Complex Refractive Index Research Articles

Pre-filter analysis for retrieval of microphysical particle parameters: a quality-assurance method applied to 3 backscatter (β) +2 extinction (α) optical data taken with HSRL/Raman lidar.

We analyze the solution space of 3β+2α optical data inferred from lidar measurements, i.e., backscatter coefficients at three wavelengths and extinction coefficients at two wavelengths. These optical data are governed by microphysical parameters that can be expressed in terms of particle size distribution, effective radius, and complex refractive index (CRI). In our analysis, we consider two scenarios of the solution space. First, it can be expressed in terms of monomodal particle size distributions represented either by fine modes or by coarse modes. Secondly, the particle size distributions contain a fine mode as well as a coarse mode. Consideration of both scenarios and different values of the effective radius and CRI allows us to find synthetic 3β+2α optical data and corresponding intensive parameters (IPs) such as lidar ratios, backscatter- and extinction-related Ångström exponents at the available measurement wavelengths. Based on interdependencies between synthetic IPs and various microphysical properties, the qualitative and quantitative criteria for the optical data quality-assurance tool are developed. We derive the conditions of smoothness, closeness, convergence, and stability of the solution space for the quantitative criteria to test the quality of the 3β+2α optical data. Our novel methodology, to the best of our knowledge, can be used not only for particles of spherical shape, but also for cases in which particles are irregularly shaped. Another strength of our methodology is that it also works for the case of a size-dependent and wavelength-dependent CRI. We show the potential of this methodology for a measurement case from the ORACLES campaign. Data were taken with NASA Langley's airborne HSRL-2 instrument on September 24, 2016.

Evaluation of a method to quantify the number concentrations of submicron water-insoluble aerosol particles based on filter sampling and complex forward-scattering amplitude measurements

Water-insoluble aerosol particles (WIAPs), such as black carbon (BC), mineral dust, and primary biological aerosol particles (PBAPs), affect climate through their interaction with radiation and clouds. However, with the exception of BC, methods to identify WIAP types and quantify their number concentrations are limited. Here, we evaluated a method that has been recently developed to measure the number concentrations of submicron WIAPs based on atmospheric aerosol measurements at an urban site in Nagoya, Japan. In this method, atmospheric aerosol particles are collected on a filter and dispersed in water. Then, the complex forward-scattering amplitudes of individual particles are measured. This complex parameter reflects the complex refractive index, volume, and shape of each measured particle, enabling the characterization of these physical properties from the signals. The WIAPs were classified as BC-like, dust-like, and PBAP-like particles based on their complex amplitude data. The number concentrations of BC-like particles were strongly correlated with those of refractory BC particles measured by a Single Particle Soot Photometer. BC-like and dust-like particles dominated the population of the submicron WIAPs, which was also confirmed using electron microscopy and Wideband Integrated Bioaerosol Sensor observations. Under the observed atmospheric conditions, the number concentrations of WIAPs were measured with their dispersion efficiency from a filter to water of approximately 50%. These results indicate that our method based on filter sampling and complex forward-scattering amplitude measurements has the potential to become a new technique for quantifying the spatio-temporal distributions of WIAPs.

Transformation of the Spatial Spectrum of Scattered Radio Waves in the Conductive Equatorial Ionosphere

The oblique radio wave incidence on a turbulent equatorial conductive collision plasma layer is considered. The “Compensation Effect” has been discovered by us. A complex refractive index of the equatorial terrestrial ionosphere has been derived for the first time. Second-order statistical moments of the spatial power spectrum (SPS) of scattered radio waves are obtained for the first time using the WKB method, taking into account the asymmetry of the problem: the inclined incidence of the wave on a plasma boundary and the asymmetry of the magneto-ionic parameters. It was established for the first time that a certain direction exists along which the inclined incidence radio wave on a plasma layer and the anisotropy parameters of a magnetoplasma compensate each other. This result will have great practical application in communication. In this case, the SPS of scattered radio waves neither widens nor is its maximum displaced. The behavior of this spectrum versus distance propagated by radio waves in the conductive equatorial ionosphere is analyzed numerically for different penetration angles and anisotropy factors of asymmetric anisotropy electron density irregularities. It was shown that the anisotropy factor of elongated plasmonic structures has a substantial influence on the “Compensation Effect” of scattered ordinary and extraordinary waves penetrating in the conductive collision ionospheric plasma the slab. Numerical calculations are carried out for the anisotropic Gaussian correlation function applying IRI experimental data.

