Complex Refractive Index Research Articles

Structural parameters, electronic structure and linear optical functions of LuXCo2Sb2 (X = V, Nb and Ta) double half Heusler alloys

Ab initio calculations based on density functional theory were performed to determine the structural parameters of the LuXCo2Sb2 (X = V, Nb and Ta) double half Heusler compounds and verify their thermodynamic stability in an orthorhombic structure as well as to predict their electronic and optical properties. The predicted electronic band structures show that LuVCo2Sb2, LuNbCo2Sb2, and LuTaCo2Sb2 are semiconductors with indirect bandgaps of 0.728, 0.845, and 0.898 eV, respectively. The effective masses calculated at the valence band maximum at the conduction band minimum show a strong anisotropy. It can be seen that electrons are lighter than holes. The nature of the electronic states involved in the formation of the energy bands has been determined thanks to the density of states calculations. The character of chemical bonds was analyzed through the charge density distribution map. Linear optical functions, namely complex dielectric function, absorption coefficient, optical reflectivity and refractive index, were calculated for all compounds in an energy range up to 20 eV. The calculated optical spectra exhibit a noticeable anisotropy. The compounds under consideration are characterized by strong absorption of incident electromagnetic radiation in a wide energy range.

Organic Synthetic Photonic Systems with Reconfigurable Parity-Time Symmetry Breaking for Tunable Single-Mode Microlasers.

Synthetic photonic materials exploiting the quantum concept of parity-time (PT) symmetry lead to an emerging photonic paradigm-non-Hermitian photonics, which is revolutionizing the photonic sciences. The non-Hermitian photonics dealing with the interplay between gain and loss in PT synthetic photonic material systems offers a versatile platform for advancing microlaser technology. However, current PT-symmetric microcavity laser systems only manipulate imaginary parts of the refractive indices, suffering from limited laser spectral bandwidth. Here, an organic composite material system is proposed to synthesize reconfigurable PT-symmetric microcavities with controllable complex refractive indices for realizing tunable single-mode laser outputs. A grayscale electron-beam direct-writing technique is elaborately designed to process laser dye-doped polymer films in one single step into microdisk cavities with periodic gain and loss distribution, which enables thresholdless PT-symmetry breaking and single-mode laser operation. Furthermore, organic photoisomerizable compounds are introduced to reconfigure the PT-symmetric systems in real-time by tailoring the real refractive index of the polymer microresonators, allowing for a dynamically and continuously tunable single-mode laser output. This work fundamentally enhances the PT-symmetric photonic systems for innovative design of synthetic photonic materials and architectures.

Excitonic Beam Steering in an Active van der Waals Metasurface.

Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising candidates for ultrathin active nanophotonic elements due to the strong tunable excitonic resonances that dominate their optical response. Here, we demonstrate dynamic beam steering by an active van der Waals metasurface that leverages large complex refractive index tunability near excitonic resonances in monolayer molybdenum diselenide (MoSe2). Through varying the radiative and nonradiative rates of the excitons, we can dynamically control both the reflection amplitude and phase profiles, resulting in an excitonic phased array metasurface. Our experiments show reflected light steering to angles between -30° and 30° at different resonant wavelengths corresponding to the A exciton and B exciton. This active van der Waals metasurface relies solely on the excitonic resonances of the monolayer MoSe2 material rather than geometric resonances of patterned nanostructures, suggesting the potential to harness the tunability of excitonic resonances for wavefront shaping in emerging photonic applications.

Numerical study of light backscattering from layers of absorbing irregular particles larger than the wavelength

We simulate light backscattering from monolayers of absorbing irregular particles larger than the wavelength using a numerically exact discontinuous Galerkin time domain (DGTD) method. Varying the real and imaginary parts of the complex refractive index m, particle size and the structure of a monolayer we study the evolution of the negative polarization (NP) feature that is observed for many particulate surfaces near backscattering. Simulations show that the NP is enhanced when increasing absorption from Im(m)=0.06 to 0.3. The real part Re(m) plays little role in this case. We confirm correlation of NP with the particle size predicted by approximate models and observed in laboratory measurements. We also demonstrate that the interplay between single- and multiple scattering can be controlled by the topography of a monolayer resulting in enhanced or damped NP.

Reflection from uniaxial crystals: symmetries, and formulae for near-normal incidence.

