Dependence Of Optical Constants Research Articles

Ab-initio investigation of the physical properties of BaAgAs Dirac semimetal and its possible thermo-mechanical and optoelectronic applications

BaAgAs is a ternary Dirac semimetal which can be tuned across a number of topological orders. In this study we have investigated the bulk physical properties of BaAgAs using density functional theory based computations. Most of the results presented in this work are novel. The optimized structural parameters are in good agreement with previous results. The elastic constants indicate that BaAgAs is mechanically stable and brittle in nature. The compound is moderately hard and possesses fair degree of machinability. There is significant mechanical/elastic anisotropy in BaAgAs. The Debye temperature of the compound is medium and the phonon thermal conductivity and melting temperature are moderate as well. The bonding character is mixed with notable covalent contribution. The electronic band structure calculations reveal clear semimetallic behavior with a Dirac node at the Fermi level. BaAgAs has a small ellipsoidal Fermi surface centered at the Γ-point of the Brillouin zone. The phonon dispersion curves show dynamical stability. There is a clear phonon band gap between the acoustic and the optical branches. The energy dependent optical constants conform to the band structure calculations. The compound is an efficient absorber of the ultraviolet light and has potential to be used as an anti-reflection coating. Optical anisotropy of BaAgAs is moderate. The computed repulsive Coulomb pseudopotential is low indicating that the electronic correlations in this compound are not strong.

DFT based comparative analysis of the physical properties of some binary transition metal carbides XC (X = Nb, Ta, Ti)

Binary metallic carbides belong to technologically prominent class of compounds. Present work theoretically explores the structural, mechanical, optoelectronic, and some thermal properties of XC (X = Nb, Ta, Ti) compounds using the density functional methodology. A number of the results obtained are novel. The computed elastic constants and moduli disclose that the compounds possess moderate elastic anisotropy and reasonably good machinability (machinability index above 1.45 for all the compounds), mixed bonding characteristics, high Vickers hardness (in the range 19–25 GPa) with very high Debye temperatures (in the range 800–925 K) and brittle characteristics. The elastic/structural stability conditions are fulfilled for all the carbides under study. The Young's modulus and bulk modulus of TiC are lower than those of NbC and TaC. All the XC (X = Nb, Ta, Ti) compounds are hard and suitable for heavy duty structural applications. The electronic band structures show finite density of states at the Fermi level revealing metallic character of the XC (X = Nb, Ta, Ti) compounds. Mixed bonding features are evident from the charge density distribution maps of the XC (X = Nb, Ta, Ti) compounds. The lattice dynamical properties, such as the phonon dispersion curves and phonon density of states for XC (X = Nb, Ta, Ti) are investigated. Absence of negative phonon modes in the Brillouin zone indicates that the carbides under study are dynamically stable. The energy dependent optical constants are nearly isotropic. The optical absorption, reflectivity spectra, and the high value of the low energy index of refraction of XC (X = Nb, Ta, Ti) compounds reveal that they hold promise to be utilized in optoelectronics. The computed melting temperature, lattice thermal conductivity, Debye temperature, and minimum phonon thermal conductivity of the XC (X = Nb, Ta, Ti) compounds exhibit conformity with the computed elastic and bonding characteristics. Extremely high melting point (4399 K) of the compound TaC suggests that it is a very good candidate for applications at high-temperatures.

Enhanced antimicrobial activity and physicochemical properties of an organic nonlinear optical crystal: L-tryptophan l-tryptophanium bromide (LTTB) for biomedical applications