Particle shapes and infrared extinction spectra of nitric acid dihydrate (NAD) crystals: optical constants of the β-NAD modification

Abstract. Satellite- and aircraft-based mid-infrared measurements of polar stratospheric clouds (PSCs) have provided spectroscopic evidence for the presence of β-NAT (nitric acid trihydrate) particles. Metastable nitric acid hydrate phases such as α-NAT and α-NAD (nitric acid dihydrate) have been frequently observed in laboratory experiments but not yet detected as a constituent of PSCs in atmospheric measurements. As for the β-NAD modification, its formation was first observed in X-ray diffraction measurements when the low-temperature α-NAD phase was warmed to a temperature above 210 K. Its infrared spectrum has been reported, but so far no optical constants have been derived that could be used as input for infrared retrievals of PSC composition. In this work, we show that β-NAD particles were efficiently formed in isothermal heterogeneous crystallisation experiments at 190 K from supercooled HNO3/H2O solution droplets containing an embedded mineral dust or meteoric smoke particle analogue. An inversion algorithm based on a T-matrix optical model was used to derive for the first time the mid-infrared complex refractive indices of the β-NAD modification from the measured extinction spectrum of the particles. In contrast to the heterogeneous crystallisation experiments, the α-NAD phase was formed when the HNO3/H2O solution droplets did not contain a solid nucleus and crystallised homogeneously. Using a light-scattering detector that recorded two-dimensional scattering patterns of the crystallised NAD particles, we were able to determine predominant shapes of the α- and β-NAD crystals. We found that α-NAD grew into elongated, needle-shaped crystals, while β-NAD particles were compact in shape. This agrees with previously reported images of α- and β-NAD particles grown on the cryo-stage of an environmental scanning electron microscope. While direct evidence for the existence of metastable NAD in the polar stratosphere is still lacking, our experiments add to the wealth of previous laboratory studies that have identified various conditions for the rapid growth of metastable compositions. In the atmosphere, these could be intermediate states that transform into thermodynamically stable NAT on longer timescales in aged PSCs.

Development of High‐Reflectivity and Antireflection Dielectric Multilayer Mirrors for AlGaN‐Based Ultraviolet‐B Laser Diodes and their Device Applications

Fabrication techniques for high‐reflectivity (HR) and antireflection (AR) dielectric multilayer mirrors for AlGaN‐based ultraviolet‐B (UV‐B) laser diodes are developed. After depositing several dielectric materials and evaluating their complex refractive indices via ellipsometry, it is determined that SiO2 as a low‐refractive‐index material and Ta2O5 as a high‐refractive‐index material are appropriate material combinations in the UV‐B region at a light wavelength of ≈300 nm due to their low extinction coefficients and large refractive index difference. Based on these results, HR mirror with a reflectance of >99% in the UV‐B region at a center wavelength of 310 nm and an AR mirror with a reflectance of ≈8% in the same wavelength range are demonstrated; a mirror with reflectance that is almost equal to the designed value is demonstrated. Furthermore, these mirrors are coated on the respective edge surfaces of the UV‐B laser diodes. A comparison of the characteristics of the same device before and after edge coating reveals a reduction in the threshold current density of laser oscillation, whereas, simultaneously, an increase in slope efficiency and external differential quantum efficiency is observed. The improvement of these device characteristics, estimated from the above reflectance values, is confirmed to be almost theoretically explainable.

Dielectric Characterization of Fabric Aggregates around the 2.45 GHz ISM Band under Various Humidity, Density, and Temperature Conditions.

