as a function of CQD radius was studied. The simulation results indicate that the SDF increases with decreasing CQD radius.

Application of adaptive optics on bit error rate of M-ary pulse-position-modulated oceanic optical wireless communication systems

An adaptive optics correction arising from the sum of tilt, focus, astigmatism and coma components is applied to the bit error rate (BER) of M-ary pulse-position-modulated (PPM) oceanic optical wireless communication systems. The percentage reduction in BER is evaluated versus the oceanic turbulence parameters of the ratio of temperature to salinity contributions to the refractive index spectrum, the rate of dissipation of mean-squared temperature and that of kinetic energy per unit mass of fluid under different data bit rates, avalanche photodiode (APD) average current gains and the M values of the M-ary PPM. Our findings indicate that the percentage reduction in BER becomes larger when the ratio of temperature to salinity contributions to the refractive index spectrum or the rate of dissipation of mean-squared temperature or the data bit rate or the M value of the M-ary PPM is smaller, and when the rate of dissipation of kinetic energy per unit mass of fluid or the APD average current gain is larger.

Highly efficient solar-heat shield based on the bipolaron-assisted PEDOT:PSS thin film

We report on an efficient solar-heat shield based on the highly transparent PEDOT:PSS thin film, which significantly reduces the temperature from 38.2 ̊C to 33.5 ̊C. To understand the superior solar-heat shielding ability, the modified PEDOT:PSS thin films were investigated by using the transmittance spectra, Raman scattering spectra, atomic-force microscopic (AFM) images and broadband refractive index spectra. The experimental results show that the molecular structure of the PEDOT chains can be manipulated by varying the Brownian motion of PEDOT polymers in the PEDOT:PSS solutions and the spin rate. The lower Brownian motion of PEDOT polymers and the higher spin rate both can change the molecular structure of the PEDOT chains from bended structures (de-doping states) to linear structures (doping states), which simultaneously increase the near-infrared absorption strength and the visible transparency due to the formation of bipolarons.

Intensity correlations of flat-topped beams in oceanic turbulence

ABSTRACT Intensity correlations of flat-topped beams are formulated and evaluated in oceanic turbulence. Variations of the intensity correlations are examined against the diagonal distance from different starting points at the receiver plane, for the various number of beams composing the flat-topped beam, for various starting points at the receiver and for various source sizes. Also, the variations of the intensity correlations are investigated against the ratio of temperature to salinity contributions to the refractive index spectrum for the various number of beams composing the flat-topped beam, against the rate of dissipation of mean-squared temperature for various starting points at the receiver plane and against the rate of dissipation of kinetic energy per unit mass of fluid for various source sizes.

A2AgCrBr6 (A = K, Rb, Cs) and Cs2AgCrX6(X = Cl, I) Double Perovskites: A Transition-Metal-Based Semiconducting Material Series with Remarkable Optics.

With an interest to quest for transition metal-based halogenated double perovskites AB′B″X6 as high performance semiconducting materials for optoelectronics, this study theoretically examined the electronic structures, stability, electronic (density of states and band structures), transport (effective masses of charge carriers), and optical properties (dielectric function and absorption coefficients, etc.) of the series A2AgCrBr6 (A = K, Rb, Cs) using SCAN + rVV10. Our results showed that A2AgCrBr6 (A = Rb, Cs), but not K2AgCrBr6, has a stable perovskite structure, which was revealed using various traditionally recommended geometry-based indices. Despite this reservation, all the three systems were shown to have similar band structures, density of states, and carrier effective masses of conducting holes and electrons, as well as the nature of the real and imaginary parts of their dielectric function, absorption coefficient, refractive index, and photoconductivity spectra. The small changes observed in any specific property of the series A2AgCrBr6 were due to the changes in the lattice properties driven by alkali substitution at the A site. A comparison with the corresponding properties of Cs2AgCrX6 (X = Cl, I) suggested that halogen substitution at the X-site can not only significantly shift the position of the onset of optical absorption found of the dielectric function, absorption coefficient and refractive spectra of Cs2AgCrCl6 and Cs2AgCrI6 toward the high- and low-energy infrared regions, respectively; but that it is also responsible in modifying their stability, electronic, transport, and optical absorption preferences. The large value of the high frequency dielectric constants—together with the appreciable magnitude of absorption coefficients and refractive indices, small values of effective masses of conducting electrons and holes, and the indirect nature of the bandgap transitions, among others—suggested that cubic A2AgCrBr6 (A = Rb, Cs) and Cs2AgCrCl6 may likely be a set of optoelectronic materials for subsequent experimental characterizations.