The following symmetries and interrelationships are established: the direct reflection amplitudes r ss,r pp are independent of the signs of the direction cosines of the optic axis. For example, they are unchanged by ϕ→π-ϕ or ϕ→-ϕ, where ϕ is the azimuthal angle of the optic axis. The cross-polarization amplitudes r sp a n d r ps are both odd in ϕ; they also satisfy the general relations r sp(ϕ)=r ps(π+ϕ) and r sp(ϕ)+r ps(π-ϕ)=0. All of these symmetries apply equally to absorbing media with complex refractive indices, and thus complex reflection amplitudes. Analytic expressions are given for the amplitudes which characterize the reflection from a uniaxial crystal when the incidence is close to normal. The amplitudes for reflection in which the polarization is unchanged (r ss a n d r pp) have corrections which are second order in the angle of incidence. The cross-reflection amplitudes r sp a n d r ps are equal at normal incidence and have corrections (equal and opposite) which are first order in the angle of incidence. Examples for normal incidence and small-angle (6°) and large-angle (60°) incidence reflection are given for non-absorbing calcite and absorbing selenium.

Simulated depolarization ratios for dust and smoke at laser wavelengths: implications for lidar application.

Polarization measurements have been widely used to detect aerosol properties by remote sensing in recent decades. To better understand the polarization characteristics of aerosols by lidar, the numerically exact T-matrix method was used to simulate the depolarization ratio (DR) of dust and smoke aerosols at typical laser wavelengths in this study. The results show that the DRs of dust and smoke aerosols have obviously different spectral dependences. Moreover, the ratio of DRs at two wavelengths has an obvious linear relationship with the microphysical properties of aerosols, including aspect ratio, effective radius and complex refractive index. At short wavelengths, we can use it to invert the absorption characteristics of particles, further improving the detection ability of lidar. Comparing the simulation results of different channels, DR, (color ratio) CR and (lidar ratio) LR have a good logarithmic fitting relationship at 532 nm and 1064 nm, which helps to classify the aerosol types. On this basis, a new inversion algorithm, "1β+1α+2δ", was presented. By this algorithm, the backscattering coefficient (β), extinction coefficient (α), DR (δ) at 532 nm and 1064 nm can be used to expand the range of inversion and compare lidar data with different configurations to obtain more extensive optical characteristics of aerosols. Our study enhances the application of laser remote sensing in aerosol observations more accurately.

Spin-Electric Coupling in Lead Halide Perovskites.

Lead halide perovskites enjoy a number of remarkable optoelectronic properties. To explain their origin, it is necessary to study how electromagnetic fields interact with these systems. We address this problem here by studying two classical quantities: Faraday rotation and the complex refractive index in a paradigmatic perovskite CH_{3}NH_{3}PbBr_{3} in a broad wavelength range. We find that the minimal coupling of electromagnetic fields to the k·p Hamiltonian is insufficient to describe the observed data even on the qualitative level. To amend this, we demonstrate that there exists a relevant atomic-level coupling between electromagnetic fields and the spin degree of freedom. This spin-electric coupling allows for quantitative description of a number of previous as well as present experimental data. In particular, we use it here to show that the Faraday effect in lead halide perovskites is dominated by the Zeeman splitting of the energy levels and has a substantial beyond-Becquerel contribution. Finally, we present general symmetry-based phenomenological arguments that in the low-energy limit our effective model includes all basis coupling terms to the electromagnetic field in the linear order.

Optical properties of mineral dust aerosols with non-absorptive coating: A numerical investigation

Mineral particles form complex mixed states with other anthropogenic species through a series of chemical reactions during the long-range transport and aging process, which modifies their optical properties. Traditional homogeneous particle models do not effectively describe the variability of particle size distribution, morphology and mixing structure, and the discrepancy between the simulations and experimental results is still apparent. In this paper, the impact of non-absorptive coating material on the optical properties of mineral dust particles with varying particle size distribution, morphology, and inhomogeneous characteristics is investigated by the core-shell spheroid model. The results reveal that the real part of the complex refractive index (RRI) of non-absorptive coating material has a more prominent effect on the scattering matrix elements than the particle size distribution, morphology, and inhomogeneity. The P11(π)/P11(30°) generally increases with the square of RRI, for any mean effective radius(r0), coating ratio(fcoating), and aspect ratio(α). However, the variation of -P12(θ)/P11(θ) with the r0, fcoating, and α is more complicated and depends on the RRI. The particle inhomogeneity promotes positive values of -P12(θ)/P11(θ) for lower RRI, and negative values for higher RRI in general. The maximum value of -P12(θ)/P11(θ) (Pmax) and the minimum value of -P12(θ)/P11(θ) (Pmin) are strongly depends on the RRI, r0, fcoating, and α. Moreover, the Pmax has an excellent Gaussian distribution relationship with RRI, r0, fcoating, and α. Furthermore, the parameterization scheme of the P11(π)/ P11(30°) and the Pmax as a function of RRI, r0, fcoating, and α are systematically studied. Considering further research, our study may be of great significance for the study of aerosol composition retrieval and remote sensing applications.