Fabric permittivity is critical for the manufacturing of wearable sensors and antennas as well as predicting how fabrics interact with electromagnetic fields. Engineers should also understand how permittivity changes under different temperatures, densities, and moisture content values, or when several fabrics are mixed in aggregates, when designing future applications such as microwave dryers. The permittivity of cotton, polyester, and polyamide fabric aggregates is investigated in this paper for a wide range of compositions, moisture content levels, density values, and temperature conditions around the 2.45 GHz ISM band using a bi-reentrant resonant cavity. The obtained results show extremely comparable responses for all characteristics investigated for single and binary fabric aggregates. Permittivity always increases as temperature, density, or moisture content levels rise. Moisture content is the most influential characteristic, causing enormous variations in the permittivity of aggregates. Fitting equations are supplied for all data, with exponential functions used to accurately model variation in temperature and polynomial functions employed to precisely model density and moisture content variations with low error levels. The temperature permittivity dependence of single fabrics without the influence of air gaps is also extracted from fabric and air aggregates by using complex refractive index equations for two-phase mixtures.

Near-Field Single-Scattering Calculations of Aerosols: Sensitivity Studies

We model the effects of the photosensitive parameters of aerosols on their optical properties to provide a solid framework for further experimental and theoretical studies. A spherical dust particle is used to study the effects of the ambient medium, size, surface roughness, wavelength, and imaginary part of the complex refractive index. Five Gaussian random spheres with different aspect ratios are simulated to study the dependence of aerosol light scattering properties on particle shape distribution. To investigate the influence of composition, we model two typical kaolinite-like particles (pure and composite) collected from Southwest Sahara, with 0 and 2% hematite at different mixing states. Using the method of discrete-dipole approximation in DDSCAT, a comparative study is performed with the Mueller matrix elements, scattering, absorption, extinction efficiencies, single scattering albedo, and linear depolarization ratio as indicators. For single, microscopic dust particles, near-field calculations are carried out. The results show that the intensity of backscattering and the scattering efficiency decreases in water compared to dry air. Light in the visible range is more efficient for aerosol scattering experiments. A small number of impurities in the sample will increase its absorbing properties, but, in general, the scattering efficiencies strongly depend on the single-particle mixing state. Smaller particles with a diameter comparable to the wavelength of incident light show higher scattering efficiencies but lower backscattering intensities than larger particles, while surface roughness is shown to strongly alter the polarizability of the particle but has a negligible effect on its single-scattering albedo. Moreover, different shapes have a strong effect on the degree of linear polarization, but, in general, using the spherical over elliptic shape model can underestimate the scattering efficiencies by up to 4%. Finally, variation in the imaginary part of the complex RI can underestimate the single scattering albedo by up to 35.8%.

Investigating electronic and optical properties of pyrochlore oxides Ta2A2O7 (A = Cd and Ge) for promising optoelectronic applications

Due to their stable crystal structure and controllable lattice parameters, rare earth zirconates with pyrochlore type structures have demonstrated a promise for usage in practical applications. We have investigated the electronic and optical properties of pyrochlore oxide compound named Ta2A2O7 (A = Cd and Ge) using density functional theory based on spin–orbit coupling and Hubbard (SOC + U) corrections to consider the existence electronic d orbitals of (A) and f elements (Ta). This study focuses on the characterization and investigation of pyrochlore oxides with the A2B2O7 composition, which have significant technological applications including quantum spin ices, solid oxide fuel cells, and nuclear waste forms. No evidence regarding magnetic moment is obtained from the first run with spin-polarization setup. Also, from the spectra of TDOS was showing a similar overall profile for both spin channels. The electronic properties (total and partial) densities of states as well as the optical properties such as the complex dielectric function, refractive index, absorption coefficient and reflectivity for wavelengths in the visible spectrum have been reported to explore the potential use of these compounds in technological applications. Consequently, the investigated compounds can be used in optoelectronic device fabrication.

Enhanced optical contrast of Ag-doped Ge-Sb-Se-Te phase change films

The phase change material (PCM) Ge2Sb2Se4Te1 film doped by Ag (AGSST) was prepared by the pulsed laser deposition (PLD) method. The crystal structure, chemical bonding and film complex refractive index were investigated. The AGSST undergoes a phase transition through annealing. The extinction coefficient of crystalline (CR) AGSST has been significantly lowered for the communication C-band at 1550 nm, with a figure of merit (FOM) of 7.48, surpassing that of undoped GSST. This outcome delivers chalcogenide PCMs that are potentially in the development of programmable nonvolatile photonics.