Characterization of thermal barrier coatings microstructural features using terahertz spectroscopy

A novel approach was presented to characterize microstructural features of thermal barrier coatings (TBCs) using terahertz spectroscopy based on machine learning algorithms. In this study, the microstructures of yttria-stabilized zirconia (YSZ) atmospheric-plasma-sprayed (APS) thermal barrier coatings were regulated by choosing different kinds of spray powders, distances and power during processing. A terahertz time-domain spectroscopy system configurated transmission mode with an incidence angle of 0° was employed to estimate terahertz properties of porous YSZ ceramic coatings, including refractive index, extinction coefficient and relative time-domain broadening ratio. The variation tendency of terahertz properties of YSZ ceramic coatings with different microstructure features (porosity, pore to crack ratio, pore size) were investigated. Principal component analysis (PCA) method was adopted to reduce the dimensions of refractive index and extinction coefficient spectra data at the range of 0.6–1.4 THz and to ensure that different terahertz properties could be treated as inputs with similar weights during modeling. Three models (multiple linear regression (MLR), back-propagation (BP) neural network and support vector machine (SVM)) were set up to conduct regression analysis. As a result, according to the contribution rates of eigenvectors, the top one principal component of refractive index spectra data and the top two principal components of extinction coefficient spectra data were selected as the model inputs. The correlation coefficient comparisons showed that the characterization accuracy of PCA-SVM reached by over 95% and outperformed the other models. Finally, this study proposed that THz nondestructive technology combined with machine learning technique is efficient and feasible for microstructural features characterization and has profound implications for the structure integrity of TBCs evaluation in gas turbine blades.

Optical properties of a hexagonal C/BN framework with sp2 and sp3 hybridized bonds

We investigated the optical properties and roles of sp2- and sp3-hybridized bonds of a hexagonal C/BN family using first-principles calculations. The calculated phonon dispersions confirm the dynamic stability of Hex-(BN)6C12 and Hex-C12(BN)6. The complex dielectric function evolves from the infrared to the ultraviolet region and has a significant anisotropy for different polarizations. The reflectivity and refractive index spectra show that the sp2-hybridized C atoms are more sensitive to the light from infrared to visible region than B-N pairs while the C atoms and B-N pairs have a similar sensitivity to high frequencies. The sharp peaks of the energy-loss spectrum are all concentrated in the 23–30 eV energy region, which can be used to identify these hexagonal structures. The calculated band structures show Hex-C24 and Hex-(BN)6C12 are metals, but Hex-C12(BN)6 and Hex-(BN)12 are semiconductors with indirect band gaps of 3.47 and 3.25 eV, respectively. The electronic states near the Fermi level primarily originate from sp2-hybridized atoms. In addition, sp2-hybridized bonds are the main elements affecting the optical and electronic structure of C/BN materials with sp2- and sp3-hybridizations. We expect that the results presented will help understand the optical properties of C/BN materials containing sp2- and sp3-hybridized C atoms and B-N pairs.

Multi-Foulant-Resistant Material Design by Matching Coating-Fluid Optical Properties.

