Complex Refractive Index Research Articles

The Effect of Salt Concentration on Dielectric Permittivity and Interfacial Polarization in Carbonate Rocks with Complex Pore Structure

Summary Broadband dielectric dispersion measurements are attractive options for the assessment of water-filled porosity. Dielectric permittivity is influenced by salinity as well as other rock/fluid properties. However, the effect of salinity on Maxwell-Wagner polarization (i.e., interfacial polarization) and dielectric permittivity in rock samples with complex pore structures requires further investigation. The objectives of this work are (a) to perform frequency-domain dielectric permittivity numerical simulations on 3D pore-scale rock samples at different salt concentration levels, (b) to quantify the effect of salinity on dielectric permittivity and interfacial polarization in the frequency range between 20 MHz and 5 GHz, and (c) to quantify the critical frequency (i.e., the frequency at which the relative permittivity becomes frequency-independent). We first perform pore-scale frequency domain dielectric permittivity simulations in fully water-saturated carbonate samples with complex pore structures to obtain the complex dielectric permittivity in the frequency range of 0.01–5 GHz and at different salinity levels. Next, we numerically create partially water/hydrocarbon-saturated water-wet samples and perform simulations at different salinity and water saturation levels to investigate the combined effect of salinity and water saturation on dielectric permittivity. Finally, we investigate how reliable conventional mixing models, such as the complex refractive index model (CRIM) and Hanai-Bruggeman (HB), are in the assessment of water saturation at different salinity levels. We used 3D pore-scale rock samples with complex pore structures from Austin Chalk, Estaillades Limestone, and Happy Spraberry formations. The increase in salinity from 2 to 50 parts per thousand (PPT) resulted in the relative permittivity to increase by 18% at 20 MHz. Similarly, an increase in salinity from 2 PPT to 50 PPT resulted in electrical conductivity to increase by 15 times at 20 MHz. However, at 5 GHz, the difference between the relative permittivity of the samples at different salinities was negligible. We demonstrated that the critical frequency was above 1 GHz. Thus, if complex dielectric permittivity at 1 GHz is being used, an accurate salinity assumption is required in the interpretation of conventional dielectric mixture models in carbonate formations. Finally, we observed 52% and 42% average relative errors in water saturation quantification when applying CRIM and HB models at all the frequencies of interest, respectively. The results also indicated that conventional models should not be used in the presence of uncertainty in salinity at lower frequencies. The results of this work quantified the frequency at which the water-filled pore volume rather than the Maxwell-Wagner polarization controls the relative permittivity of rock samples saturated with a wide range of brine salinity. Moreover, the results demonstrated that the relative permittivity of the rock samples with complex pore structures may still be significantly affected by the interfacial polarization even at 1 GHz. Moreover, the results suggested that the conventional mixture methods cannot reliably take into account the salt concentration of formation water, and this can lead to significant errors in reserves assessment.

First complex refractive indices retrieval from FIR to UV: Application to kaolinite particles

First complex refractive indices retrieval from FIR to UV: Application to kaolinite particles

Measurement of nanocomposite scintillators by terahertz time domain spectroscopy.

Nanocomposite scintillators are expected to combine the advantages of inorganic and plastic scintillators, such as high detection efficiency, high light yield, fast decay time, low cost, and ease of processing. They are currently the forefront and hot field of scintillator research. In this study, a non-destructive method was developed for measuring the content of inorganic components in nanocomposite scintillators by terahertz time-domain spectroscopy. The complex refractive index of B a F 2 nanocomposite scintillators with different mass contents was measured in the terahertz band. As the mass content of B a F 2 nanoparticles increases, the refractive index and extinction coefficient of B a F 2 nanocomposite scintillators also gradually increase in the terahertz band. By combining the effective medium theory, the expected mass content was obtained, proving the feasibility of this measuring method.