Label-free superior contrast with c-band ultra-violet extinction microscopy

In 1934, Frits Zernike demonstrated that it is possible to exploit the sample’s refractive index to obtain superior contrast images of biological cells. The refractive index contrast of a cell surrounded by media yields a change in the phase and intensity of the transmitted light wave. This change can be due to either scattering or absorption caused by the sample. Most cells are transparent at visible wavelengths, which means the imaginary component of their complex refractive index, also known as extinction coefficient k, is close to zero. Here, we explore the use of c-band ultra-violet (UVC) light for high-contrast high-resolution label-free microscopy, as k is naturally substantially higher in the UVC than at visible wavelengths. Using differential phase contrast illumination and associated processing, we achieve a 7- to 300-fold improvement in contrast compared to visible-wavelength and UVA differential interference contrast microscopy or holotomography, and quantify the extinction coefficient distribution within liver sinusoidal endothelial cells. With a resolution down to 215 nm, we are, for the first time in a far-field label-free method, able to image individual fenestrations within their sieve plates which normally requires electron or fluorescence superresolution microscopy. UVC illumination also matches the excitation peak of intrinsically fluorescent proteins and amino acids and thus allows us to utilize autofluorescence as an independent imaging modality on the same setup.

Theoretical Derivation of and Experimental Investigations into the Dielectric Properties Modeling of Concrete

Investigations into the dielectric properties of concrete form the basis for concrete quality testing and condition monitoring of internal structures when using ground penetrating radar (GPR). The formula used to describe the quantitative relationship between the dielectric properties of composite materials and the dielectric constant and volumetric ratio of each component is known as a dielectric model. In this study, a new dielectric model was deduced from circuit theory on the basis of structural abstraction to calculate the dielectric permittivities of concrete. To validate the applicability and accuracy of the proposed model, because concrete consists of matrix (cement mortar), aggregates (limestone), and air, the dielectric traits of concrete and its components were acquired using an open-ended coaxial probe. Compared with the Brown model, complex refractive index method (CRIM) model, Looyenga model, and Rayleigh model (i.e., the classical models), the results showed that among overall predictions of concrete dielectric constants, the proposed model has the greatest accuracy, because the theoretical results matched the test results more closely. This indicates that the proposed model can better interpret the dielectric properties of concrete, which lays the groundwork for future research into the application of GPR to the nondestructive testing and evaluation of concrete pavement.

Light-Matter Interactions Revealing Load-Induced Phase Mobility in Elastomers.

Elastomers with segmental microstructure are a fascinating class of shock-tolerant and impact-resistant materials. However, their technological potential remains untapped due to a vague understanding of the molecular contributions to their superior mechanical behavior. Herein, in situ light-matter interactions, to reveal the extent of microstructural mobility by temporally exploiting molecular processes during creep response, are leveraged. The segmental microstructure comprises aromatic hard domains embedded within an aliphatic soft matrix. High-resolution digital image correlation reveals the development of strain striations, mild anisotropy, and the mechanisms responsible for domain mobility, where the rate of hard segment mobility is found to be 60% slower than that of the soft segment. Terahertz spectral analyses pinpoint the contributions of interchain hydrogen bonding of the hard segments and their significant conformational changes by observing spectral features at ≈1.2THz and ≈1.67THz. Moreover, the domain mobility is examined using experimental and computational light scattering approaches, uncovering dynamic scattering and elucidating the difference in the complex refractive index of the soft and hard segments. The study unlocks the pathway for quantitative measurements of elusive molecular mobility and conformational changes during mechanical loading and sheds light on the origin of the shock tolerance in some elastomeric polymers with segmental microstructure.