The buildup of corrosion deposits, known as fouling, seriously hinders large-scale energy production. From nuclear power plants to geothermal reservoirs, fouling increases system pressure drops, impedes heat transfer, and accelerates corrosion, leading to derating and early failure. Here, we investigate the collodial interactions between multiple foulants and coated surfaces, with the aim of discovering principles for minimizing the adhesion of foulants to them. We hypothesize that matching the full refractive index spectrum of a coating to its surrounding fluid minimizes the adhesion of all foulants entrained within and that the Lifshitz theory is sufficient to predict which materials will be multi-foulant-resistant. First-principle calculations of Hamaker constants and refractive indices of six foulants on six coatings in water correlate well to direct measurements of adhesion by atomic force microscopy (AFM)-based force spectroscopy. Amorphous 2% fluorine-doped tin oxide, crystalline SiO2, CaF2, and Na3AlF6, which all nearly match the refractive index spectrum of water, successfully resisted adhesion of six diverse foulant materials in aqueous AFM measurements. The validation of this design principle may be expanded to design multi-fouling-resistant coatings for any system in which van der Waals forces are the dominant adhesion mechanism.

Spectral and Physical Studies on Some Metal Complexes of Lysine Amino Acid and the Adsorption Properties of their Nano Oxides

AbstractLys-Zn, Lys-Co, Lys-Cu and Lys-Ni complexes prepared by reaction of metal acetates with lysine (Lys). Complexes were characterized by elemental analysis (C, H, N and M), FT-IR, (UV/Vis) electronic spectra, 1H-13C NMR, thermal analyses (TGA), (ESR) spectroscopy and magnetic moments measurements. Physical studies were applied on the synthesis complexes such as solubility, density, molar volume, refractive index and excess refractive index and electronic (UV/Vis) spectra measurements. NiO nanoparticles were synthesized from calcinations of complex at 800 oC. Nanoparticles were characterized by X-ray powder diffraction (XRD) and transmittance electron microscope (TEM). Particle size is at range 43-52 nm. Nanoparticles were used to remove dyes from aqueous solution such as methylene blue (MB) and the percent of removal is 96 %. The best model which represents the adsorption of MB is Langmuir isotherm. The best model for adsorption kinetic of MB is pseudo-second-order model with determination coefficient (R2) close to the unity.

Theoretical investigation of the structural, elastic, electronic, and optical properties of the ternary tetragonal tellurides KBTe2 (B = Al, In)

Structural parameters, elastic constants, electronic structure and optical functions of the ternary tetragonal chalcogenides KAlTe2 and KInTe2 are predicted via ab initio calculations. The calculated structural parameters and interatomic distances agree very well with the existing experimental results. The computed band structures show that the two studied materials are semiconductors with indirect band-gaps. The computed partial density of states diagrams, charges transfer and distribution of charge density show that the interatomic bonds are mainly of a covalent nature. The calculated elastic constants indicate that the title compounds are mechanically stable and elastically strongly anisotropic. The calculated optical spectra for both explored compounds exhibit a significant optical anisotropy. Both compounds have a low absorption in the visible range compared to the infrared (IR) domain. The refractive index spectrum of KInTe2 is more anisotropic than that of KAlTe2.

Temperature-dependent terahertz time-domain spectroscopy study of Mg-doped stoichiometric lithium niobate

Terahertz time-domain spectroscopy measurements were performed on 0.7 mol% Mg-doped stoichiometric lithium niobate crystal with ordinary and extraordinary polarization in the 4-460 K temperature range. The absorption coefficient and refractive index spectra were recorded in the terahertz frequency range from 0.5 to 1.8 THz. The data extracted from the measurements are given in simple and concise form in order to provide easily usable practical information for those who would like to use this material in the terahertz range. Through a practical example it was also pointed out, that the effect of the temperature must not be neglected during the design and adjustment of terahertz sources if the goal is to maximize the optical-to-THz conversion efficiency.

Optical Characteristics of Parallel-Sided Quartz Plates at Brewster Angles Determined by Spectral Ellipsometry

A method is proposed for determining the optical characteristics of glass plates after physicochemical surface treatment by measuring spectra of ellipsometric angles at incidence angles equal to Brewster’s angle. The dispersion dependence of the refractive index could be determined and the surface layer thickness during the plate manufacturing process could be estimated using the coordinates of the ellipsometric-angle spectral minima. Refractive-index spectra calculated by the Brewster formula for KU-1 optical quartz plates in the non-absorbing region were in satisfactory agreement with spectra determined by numerical methods in the DeltaPsi2 software of a UVISEL 2 spectral ellipsometer (Horiba). The relative error of a refractive-index determination of the quartz plates by the proposed method was ≤0.1%.