A good quality biologically active single crystal of l -tryptophan l -tryptophanium bromide (LTTB) with molecular formula C 22 H 27 BrN 4 O 5 was successfully synthesized and grown from aqueous solution using slow evaporation solution growth technique. The synthesized crystal was subjected to various characterization techniques. Single crystal XRD study revealed that the grown crystal corresponds to monoclinic system of space group P2 1 and its crystallinity was tested by powder XRD analysis. The elemental composition of LTTB was confirmed by EDS analysis. The optical characteristics of the synthesized crystal were assessed by UV–Vis-NIR and Photoluminescence analyses. Moreover, the frequency dependent optical constants were computed and discussed in detail. The mechanical strength and the mechanical factors of the synthesized crystal were calculated based on Vickers microhardness test. The thermal stability and its characteristics were computed by TG-DTA-DSC analysis. Dielectric studies of LTTB were observed at various frequencies and temperature. The nonlinear optical performance of the synthesized crystal was examined by Kurtz-Perry technique. The antimicrobial capacity of the synthesized crystal was tested in-vitro against various bacterial and fungal species by Kirby-Bauer agar disk diffusion technique. The susceptibility test showed that the compound has the greatest activity against Aspergillus Niger and Staphylococcus aureus. Finally, all of these studies suggested that LTTB might be useful in biomedical and nonlinear optics applications.

Elucidation of optical, thermal, morphological and antimicrobial efficacy of L-tryptophan hydrochloride single crystals

An efficient semi-organic nonlinear optical single crystal of L-tryptophan hydrochloride (LTHC) of molecular formula C11H13N2O2Cl has been synthesized and its elemental composition was confirmed by EDS analysis. The crystallographic system and lattice parameters were found by single crystal XRD analysis which indicates the synthesized crystal corresponds to monoclinic system of space group P21 and its crystalline nature was identified by powder XRD analysis. The typical characteristic vibrational modes of the grown LTHC crystal were identified by FTIR spectral analysis. The grown crystal’s optical properties were analyzed by UV-Vis-NIR and Photoluminescence analyses. The wavelength dependent optical constants were also measured, and its refractive index at 532 nm was found to be 1.67. The grown crystal’s mechanical stability was determined with the help of Vicker’s hardness tester, which revealed that LTHC has soft material behaviour. TG-DTA-DSC tests were used to examine the thermal behaviour such as decomposition phenomena, phase transition, and melting point of the grown crystal, which revealed that it has high thermal stability up to 205 degrees. The dielectric behaviour, a.c. conductivity of LTHC crystal was studied by dielectric studies and its nonlinear efficiency was analyzed by Kurtz-Perry technique. The crystalline morphology of LTHC was examined by TEM and SEM analyses. Its optical damage property was analyzed by laser damage threshold analysis. The antimicrobial property of LTHC was also scrutinized against bacterial as well as fungal strains by Agar disk diffusion method. Finally, all these findings recommended that LTHC crystals could be useful in biomedical and nonlinear optics applications.

Elastic, electronic, bonding, and optical properties of WTe2 Weyl semimetal: A comparative investigation with MoTe2 from first principles

Td-WTe2 is a topological Weyl semimetal. WTe2 in the orthorhombic structure is stable at room temperature. Elastic, electronic, bonding, and optoelectronic properties of WTe2 have been investigated in detail in this work using the density functional theory. Elastic behaviour together with anisotropy indices of WTe2 have been investigated for the first time. Bonding nature among the constituent atoms and electric field polarization dependent optical constants are also explored theoretically for the first time. WTe2 is elastically anisotropic; optical anisotropy on the other hand is low. The electronic band structure reveals quasi-linear dispersions along certain direction in the Brillouin zone with semi-metallic features. The Fermi level is located at a pseudogap separating bonding and anti-bonding density of states. The electronic effective mass tensor is predicted to be highly direction dependent. The energy dispersion is significantly weaker in the c-direction. The bonding in WTe2 is an admixture of covalent and metallic bonds. Optoelectronic properties show strongly reflecting character over a wide band of photon energies. The compound is a strong absorber of ultraviolet radiation. The Debye temperature has been calculated from the elastic constants. We have compared all the calculated physical properties of WTe2 with those of isostructural MoTe2 Weyl semimetal studied in a previous work. The properties of WTe2 and MoTe2 have been compared and contrasted. The calculated parameters of WTe2 have also been compared with those already available in the literature. Good agreements have been found.