A study of the correlation between polarization maximum to minimum ratio and scattering parameters in a spheroid dust model

This paper investigates the correlation between the ratio of polarization maximum to polarization minimum (Pz) and different scattering parameters such as complex refractive indices (m=n+ik), effective radius (reff), effective variance (veff), and axial ratio (E) for spheroidal particles. The simulations are executed using the double-precision T-matrix code for randomly oriented spheroidal particles, considering different sets of scattering parameters. For a specific range of n, k, E, reff, and veff, a strong correlation between Pz and other scattering parameters is observed in all cases, which can be fitted by a second-degree polynomial equation. The simulations are conducted at two wavelengths (λ=0.50 µm and 0.65 µm). Additionally, we compare our results obtained from numerical simulation with the experimental data for a few forsterite samples conducted by Muñoz et al. (2021). A set of conclusions is presented based on our work.

Optical properties of pulsed dc magnetron sputtered thin film tantalum nitride by spectroscopic ellipsometry

Ellipsometric spectra of polycrystalline thin film tantalum nitride (TaN) have been collected and analyzed to obtain the complex dielectric function (ɛ = ɛ1 + iɛ2) and complex refractive index (N = n + ik) spectra over the photon energy range of 0.059–8.500 eV. Complex optical properties are obtained for TaN in the as-deposited state and rapid thermal annealed at 750 °C for 30 s post deposition. A parametric expression including the contribution from intraband and interband transitions along with a structural model is used and fitted to experimental ellipsometric spectra using iterative least-square regression, which minimizes the unweighted error function or mean square error to extract complex optical properties. The parametric expression developed in this work is successful in describing and differentiating the optical response of measured as-deposited and annealed TaN films and can potentially be used to analyze the optical properties of similar TaN films regardless of their deposition techniques.

An Investigation of Laser Absorption Enhancement of Energetic Materials Doped with Metallic Nano-Particles

The influence onto laser absorption enhancement of energetic materials from doping in metallic nanoparticles was investigated. The FEM analyzation using complex refractive index of Al and Au shows that the doping metal types, the structure of nanoparticles, and the wavelength of electromagnetic waves, takes an influence on the absorption efficiency. The laser ignition experiment was carried, the influence on the ignition properties of pyrotechnics of the energetic material types, the amount of doping particles, and ignition laser parameters were analysed.

A double-layer scheme for extrapolating terahertz complex refractivity from reflectance of aerocraft materials

The complex refractive indexes of aero craft materials are seldom investigated. In this paper, we fabricate two aero craft materials: a white painted aluminum plate and a coated window glass, and measure the reflectance spectra in the region of 3–50 THz. Different from the acquainted metal materials, the reflectance spectra of the two materials have apparent peaks and valleys, which is known as the spectral fingerprint. With the preliminary analysis by using Lorentz-Drude/Lorentz dispersion oscillator models and Kramers-Kronig (KK) algorithm, we find that the errors of the extracted complex refractivity from the dispersion oscillator models are apparent at the abrupt regions, and the accuracy of the real refractive index n and extinction coefficient κ from KK algorithm differs dramatically. To reduce the inevitable errors, we design a double-layer extraction scheme to reestablish the oscillator models based on the KK relation. Compared with the inversed data from KK relation and dispersion oscillator models, the procedure assures the accuracy of the real refractive index and extinction coefficient concurrently, the error of n/κ values at the abrupt region is also improved significantly. The method is reliable and convenient to obtain the precise broadband refractive index from spectra with apparent spectral fingerprints. And the obtained complex refractivity can support the further THz scattering analysis for aero craft targets.

Compact and fast-response optical switch based on complex refractive index engineering.

The optical switch is a crucial device in integrated photonic circuits. Among the various types of optical switches available, the on-off Mach-Zehnder interferometer is one of the most widely used structures. However, compared with other structures, such as a microring, the large footprint of a Mach-Zehnder interferometer significantly restricts the integration density. In this paper, we propose a compact Mach-Zehnder interferometer based on complex refractive index engineering. By manipulating the complex index of the material in the structure, the lateral size of the device can be compressed down to only 3.25 µm. Moreover, the reducing of the space between heaters and waveguides leads to a fast response of only 1.9 µs. Our work offers a new, to the best of our knowledge, approach of a compact integrated optical switch, and opens a new avenue for application of absorbing materials.