Spectroscopic Evaluation of Epidermis‐Equivalent Phantom in Terahertz‐Frequency Region

AbstractThe complex refractive index and reflectance of an epidermis‐equivalent phantom were evaluated in the terahertz‐frequency region. The complex refractive indices of the epidermis and the epidermis‐equivalent phantom, made using ultrapure water, mineral oil, glycerin fatty acid ester, and agar, were measured using a terahertz time‐domain spectrometer. The complex refractive indices of the epidermis and the epidermis‐equivalent phantom were in agreement. However, their mean reflectances had a difference of approximately 3%. The difference disappeared on adding surface roughness to the epidermis‐equivalent phantom. Thus, we found that roughness of the surface of the epidermis‐equivalent phantom was required to ensure a match of the reflectance of the phantom with that of the epidermis at frequencies from 0.2 THz to 0.6 THz.

Structural, phononic, opto-electronic and elasto-mechanical properties of LiXBe (X = N, P) half Heusler compounds

The plethora of properties associated with half Heusler compounds has made them apposite materials for functional applications significantly in thermoelectrics, solar harvesting, semiconducting and spintronics. The current study invested the exertions in unveiling the details of physical properties of LiXBe (X = N, P) compounds using DFT based VASP and WIEN2k codes considering PBE-GGA and TB-mBJ exchange correlation functionals. Titled compounds successfully assured the stability checks regarding energetic, lattice dynamic and mechanical. By means of electronic structure calculations, TB-mBJ functional disclosed the precise band gaps of 6.912 and 2.340 eV for LiNBe and LiPBe compounds respectively. Fat-band character analysis and partial charge density distributions in HOMO and LUMO orbitals revealed the governess of band edges around Fermi-level by homo-site (N-2p & N-2s) states in LiNBe and hetero-site (P-2p & Li-2s) states in LiPBe compound. The COHP examination and bond order characteristics evidenced comparatively strongest bonding interactions of X–Be than Li–Be bonding pair. The calculated opto-electronic properties; complex dielectric function, optical conductivity, loss function and refractive index have been discussed with substantial details. Physio-mechanical parameters and mechanical anisotropy have been comprehensibly elaborated.

Modeling the Impact of Dye Concentration on Polymer Optical Properties via the Complex Refractive Index: A Pathway to Optical Engineering.

This work investigates the potential to rely on the complex refractive index to correlate the chemical composition of polymers with their optical properties, including transmittance, reflectance and absorbance. The optical properties of polycarbonate slabs with various controlled concentrations of two dyes were initially measured and analyzed. The reflection and transmission measurements obtained were used to determine the corresponding complex refractive index over a wide range of wavelengths. Comparing it with that of a clear material provided the spectral deviation of the complex refractive index induced by the dye concentrations and resulted in assigning a spectral efficiency to both of them. A modification function of the complex refractive index was established based on this spectral efficiency, which acts as a spectral fingerprint related to each dye. Finally, two samples doped with the two dyes mixed were studied to assess the model's capabilities. On the one hand, based on the measured transmittance, the dye concentrations were determined within a deviation below 8% in comparison with the values provided by the manufacturer. On the other hand, when the dye concentrations were known, the model reproduced the optical properties with good accuracy beyond the limitations of the experimental setup. The model's effectiveness in correlating the chemical composition of polymer with its optical properties through the complex refractive index makes it a valuable asset in analyzing and formulating plastics with intended optical properties.

Recent Advances and Current Trends in Transmission Tomographic Diffraction Microscopy.

Optical microscopy techniques are among the most used methods in biomedical sample characterization. In their more advanced realization, optical microscopes demonstrate resolution down to the nanometric scale. These methods rely on the use of fluorescent sample labeling in order to break the diffraction limit. However, fluorescent molecules' phototoxicity or photobleaching is not always compatible with the investigated samples. To overcome this limitation, quantitative phase imaging techniques have been proposed. Among these, holographic imaging has demonstrated its ability to image living microscopic samples without staining. However, for a 3D assessment of samples, tomographic acquisitions are needed. Tomographic Diffraction Microscopy (TDM) combines holographic acquisitions with tomographic reconstructions. Relying on a 3D synthetic aperture process, TDM allows for 3D quantitative measurements of the complex refractive index of the investigated sample. Since its initial proposition by Emil Wolf in 1969, the concept of TDM has found a lot of applications and has become one of the hot topics in biomedical imaging. This review focuses on recent achievements in TDM development. Current trends and perspectives of the technique are also discussed.