Conductor-backed dielectric metasurface thermal emitters for mid-infrared spectroscopy

A conductor-backed dielectric metasurface thermal emitter at mid-IR frequencies with narrowband emissivity is experimentally demonstrated. The metasurface emitter consists of a high permittivity silicon resonator on top of a ground plane, whose resonant mechanism is explained using image theory. The resonator, placed close to a copper ground plane, is designed to produce a magnetic resonance, resulting in a low-profile device with a single emission peak in its subwavelength frequency range. The thermal emitter is next fabricated using common CMOS processes. Frequency dependent optical constants of plasma-enhanced chemical vapor deposited films of Si, SiO2, and evaporated Cu are also reported in the mid-IR range. Narrowband thermal emission is successfully obtained at around 7.22μm (41.5 THz), which corresponds to the absorption band of SO2. The Q-factor of about 37 is achieved with a peak emissivity of 0.65, which is significantly higher compared to the reported Q-factors of state-of-the-art plasmonic resonators.

Analysis of thickness influence on refractive index and absorption coefficient of zinc selenide thin films.

Linear optical constants of thermally evaporated Zinc Selenide thin films are computed on the account of only transmittance experimental data. The refractive index and absorption coefficient dispersions of Zinc Selenide layers of small to close-to-bulk (50 nm ÷ 800 nm) thicknesses deposited on transparent quartz substrates are reckoned over a broad wavelength spectrum (300 nm ÷ 2500 nm). To distinct optical thickness classes, distinct analytical approach methods are implemented to compute the refractive index dispersions (Sellmeier equations) and absorption coefficient values, with the substrate's influence accounted for. The implemented methods are exploited to infer the linear optical constants for practically any thin film thickness, with the premise that all considered films are deposited by a same deposition method. Although different deposition methods may lead to different thickness dependencies of the linear optical constants (thin films of the same material having to be regarded accordingly to the deposition process), the implemented approach stays. The study substantiates the possibility and uplifts the importance of prior knowledge on thickness dependent linear optical constants to design thin film structures with particular, custom features for linear and nonlinear photonics applications.

Temperature dependent optical properties of SnO2 film study by ellipsometry

In this study, columbite phase tin oxide film was deposited onto quartz glass using the thermal evaporation method. An ellipsometry experiment was performed at the temperature of 25 °C-600 °C. B-spline with K-K consistence was used to describe the optical constants of SnO2 film to obtain the temperature dependent film thickness and optical constants. Results of X-ray diffraction pattern (XRD) confirmed an irreversible phase transition from columbite to the rutile structure at the temperature range of 100 to 300 °C, which had remarkably reduced the film thickness and resulted in the blue shift of the absorption edge. Besides, the total and partial densities of states (TDOS and PDOS) for both rutile and columbite phase SnO2 were also calculated based on the first-principles in accordance with the density functional theory, so as to clarify the structure properties of these two phases.

Anomalous terahertz dielectric phase in charge-ordered La1/3Sr2/3FeO3 thin film

Orthoferrites are the model systems for understanding the origin of complex valance-skipping charge ordering (CO) associated with antiferromagnetic spin ordering as a sequence of Fe+3Fe+3Fe+5Fe+3Fe+3Fe+5 … along the pseudocubic (111) direction. Here, we have investigated the low-energy dynamics of one such system La1/3Sr2/3FeO3 along the (111) crystal orientation using terahertz (THz) time domain spectroscopy. The temperature dependent THz optical constants reveal a purely electronic semiconductor to a charge-ordered Mott insulator transition, which is contrary to the reported metal–insulator transition. Above the transition of ∼180 K, THz conductivity shows a thermally activated charge-carrier hopping type conduction with activation energy 92.7 meV (87.3 meV) when measured in cooling (heating). The dielectric constant also indicates that the charge ordering formation starts at 200 K, which exists down to 20 K. Further analysis shows a reduction in the effective number of carriers by a fraction of about six as temperature decreases from 300 K to the CO transition at 200 K; this depletion of carriers triggers the intersite interactions required for CO. These findings suggest that a high dielectric constant associated with charge order in the THz region could be useful for THz applications in communication.