Theoretical investigations of optoelectronic and transport properties of halide based K2YAuX6 (X = Cl, Br) double perovskites

Intriguing optoelectronic and transport properties with huge compositional range and structural stability make double perovskites interesting for energy harvesting technologies. Here we theoretically investigate the structural, optical, mechanical, and transport properties of K2YAuX6 (X = Cl, Br) double perovskites using the WIEN2K code. Based on the calculated values of the tolerance factor and enthalpy of production, the thermodynamic and structural stability is affirmed. The ductile nature of these compositions is revealed in the computation of Poisson’s ratio (>0.26) and Pugh’s (1.75) ratio. The involvement of s-states of K, p-states of Cl/Br, and d-states of Y and Au in the formation of valence and conduction band edges is exhibited from the density of state plots. Using the Tran-Blaha mBJ potential and spin-orbital coupling, the energy bandgap value for K2YAuCl6 and K2YAuBr6 is reported as 3.20, and 2.70 eV, respectively, which is suitable for the fabrication of light-emitting diodes. Optical behavior is further explored regarding complex dielectric constant, refractive index, optical conductivity, optical loss, and absorption factor. The small value of thermal conductivity with large value of electrical conductivity, Seebeck coefficient, power factor, and figure of merit revealed the potential of these materials for the fabrication of light-emitting diodes and thermoelectric generators.

Accurate determination of optical parameters of non transparent materials: The ε–GaSe case

The aim of this study is to introduce a precise approach for the determination of the optical parameters of mechanically exfoliated gallium selenide (ε–GaSe) thin films from both the measurements of transmittance (Tmeas) and reflectance (Rmeas). Specifically, the absorption α(λ) and the reflection R(λ) coefficients can be accurately calculated at any wavelength. Our approach takes into consideration the multiple reflections and transmissions occurring on both sides of the film, providing a comprehensive analysis of the optical behavior. Then, the fundamental optical constants that are the complex refractive index N(λ) and the complex dielectric constant ε(λ) can be accurately determined. The modeling of the imaginary part of this last parameter allows a precise determination of the position of each resonance observed in the transmission and reflectance spectra. The modeling, based on the Lorentz model of interband oscillators, allows us to investigate up to five resonances, at and above the band gap, associated with the critical points of the Brillouin zone. Simultaneously, we consider the splitting on the valence band and the closeness of the indirect-direct minima of the configuration in the conduction band in our analysis. Our investigation reveals that the refractive index of GaSe material varies from 2.8 to 3.3 with strong peaks observed above the band gap at the resonances of 3.2 eV and 3.7 eV. Through the analysis of the oscillator strengths and lifetimes of the observed transitions, we were able to clearly identify the indirect nature of the fundamental band gap of GaSe. Its value of 1.97 eV is largely separated from the direct band gap of about 230 meV. Furthermore, the modeling allows us to estimate the transitions lifetimes. Their values are ranging between 0.6 fs and 24 fs.

Study on the effects of oxidation temperature and ambient humidity on the terahertz spectroscopy characteristics of lignite

In this paper, the terahertz (THz) spectroscopy characteristics of lignite with different oxidation temperatures under different ambient humidity were investigated by terahertz time-domain spectroscopy (THz-TDS), and the mechanism was analyzed by combining the synchronous thermal analyzer. Results indicate that the refractive index of lignite has abnormal dispersion characteristics. THz waves have different transmission characteristics in lignite with different oxidation temperatures, which is reflected in the amplitude difference of time-domain and frequency-domain signals. The complex refractive index of lignite shows obvious oxidation temperature dependence, which is closely related to the physicochemical changes that occur during coal oxidation. Besides, ambient humidity has a significant effect on the THz spectroscopy measurement, which follows the Beer-Lambert law. As humidity increases, the amplitude of time-domain and frequency-domain signals decays linearly, while the complex refractive index increase linearly. Humidity has a similar effect on coal samples with different oxidation temperatures, which means that based on linear effects and good fit results, THz spectroscopy measurements in humid environments can be corrected by a simple linear empirical formula. The results of this study are helpful to study the spectroscopy characteristics of coal in the THz band, and provide guidance for the application of terahertz technology in coal mines.