First-principles study on electronic, mechanical, and optical properties of pressure-induced vanadium-based perovskite KVO3

Vanadium-based compounds exhibit a variety of excellent characteristics due to their variable oxidation states. In this study, Density Functional Theory (DFT) is performed to investigate the structural, electronic, mechanical, and optical properties of KVO3 under high pressure. The thermodynamical and mechanical stabilities under high pressure are probed by the negative value of formation enthalpy and Born-mechanical stability criteria, respectively. KVO3 displays a nearly metallic nature in both pressurized and unpressurized systems owing to its remarkably narrow band gap. Furthermore, a positive pressure coefficient of the band gap is revealed from electronic properties. A lower effective mass value for charge carriers indicates KVO3's suitability for optoelectronic applications. The mechanical properties indicate that increasing pressure results in a slight increase in elastic constants, elastic moduli, hardness, and machinability index. Additionally, it has been disclosed that it is capable of enduring localized plastic deformation and fracturing under intense pressure in a prospective application. The calculations of the complex dielectric function, absorption, reflectivity, conductivity, refractive index, and loss function for KVO3 with and without pressure reveal a substantial behavior change. The absorption edge exhibits a significant blue shift with the applied pressure. The high peaks in the UV region suggest that it can be used in UV detectors and anti-reflection coatings. The refractive index shows promising application in waveguides.

A-Si/SiO2 nanolaminates for tuning the complex refractive index and band gap in optical interference coatings.

A-S i/S i O 2 nanolaminates are deposited by magnetron sputtering and show a decreasing absorption when the a-Si single-layer thickness is reduced from 2.4n m to 0.7n m. Moreover, an increase of the Tauc band gap by 0.18e V is measured. Experimental Tauc band gaps are compared to calculated effective band gaps, utilizing a numerical Schrödinger solver. Further, it is demonstrated that the refractive index can be controlled by adjusting the a-Si and S i O 2 single-layer thicknesses in the nanolaminates. The nanolaminates are optically characterized by spectroscopic ellipsometry, transmittance, and reflectance measurements. Additionally, TEM images reveal uniform, well-separated layers, and EDX measurements show the silicon and oxygen distribution in the nanolaminates.

Method to derive the infrared complex refractive indices n(λ) and k(λ) for organic solids from KBr pellet absorption measurements.

Obtaining the complex refractive index vectors n(ν~) and k(ν~) allows calculation of the (infrared) reflectance spectrum that is obtained from a solid in any of its many morphological forms. We report an adaptation to the KBr pellet technique using two gravimetric dilutions to derive quantitative n(ν~)/k(ν~) for dozens of powders with greater repeatability. The optical constants of bisphenol A and sucrose are compared to those derived by other methods, particularly for powdered materials. The variability of the k values for bisphenol A was examined by 10 individual measurements, showing an average coefficient of variation for k peak heights of 5.6%. Though no established standards exist, the pellet-derived k peak values of bisphenol A differ by 11% and 31% from their single-angle- and ellipsometry-derived values, respectively. These values provide an initial estimate of the precision and accuracy of complex refractive indices that can be derived using this method. Limitations and advantages of the method are discussed, the salient advantage being a more rapid method to derive n/k for those species that do not readily form crystals or specular pellets.

Monitoring the Electrochemical Failure of Indium Tin Oxide Electrodes via Operando Ellipsometry Complemented by Electron Microscopy and Spectroscopy.

Transparent conductive oxides such as indium tin oxide (ITO) are standards for thin film electrodes, providing a synergy of high optical transparency and electrical conductivity. In an electrolytic environment, the determination of an inert electrochemical potential window is crucial to maintain a stable material performance during device operation. We introduce operando ellipsometry, combining cyclic voltammetry (CV) with spectroscopic ellipsometry, as a versatile tool to monitor the evolution of both complete optical (i.e., complex refractive index) and electrical properties under wet electrochemical operational conditions. In particular, we trace the degradation of ITO electrodes caused by electrochemical reduction in a pH-neutral, water-based electrolyte environment during electrochemical cycling. With the onset of hydrogen evolution at negative bias voltages, indium and tin are irreversibly reduced to the metallic state, causing an advancing darkening, i.e., a gradual loss of transparency, with every CV cycle, while the conductivity is mostly conserved over multiple CV cycles. Post-operando analysis reveals the reductive (loss of oxygen) formation of metallic nanodroplets on the surface. The reductive disruption of the ITO electrode happens at the solid-liquid interface and proceeds gradually from the surface to the bottom of the layer, which is evidenced by cross-sectional transmission electron microscopy imaging and complemented by energy-dispersive X-ray spectroscopy mapping. As long as a continuous part of the ITO layer remains at the bottom, the conductivity is largely retained, allowing repeated CV cycling. We consider operando ellipsometry a sensitive and nondestructive tool to monitor early stage material and property changes, either by tracing failure points, controlling intentional processes, or for sensing purposes, making it suitable for various research fields involving solid-liquid interfaces and electrochemical activity.