Semiconductors for high selectivity thermal emitters

Selective emitters limit thermal radiation to a narrow spectral band enabling applications including energy conversion and sensing. While applications demand high spectral selectivity, the materials used for selective emitters limit the ultimate selectivity. At high temperatures (>600 K), the optical properties of materials degrade, resulting in poor spectral selectivity, and thus leaving an open question—what material platform makes a good selective thermal emitter? Here, we show that doped semiconductors make a good choice by predicting their temperature dependent optical constants in a wide doping range. We build semi-empirical models to predict the temperature dependences of microscopic processes in GaAs and Si, a polar and a non-polar semiconductor respectively, and demonstrate that semiconductors with small and tunable optical losses enable very high spectral selectivity in thermal emitters. Our predictions agree reasonably well with more complex and accurate methods such as variational and second order perturbation theories, and serve as a guide in the discovery and development of materials for high temperature nanophotonics.

Exact interband transition energies of VO2 films

The temperature dependent optical constants of VO2 thin films with different thicknesses on variable substrates were measured by spectroscopic ellipsometry. The exact interband transition energies crossover metal-to-insulator transition of VO2 were obtained using a standard critical point model fitting based on the measured optical constants. The dynamic image of all critical points was obtained during the heating and cooling processes within the measured energy region. By introducing the fine orbital structure into the band model, every critical point was assigned to the corresponding band transition. The critical point of 4.7eV predicted theoretically, which is response to the transition from O2P to upper a1g, was obtained experimentally for the first time. Our work provides a new view for the complex band scheme of the monoclinic and rutile phase VO2. Moreover, the insulator-to-metal transition temperature is modified by regulating both film thickness and substrate.

Spectral emissivity of oxidized and roughened metal surfaces

The effects of materials’ surface oxidation and roughness on spectral emissivity have been investigated for design improvement and effective thermal management of energy systems. Experiments confirm that both surface oxidation and surface roughness affect spectral emissivity with surface oxidation having a more dominant effect. The surface oxidation layer effect is substantial at low wavelengths, and the affected wavelength range widens as oxidation temperature and time are increased. Increasing surface roughness increases spectral emissivity independent of wavelength. To describe the experimental results, a computational model based on Kirchhoff’s Law is used. The dependence of optical constants on oxidation conditions were measured, and included in the computational model. Surface morphological characteristics for the roughened surfaces are described by multiple superimposed cosine function Fourier series. There is good agreement between computational predictions and experimental results.

Temperature dependent optical constants for SiO2 film on Si substrate by ellipsometry

In situ ellipsometry measurements in the spectra range from 246 to 1000 nm were performed on a SiO2/Si system at temperatures varying from 25 to 600 °C. By using Cauchy dispersion model to describe SiO2 layer and B-spline to parameterize Si substrate, the temperature dependent optical constants of SiO2 film and Si substrate were precisely determined and analyzed which is helpful to understand the temperature effect of substrate for ellipsometry measurement. The temperature coefficient of refractive index of SiO2 at elevated temperature was obtained by ellipsometry. The results indicate that the temperature coefficient increases with increasing wavelength and decreasing temperature.

Optical properties of nonstoichiometric ZrO x according to spectroellipsometry data

Amorphous nonstoichiometric ZrOx films of different composition have been synthesized by the method of ion-beam sputtering deposition of metallic zirconium in the presence of oxygen at different partial oxygen pressures in the growth zone, and their optical properties have been studied in the spectral range of 1.12–4.96 eV. It is found that light-absorbing films with metallic conductivity are formed at the partial oxygen pressure below 1.04 × 10–3 Pa and transparent films with dielectric conductivity are formed at the pressure above 1.50 × 10–3 Pa. It is shown that the spectral dependences of optical constants of ZrOx films are described well by the corresponding dispersion models: the Cauchy polynomial model for films with dielectric conductivity and the Lorentz–Drude oscillator model for films with metallic conductivity.