Refractive index inversion method for metals based on circular polarization detection of light

Among many optical detection methods, the polarization detection method of light has been proven to be capable of providing outstanding, even unique sometimes, information of targets under test, this includes detecting objects that are difficult to be distinguished by the light intensity or spectrum information alone. However, as for materials identification, the current polarization detection method which usually uses linearly polarized light can only roughly distinguish metallic materials from non-metallic materials but cannot further differentiate among types of metallic materials. Note that the phase of a linearly polarized light may change after being reflected by a metal due to its complex refractive index, and the circular polarization information of the reflected light can be used to sense the phase change property of the metal, therefore, different types of metallic materials can be identified according to their characteristic circular polarization information due to their diverse complex refractive indices. In this paper, based on the circular polarization detection of light, a complex refractive index inversion method for metals is proposed accordingly, and experimental measurements and fitting estimations are carried out for different types of metallic materials using a Bidirectional Reflectance Distribution Function (BRDF) measurement equipment. The experimental results show that the method can accurately invert the complex refractive indices of different metals and distinguish different types of metal samples. Compared to the Mueller-matrix-based measurement method, the proposed method is computationally simple, needless of measuring a large number of parameters and performing complex calculations, it only needs to invert the results of twice measurements. In addition, it also has the advantages of high sensitivity, good stability and intuitive algorithm, which are valuable for identification applications of metallic materials.

Analysis of target surface polarization characteristics and inversion of complex refractive index based on three-component model optimization

Traditional imaging detection techniques are used to collect intensity information and spectral information of the target to generate high-resolution target images, but the imaging system is susceptible to external environmental factors, resulting in system instability and image quality degradation. Polarization detection techniques use the polarization properties of light to determine the polarization state and spatial profile of a target, thereby improving the contrast of the target in the image. In this study, the reflection process of the target surface is investigated from a microscopic perspective. A polarization characteristics model was developed to illustrate the link between the properties of the target surface and the degree of polarization of the reflected light. The results show that the degree of polarization is strongly related to the detection of zenith and azimuth angles. Experimental data are used to invert the complex refractive index of the target. The polarization of the target is greatest in the direction of specular reflection and smallest in the direction of light source incidence. This suggests that the established Polarimatric Bidirectional Reflectance Distribution Function (pBRDF) model can provide theoretical support for the analysis of the polarization properties of the target surface. Targets of different materials have different polarization properties and the extinction coefficient can effectively classify the targets.

Light energy distribution of submicron charged particles based on the scattering-enhancing effect

Magnetic microspheres, charged soot and other charged sub-micron particles are widely used, and the accurate measurement of their particle size is crucial to biomedical, environmental monitoring and other fields. During light scattering measurements, the light energy distribution projected onto each ring surface of the photodetector is the core data for particle size inversion calculations. When the measured particle is charged, the light energy distribution will be different from its neutral state due to the scattering-enhancing effect of surface charges. Therefore, it is necessary to confirm the extent of the scattering-enhancing effect on the light energy distribution by numerical simulation, before actual measurement. In this paper, calculation models of scattered light energy distribution are established for charged single particle, monodisperse particle system and polydisperse particle system respectively. Considering the difference between the complex refractive indices of charged particles, the forward scattering of silica and polystyrene with different monodisperse degrees are taken as an example to analyze the light energy distribution. The results show that the scattering-enhancing effect of surface charges can affect the light energy distribution of submicron particles, and the smaller the median diameter of the particle system and the lower the monodisperse degree, the more significant the effect is. At a median diameter of 0.1 μm, the light energy of charged particles with different monodisperse degrees increased by an average of about 3.5 times, near the scattering angle of 45° This research realizes the forward deduction of the light energy distribution for charged particles, and provides the necessary theoretical support for the visible light measurement of charged submicron and nanoparticles.

Structure, electronic and optical properties of B single- and double-doped graphene

We investigate the structural, electrical, and optical properties of pure, B single- and double-doped graphene by the first-principles calculation. The results of the formation energy are used to obtain the most stable doped configuration. The energy band of graphene can be opened by B-doped and exhibit a direct bandgap. The Fermi level is mainly contributed by the C 2p orbitals and the B 2p orbitals. Both the complex refractive index and the complex dielectric function satisfy the Kramers-Krönig (K-K) relationship. The peak of the photoconductivity and absorption coefficient of B-doped graphene is weakened. We also found that the electronic transition of B-doped graphene complies with the parity selection rule, and its optical transition is an allowable direct transition. This result is helpful to understand the basic electrical and optical properties of B-doped graphene and has a guiding role in the graphene-based device application.

Combined use of optical technologies for complex diagnostics of opaque nonlinear optical materials

This paper presents the results of creating a system for diagnosing the defective structure of materials that are opaque in the visible region, based on the combination of methods: thermal imaging detection of large defects, digital holography of defects, wideband radiometry in the area of continuum absorption to measure transmission, determination of the complex refractive index of the dispersion medium and constants of interpolation polynomials for dispersion. It is shown that the combined use of interpolation polynomial constants and cluster characteristics allows determining the refractive index of the dispersed phase to further interpret its composition.