Optical constants and critical point energies of (AgInSe2)0.75–(In2Se3)0.25 single crystals

AgInSe2 and In2Se3 are two attractive semiconducting materials for various technological applications especially for photovoltaic applications. In the present study, structural and optical properties of (AgInSe2)x–(In2Se3)1−x crystals for composition of x = 0.75 corresponding to chemical formula of Ag3In5Se9 were characterized by X-ray diffraction, energy-dispersive spectroscopy, room temperature transmission, and ellipsometry experiments. The transmittance spectrum was analyzed to reveal energy band gap. The derivative spectrophotometry analysis resulted in band gap energy of 1.22 eV. The spectra of complex dielectric function, refractive index and extinction coefficient were presented between 1.6 and 6.2 eV from the outcomes of ellipsometry analyses. Critical point energies have been determined using the derivative analyses of dielectric function. Five critical points at 2.70, 3.30, 4.05, 4.73, and 5.42 eV were revealed from the analyses. Crystal structure and atomic composition in semiconducting compound were also reported in the present work. The obtained results were compared with those reported for constituent compounds.

External Cavity Quantum Cascade Laser-Based Mid-Infrared Dispersion Spectroscopy for Qualitative and Quantitative Analysis of Liquid-Phase Samples.

Acquisition of classical absorption spectra of liquids in the mid-IR range with quantum cascade lasers (QCLs) is often limited in sensitivity by noise from the laser source. Alternatively, measurement of molecular dispersion (i.e., refractive index) spectra poses an experimental approach that is immune to intensity fluctuations and further offers a direct relationship between the recorded signal and the sample concentration. In this work, we present an external cavity quantum cascade laser (EC-QCL) based Mach-Zehnder interferometer setup to determine dispersion spectra of liquid samples. We present two approaches for acquisition of refractive index spectra and compare the qualitative experimental results. Furthermore, the performance for quantitative analysis is evaluated. Finally, multivariate analysis of a spectrally complex mixture comprising three different sugars is performed. The obtained figures of merit by partial least squares (PLS) regression modelling compare well with standard absorption spectroscopy, demonstrating the potential of the introduced dispersion spectroscopic method for quantitative chemical analysis.

Optical properties of deoxyribonucleic acid thin layers deposited on an elastomer substrate

The paper presents our research on the optical properties of thin-film deoxyribonucleic acid (DNA) complexes based on cetyltrimethylammonium (DNA-CTMA) deposited onto an optical elastomer substrate, which may be suitable for the construction of biophotonic devices. The study involved the measurement of Raman spectra, absorption spectra from the visible to the near-infrared region and the values of the refractive indices by m-line spectroscopy at five wavelengths (473, 632.8, 964, 1311, and 1552 nm). The samples were proved to have waveguiding properties from visible to infrared spectrum and high contrast of refractive index - the value is 0,0457. Photoluminescence measurement was done under excitation at three wavelengths (355, 405 and 450 nm) and showed one broadband with maxima at 437, 520 and 530 nm depending on the excitation wavelengths. The study has demonstrated that this combination of polymers, because of its unique properties, has great potential for the implementation of all-polymer structures in the applications of high-density photonics and biocompatible optical devices.

Adaptive optics correction of scintillation in underwater medium

ABSTRACT Adaptive optics correction of the scintillation index of a Gaussian laser beam in underwater turbulence is studied. To introduce the adaptive optics correction, filter functions providing the piston, tilt and astigmatism effects are adapted to promote the spectrum of underwater turbulence. The reduction of the scintillation index due to the individual piston, tilt, astigmatism effects and their sum is examined versus the ratio of temperature to salinity contributions to the refractive index spectrum, the rate of dissipation of mean squared temperature, the rate of dissipation of kinetic energy per unit mass of fluid, receiving aperture diameter, source size, link length and the wavelength. For any value of underwater turbulence parameter, the most effective adaptive optics corrections are found to be the piston, tilt and astigmatism, respectively.