Temperature dependent optical properties of silver from spectroscopic ellipsometry and density functional theory calculations

A detailed analysis of temperature dependence of optical constants of silver thin film measured by spectroscopic ellipsometry in the energy range 1.4 to 5 eV for temperatures ranging from 300 K to 650 K in steps of 50 K is presented in this paper. A microstructure modeling based on Bruggeman effective medium approximation is carried out on the temperature dependent optical constants. A red shift of ∼300 meV accompanied by a broadening of the interband transition is observed and an additional weak absorption edge emerges from the lower energy side of main interband absorption transition at 550 K. Drude model is applied to perform the analysis of dielectric function in the low energy region. The energy loss function −Im(1/ε) shows large damping with increasing temperature and this is a consequence of increasing magnitude of ε2 resulting from the composite transition in the region where ε1 is very small. These experimental results are corroborated with first principles electronic structure calculations using the full potential linearized augmented plane wave method within the frame-work of density functional theory.

Optical properties of vanadium oxides-an analysis

In this study, the optical properties of bulk and thin films of VO2, V2O3, and V2O5, deposited on Al2O3 substrates, have been analyzed from infrared to vacuum ultraviolet range (up to 12 eV). Utilizing the available data of wavelength dependent optical constants of these materials in the literature, the energy corresponding to the peaks in the imaginary part of the dielectric function (e2–R spectra), have been interpreted and compared as a function of structure, polarization, and temperature. The energies corresponding to the peaks in reflectivity-energy (R–E) spectra are explained in terms of the Penn gap (Ep). Ep values for VO2 and V2O5 are close to the average of energies corresponding to the peaks (\( \bar{E} \)) while, their values are even closer in V2O3, reflecting the degree of anisotropy in the order of V2O3 < VO2 < V2O5. The first order reversible, insulator to metal phase transition (IMT) of both bulk and thin films of the V–O systems are studied as an effect of temperature change. The effective number of electrons, neff, participating in the optical transitions is described from the numerical integration using the well-known sum rule. The change in neff with respect to the energy of incident photons is also calculated and it is found that this change is consistent with the peaks observed in the e2–E spectra.

Single-layer terahertz metamaterials with bulk optical constants

We investigate the conditions under which single layer metamaterials may be described by bulk optical constants. Terahertz time domain spectroscopy is utilized to investigate two types of geometries, both with two different sizes of embedding dielectric---cubic and tetragonal unit cells. The tetragonal metamaterials are shown to yield layer dependent optical constants, whereas the cubic metamaterials yielded layer independent optical constants. We establish guidelines for when $\ensuremath{\epsilon}$ and $\ensuremath{\mu}$ can be used as material parameters for single layer metamaterials. Experimental results at terahertz frequencies are presented and supported by full wave three-dimensional electromagnetic simulations.

近红外波段锑基铋掺杂薄膜厚度对光学常数与光学带隙的影响

近红外波段锑基铋掺杂薄膜厚度对光学常数与光学带隙的影响

Spectra of optical parameters in bulk and film amorphous alloys of the Se95As5 system containing samarium (Sm) impurities

Reflectance spectra of bulk and film amorphous alloys of the Se95As5 system containing samarium (Sm) impurities are studied in the energy range of 1–6 eV. Spectral dependences of optical constants and derivatives of optical dielectric functions are calculated by the Kramers-Kronig method. Changes in spectra of optical parameters depending on the content of impurities introduced into Se95As5 and conditions of their preparation are explained based on the cluster model. According to the latter, changes in the electron density of states depends on changes in atomic configurations in clusters, i.e., short-range order changes.

Absorption and spectra of optical parameters in amorphous solid solutions of the Se-S system

A study of the optical properties of the Se-S system has revealed a correlation between the dependences of optical absorption coefficient α, effective concentration of charged defects Nt, and characteristic energy E0 corresponding to the Urbach optical absorption in the spectral region where the Urbach rule works for the Se-S system on the S concentration. These optical properties are controlled by charged defects. It is shown that concentrations of intrinsic charged defects can be changed by variation in composition of the Se-S system. Reflectance spectra of amorphous solid solutions of the Se-S system are studied within the energy range 1–6 eV. Using the Kramers-Kronig method, spectral dependences of optical constants and derivative optical and dielectric functions are calculated. Variation in the spectra of optical parameters with composition of the Se-S system are explained within a cluster model in which the density of electron states is a function of atomic configurations in clusters, i.e., of the character of a short-range order.