Identification of the spectral complex refractive indices of micrometric phases within a semi-transparent medium up to elevated temperatures

Complex refractive indices of micrometric aluminosilicate fibers and of silica aerogel, components of a biphasic and semi-transparent material, were identified in the mid-infrared range from 295 to 1350 K. The identification was carried out by successive applications of a methodology combining infrared spectroscopy (FTIR), X-ray microtomography and Monte Carlo Ray Tracing. This identification technique proves to be an interesting tool for material design since it is now possible to forecast the radiative properties of heterogeneous and semi-transparent materials composed of identical microphases but involving different proportions and spatial distributions. With this approach, it is no longer necessary to identify effective indices or perform systematic FTIR spectroscopy measurements to get a first overview of the radiative properties of a new spatial layout of the microphases.

Broadband-Tunable Vanadium Dioxide (VO2)-Based Linear Optical Cavity Sensor.

Sensors fabricated by using a silicon-on-insulator (SOI) platform provide promising solutions to issues such as size, power consumption, wavelength-specific nature of end reflectors and difficulty to detect ternary mixture. To address these limitations, we proposed and investigated a broadband-thermally tunable vanadium dioxide (VO2)-based linear optical cavity sensor model using a finite element method. The proposed structure consists of a silicon wire waveguide on a silicon-on-insulator (SOI) platform terminated with phase-change vanadium oxide (VO2) on each side to provide light confinement. A smooth transmission modulation range of 0.8 (VO2 in the insulator state) and 0.03 (VO2 in the conductive phase state) in the 125 to 230 THz spectral region was obtained due to the of Fabry-Pérot (FP) effect. For the 3.84 μm cavity length, the presented sensor resulted in a sensitivity of 20.2 THz/RIU or 179.56 nm/RIU, which is approximately two orders of magnitude higher than its counterparts in the literature. The sensitivity of the 2D model showed direct relation with the length of the optical cavity. Moreover, the change in the resonating mode line width Δν of approximately 6.94 THz/RIU or 59.96 nm/RIU was also observed when the sensor was subjected to the change of the imaginary part k of complex refractive index (RI). This property of the sensor equips it for the sensing of aternary mixture without using any chemical surface modification. The proposed sensor haspotential applications in the areas of chemical industries, environmental monitoring and biomedical sensing.

Microfluidic–nanophotonic sensor for on-chip analysis of complex refractive index

Photonic biosensors based on photonic integrated circuits (PICs) and microfluidic channels (MFCs) have become the subject of intensive research for point-of-care (POC) device applications. In the presented work, we demonstrate the possibility of identifying the complex refractive index (RI) of analyzed liquids through the optimization of the geometry configuration of MFCs under PICs by experimental and numerical approaches. Our results suggest that the real and imaginary parts of the RI for analytes under study can be determined from spectrum of devices with optimized MFCs width. This work paves the way for promising opportunities to identify the presence and concentration of biological markers by using RI sensors for in situ POC applications.