Optical simulation and experimental investigation of the crystalline silicon/black silicon/perovskite tandem structures

The use of a black silicon (b-Si) interlayer is a promising solution for improving the optical properties of crystalline Si/perovskite solar cells. Optical numerical simulation of the crystalline Si/b-Si/perovskite structures can efficiently optimize their parameters at the initial tandem solar cell design stage. In this work, the optical properties of these structures were modeled using the transfer matrix method, where an array of cone-like nanoneedles of the b-Si interlayer is approximated as a homogeneous optical medium, for which the effective complex refractive index is the weighted average of those for Si and perovskite. Analytical equations are obtained for tandem structures' reflection, transmission, and absorption. The simulated reflectance spectra of the structures were compared with those of structures without the b-Si layer, as well as with experimental data. Numerical calculations and experiments confirm that nanotexturing significantly improves the optical properties of tandem structures. In addition, the nanotextured tandem structures have a wide-angle antireflective characteristic at incidence angles less than 60°, indicating their omnidirectional light-trapping capability. The proposed simulation model satisfactorily describes the optical behavior of crystalline Si/b-Si/perovskite tandem structures and can be used to estimate their optical properties depending on various parameters. As a demonstration, a numerical analysis was performed to study the effect of the b-Si interlayer thickness on the reflection of the tandem structure.

Investigation of optoelectronic & thermoelectric features of ZnCrX2 (X = S Se Te) chalcopyrite semiconductor using mBJ potential

In the present investigation, we have examined the structural, optoelectronic and thermoelectric properties of the compound ZnCrX2 (X = S, Se, Te) using the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA-PBEsol) and the modified Becke–Johnson (mBJ) implemented on Wien2k code. By calculating the electronic structure, we observed that the indirect band gap was 1.76 eV, 1.68 eV, and 1.59 eV, for these selected compounds and the type of chemical bonding was ionic between Zn-X and Cr-X. For the three ternary chalcopyrites their optical properties including optical conductivity, complex dielectric functions, complex refractive index, reflectivity, energy loss, and absorption coefficient were examined. The calculated optical conductivity indicates that the studied compounds are suitable for optoelectronic applications as they have good absorbance and less energy loss in the low energy range of the electromagnetic spectrum. For ZnCrTe2, the maximum reflectivity was in the low energy range (0.48 or 48% at 9 eV). The BoltzTrap code was executed for the calculation of the thermoelectric properties and thermal efficiency of the compounds investigated, depending upon the Seebeck coefficient, thermal conductivity and electrical conductivity. The high value of the figure of merit and the Seebeck coefficient (260 uV/k) defines the high efficiency of the materials studied. Hence, the studied chalcopyrite compounds offer applications in solar cell devices and p-type semiconducting nature predictive in transport investigations.

Complex Refractive Index Extraction for Ultrathin Molybdenum Oxides Using Micro‐Photonic Integrated Circuit Chips

AbstractThe emerging 2D nanomaterials with unique optical properties are promising for next‐generation miniatured on‐chip devices. One of the prerequisites is to precisely measure their optical parameters during their implementation. However, the inherent features of 2D layers, including limited lateral dimensions and ultra‐small thicknesses, are not favorable to the conventional characterization techniques applied in the bulk system, especially for optical complex refractive indices measurement. Here, this work proposes a silicon photonics‐enabled platform to evaluate the complex refractive indices of ultrathin 2D materials in a facile and reliable manner. Ultrathin molybdenum oxides (MoOx) with multiple stoichiometric states are selected as the target 2D material to provide sufficient complexity of the system for investigation. Upon the integration of ultrathin MoOx, the silicon photonic chip, in the form of a Mach‐Zehnder interferometer, exhibits wavelength shifts which are used for calculating the optical complex refractive indices. Compared with the theoretical calculation, the deviation is as low as 1% and generally less than 5%. This work demonstrates a highly accurate and reliable approach for measuring the complex refractive index of 2D films, possibly assisting future advances in 2D materials‐enabled optical and photonic applications.