Enhanced surface effects and optical property modulation of Ge2Sb2Te5 by pulsed laser irradiation

In this work, Ge2Sb2Te5 (GST) thin films are irradiated by a 1064 nm pulsed laser heat treatment system with different beam profiles. The surface effects induced by different laser conditions are studied systematically by atomic force microscope, spectroscopic ellipsometry, and Raman spectroscopy. It is found that a top-hat beam profile with uniform intensity distribution demonstrates the advantages of a non-destructive and homogeneous surface, which is critical for large-scale processing uniformity. The threshold laser fluence for the amorphization process is predicted by simulation and further proved by the laser irradiation experiment to be 27.9 mJ/cm2 at 1 ns pulse width. We further show that modulation of complex refractive indices of GST thin films can be achieved with different duty ratios (spatial ratio of amorphization part) from 0% to 100%. Our approach paves the way for the precise control of the optical properties of PCMs in emerging optical applications such as photonic switches, optical memories, and all-optical neural networks.

Mapping spectroscopic micro-ellipsometry with sub-5 microns lateral resolution and simultaneous broadband acquisition at multiple angles.

Spectroscopic ellipsometry is a widely used optical technique in both industry and research for determining the optical properties and thickness of thin films. The effective use of spectroscopic ellipsometry on micro-structures is inhibited by technical limitations on the lateral resolution and data acquisition rate. Here, we introduce a spectroscopic micro-ellipsometer (SME), capable of recording spectrally resolved ellipsometric data simultaneously at multiple angles of incidence in a single measurement of a few seconds, with a lateral resolution down to 2μm in the visible spectral range. The SME can be easily integrated into generic optical microscopes by the addition of a few standard optical components. We demonstrate complex refractive index and thickness measurements by using the SME, which are in excellent agreement with a commercial spectroscopic ellipsometer. The high lateral resolution is displayed by complex refractive index and thickness maps over micron-scale areas. As an application for its accuracy and high lateral resolution, the SME can characterize the optical properties and number of layers of exfoliated transition-metal dichalcogenides and graphene, for structures that are a few microns in size.

Size-dependent optical properties of small titanium nitride nanoclusters

The calculation of optical properties of nanoclusters is a challenging and intriguing work. Depending upon the size and shape of the structure, they show some unique behavior. One of the most promising alternative plasmonic materials, titanium nitride has already been explored rigorously in bulk and films in terms of various properties. Here for the first time, the optical property calculation of small titanium nitride nanocluster systems are done using first-principles method. The calculated complex dielectric function, refractive index, extinction coefficient, loss function, reflectivity spectra, and absorption spectra provides information about the response of the clusters to the external field. A significant variation in the optical properties is observed with the change in the cluster size. The tiny size of the clusters is playing a significant role in giving out some fascinating results.

Photochemical Hazes Can Trace the C/O Ratio in Exoplanet Atmospheres

Photochemical hazes are suspected to obscure molecular features, such as water, from detection in the transmission spectra of exoplanets with atmospheric temperatures <800 K. The opacities of laboratory produced organic compounds (tholins) from Khare et al. have become a standard for modeling haze in exoplanet atmospheres. However, these tholins were grown in an oxygen-free, Titan-like environment that is very different from typical assumptions for exoplanets, where C/O ∼ 0.5. This work presents the 0.13–10 μm complex refractive indices derived from laboratory transmission measurements of tholins grown in environments with different oxygen abundances. With the increasing uptake of oxygen, absorption increases across the entire wavelength range, and a scattering feature around 6 μm shifts toward shorter wavelengths and becomes more peaked around 5.8 μm, due to a C = O stretch resonance. Using GJ 1214 b as a test case, we examine the transmission spectra of a sub-Neptune planet with C/O ratios of solar, 1, and 1000 to evaluate the effective differences between our opacities and those of Khare. For an atmosphere with solar hydrogen and helium abundances, we find a difference of 200–1500 ppm, but for high-metallicity (Z = 1000) environments, the difference may only be 20 ppm. The 1–2 μm transmission data for GJ 1214 b rule out the Titan-like haze model, and are more consistent with C/O = 1 and C/O = solar haze models. This work demonstrates that using haze opacities that are more consistent with underlying assumptions about bulk atmospheric composition are important for building self-consistent models that appropriately constrain the atmospheric C/O ratio, even when molecular features are obscured.