The Optical and Structural Properties of Cu Nanoparticles: Graphene Prepared by Pulsed Laser Ablation in Deionized Water

Background: In this paper, graphene and copper oxide nanoparticles and graphene-based copper oxide nanoparticles have been produced by means of a pulsed laser ablation process (PLA) in a deionized water solution. Methods: The composition ratio of materials has been investigated in the structure of the prepared materials and their optical properties. The absorbance of the samples was obtained by the UV-VIS single beam spectrophotometer in the wavelength range of 290 to 800 nm. Spectroscopic ellipsometry method was used to investigate the linear optical properties of the samples including the real and imaginary parts of refractive index and dielectric function of the samples. The preferred model in the dielectric function modeling was Tauc-Lorentz. Also, the energy band gap of the samples has been calculated using Tauc relation. In addition, the nonlinear optical properties of graphene based copper oxide have been studied by Z-scan technique. Structure of the samples was studied using TEM image. Results: The most and the least absorbance at 532 nm wavelength, and also band gap energy belong to 1.4 ml Gr-0.6 ml Cu and copper oxide, respectively. Conclusion: The band gap energies of the samples were calculated between 3.30 eV and 3.43 eV. The real and imaginary parts of the complex refractive index were obtained in the order of 10-8cm2/W and 10-5cm/W. The results for nonlinear properties show that these samples are suitable for all-optical switching devices.

Optical and Microphysical Properties of the Aerosols during a Rare Event of Biomass-Burning Mixed with Polluted Dust

A rare event of mixed biomass-burning and polluted dust aerosols was observed over Athens, Greece (37.9° N, 23.6° E), during 21–26 May 2014. This event was studied using a synergy of a 6-wavelength elastic-Raman-depolarization lidar measurements, a CIMEL sun photometer, and in situ instrumentation. The FLEXPART dispersion model was used to identify the aerosol sources and quantify the contribution of dust and black carbon particles to the mass concentration. The identified air masses were found to originate from Kazakhstan and Saharan deserts, under a rare atmospheric pressure system. The lidar ratio (LR) values retrieved from the Raman lidar ranged within 25–89 sr (355 nm) and 35–70 sr (532 nm). The particle linear depolarization ratio (δaer) ranged from 7 to 28% (532 nm), indicating mixing of dust with biomass-burning particles. The aerosol optical depth (AOD) values derived from the lidar ranged from 0.09–0.43 (355 nm) to 0.07–0.25 (532 nm). An inversion algorithm was used to derive the mean aerosol microphysical properties (mean effective radius (reff), single scattering albedo (SSA), and mean complex refractive index (m)) inside selected atmospheric layers. We found that reff was 0.12–0.51 (±0.04) µm, SSA was 0.94–0.98 (±0.19) (at 532 nm), while m ranged between 1.39 (±0.05) + 0.002 (±0.001)i and 1.63 (±0.05) + 0.008 (±0.004)i. The polarization lidar photometer networking (POLIPHON) algorithm was used to estimate the vertical profile of the mass concentration for the dust and non-dust components. A mean mass concentration of 15 ± 5 μg m−3 and 80 ± 29 μg m−3 for smoke and dust was estimated for selected days, respectively. Finally, the retrieved aerosol microphysical properties were compared with column-integrated sun photometer CIMEL data with good agreement.

Method for bandgap interpolation of perovskite's spectral complex refractive index.

Lead halide perovskites are a promising class of materials for solar cell applications. The perovskite bandgap depends on the material composition and is highly tunable. Opto-electrical device modelling is commonly used to find the optimum perovskite bandgap that maximizes device efficiency or energy yield, either in single junction or multi-junction configuration. The first step in this calculation is the optical modelling of the spectral absorptance. This requires as input the perovskite's complex refractive index N as a function of wavelength λ. The complex refractive index consists of real part n(λ) and imaginary part k(λ). For the most commonly used perovskites, n and k curves are available from spectroscopic ellipsometry measurements, but usually only for a few discrete bandgap energies. For solar cell optimization, these curves are required for a continuous range of bandgap energies. We introduce new methods for generating the n and k curves for an arbitrary bandgap, based on interpolating measured complex refractive index data. First, different dispersion models (Cody-Lorentz, Ullrich-Lorentz and Forouhi-Bloomer) are used to fit the measured data. Then, a linear regression is applied to the fit parameters with respect to the bandgap energy. From the interpolated parameters, the refractive index curve of perovskite with any desired bandgap energy is finally reconstructed. To validate our method, we compare our results with methods from literature and then use it to simulate the absorptance of a single junction perovskite and a perovskite/silicon tandem cell. This shows that our method based on the Forouhi-Bloomer model is more accurate than existing methods in predicting the complex refractive index of perovskite for arbitrary bandgaps.

RI−Calc: A user friendly software and web server for refractive index calculation

The refractive index of an optical medium is essential for studying a variety of physical phenomena. One useful method for determining the refractive index of scalar materials (i.e., materials which are characterized by a scalar dielectric function) is to employ the Kramers−Kronig (K−K) relations. The K−K method is particularly useful in cases where ellipsometric measurements are unavailable, a situation that frequently occurs in many laboratories. Although some packages can perform this calculation, they usually lack a graphical interface and are complex to implement and use. Those deficiencies inhibit their utilization by a plethora of researchers unfamiliar with programming languages. To address the aforementioned gap, we have developed the Refractive Index Calculator (RI−Calc) program that provides an intuitive and user−friendly interface. The RI−Calc program allows users to input the absorption coefficient spectrum and then easily calculate the complex refractive index and the complex relative permittivity of a broad range of thin films, including of molecules, polymers, blends, and perovskites. The program has been thoroughly tested, taking into account the Lorentz oscillator model and experimental data from a materials' refractive index database, demonstrating consistent outcomes. It is compatible with Windows, Unix, and macOS operating systems. You can download the RI−Calc binaries from our GitHub repository or conveniently access the program through our dedicated web server at nanocalc.org.

Sophisticated Attenuated Total Reflection Correction Within Seconds for Unpolarized Incident Light at 45°.

The most common mid-infrared (MIR) attenuated total reflection (ATR) accessory has a nominal angle of incidence of 45° and does not have a polarizer. A spectrum recorded with such an accessory does not hold enough information for the sophisticated ATR correction of MIR spectra with strong peaks, which are often strongly affected by refractive index changes due to anomalous dispersion. Here we show that a 45° ATR spectrum recorded without a polarizer and the polarization angle for the same ATR Fourier transform infrared spectroscopy system provide enough information to determine the ATR s-polarized spectrum. Further analysis with an improved non-iterative Kramers-Kronig analysis immediately yields the complex refractive index function. The analysis is about two orders of magnitude faster than iterative formalism and runs within seconds on a typical office PC. The effectiveness of our advanced ATR correction formalism is showcased through its application to water, employing diamond, ZnSe, and Ge ATR crystals, along with two distinct ATR accessories. Additionally, the formalism is applied to octadecane spectra. Potential sources of errors such as incidence angle spread, dispersion of the polarization angle, and the influence of reflection at the air/ATR crystal interface are investigated by simulations.

Coupling radiative properties with detailed char conversion kinetics

In pulverized solid fuel combustion, radiation is the primary heat transfer mechanism. Especially in the near-burner region with high particle loadings, particle radiation dominates gas radiation. Thus, adequate modeling of particle-radiation interactions in full-scale simulations of combustion chambers may primarily affect the overall accuracy. The radiation behavior depends strongly on the particles? optical properties, characterized by the complex index of refraction (IOR). The IOR depends on particle morphology and material composition which change during pyrolysis and char conversion. For char conversion, the focus of the present paper, the change depends on how the structural transformations take place inside the particle, characterizes by the reaction regimes, boundary layer diffusion limited, pore diffusion limited or kinetically controlled. The formation of an outer ash film layer with progressing burnout can complicate the situation even more.To quantify the effects of the structural changes during the char conversion process on the radiative properties, the output data of the char conversion kinetics (CCK) model (particle size, ash/carbon content, pore fraction) are used to build artificial particles at different conversion degrees and for the different regimes. Finally, the radiation interaction is calculated by applying an extension of the Mie theory for coated particles, and these results are compared to simplified approaches.Simplified approaches were tested to derive the IOR by applying mixing rules based on the available char and ash content with progressing char conversion. The results indicate that only a non-linear mixing rule for the IORs of ash and char lead to appropriate approximations of the radiation behavior of the particle. Furthermore, it is not necessary to model the spatial arrangement of carbon core and ash film for the radiative properties. Modeling a sphere and calculating an effective IOR depending on the carbon, ash, and pore volume fraction results in an average deviation of less than 3% to the exact solution.