Frequency-dependent Refractive Index Research Articles

Optical and dielectric properties of β-Ta2O5

Tantalia (Ta2O5) is an interesting material which is used in several technological applications. In particular, this wide gap semiconductor is used as a new material for memristive devices, which could provide a paradigm shift on the memory devices fabrication. These new devices would be faster, denser and less power consuming than those available today. Because of this, we present a study of the optical and dielectric properties of β-Ta2O5 polymorph by means of first-principles Density Functional Theory calculations. Taking as a starting point the Nashed’s model of the crystal structure of β-Ta2O5 and the generalized gradient approximation (GGA-PBEsol), we calculated the equation of state (EOS) of this polymorph. From EOS we identify the equilibrium volume and for this we calculated the frequency dependent dielectric tensor, refractive index (n = 2.63), absorption coefficient and transmittance. Also, we calculated the electronic (ε∞ = 6.98) and ionic contribution (ε0 = 43.51) to static dielectric constant and it was compared with some reported values in the literature.

Optical properties of Meloxicam in the far-infrared spectral region

One of the most commonly used nonsteroidal anti-inflammatory active pharmaceutical ingredient called Meloxicam has been characterized spectroscopically both by Terahertz (THz) time domain spectroscopy (THz-TDS) and by Fourier Transform Infrared (FTIR) spectroscopy in far-IR regions of electromagnetic spectrum; 0.2 THz–20 THz. While many relatively sharp features are observed in the far-IR range between 2 THz–20 THz as expected for being an organic substance, very distinct and relatively strong absorption bands are also observed at 1.00, 1.66, 2.07 and 2.57 THz in the THz range. These well separated, defined, and fairly strong spectral features can be used for discrimination and quantification of Meloxicam in drug analysis. Frequency dependent refractive index of the drug was determined in a range of 0.2 THz–2.7 THz, where an almost constant index was observed with an average index of 1.75. Powder XRD, and solid-state Density Functional Theory (SS-DFT) calculations were utilized to determine the crystalline form of the Meloxicam sample in its enolic crystalline form. Single molecule DFT calculations were also performed in all four possible structures of Meloxicam. In addition, the capability of THz waves transmission through common packaging materials is demonstrated for possibility of future on-site analysis. The results suggest that drug analysis will be possible to perform not only at every stage of manufacturing without destruction but also directly at the shelf of a market after development of portable THz technologies.

Enhanced thermoelectric properties of ASbO3 due to decreased band gap through modified becke johnson potential scheme

Using density functional theory (DFT) calculations, we have explored the effect of Ag and Cs atoms on the electronic transport properties of ASbO3 (A = K, Ag and Cs). We employed first principle calculations for investigation of electronic, optical and thermoelectric properties of ASbO3 compounds. The exchange and correlation potential (EXC) was treated by the modified Becke Johnson functional (mBJ). The optimized lattice constants and internal cell parameters were in agreement with the available experimental data. The self-consistence density of state and band-structure calculations show that Cs-d and Ag-d states remains in valence band and dominate below the Fermi level, while Sb-s/p states mainly contribute in conduction band. When Ag and Cs for K substitution take place, there is a gradual hybridization of Cs-d and Ag-d states results. Optical spectra show the main absorption peak in between 8.0 and 13.0 eV depends on the substituent nature and could be due to transition from hybridized band (Sb-d and Sb-s), below EF to free Ag and Cs-s/p/d states. Frequency-dependent refractive index, n(ω), and the extinction coefficient, k(ω), of ASbO3 were also calculated for the radiation upto 40 eV. We combined the outputs of DFT to transport theory based on Boltzmann equation to calculate the potential use of the ASbO3 as thermoelectrics. The change of the alkaline element affected both of electrical conductivity, Seebeck coefficient and thermal conductivity.

Dynamics of a broad-band quantum cascade laser: from chaos to coherent dynamics and mode-locking.

The dynamics of a multimode quantum cascade laser, are studied in a model based on effective semiconductor Maxwell-Bloch equations, encompassing key features for the radiation-medium interaction such as an asymmetric frequency dependent gain and refractive index as well as the phase-amplitude coupling provided by the linewidth enhancement factor. By considering its role and that of the free spectral range, we find the conditions in which the traveling wave emitted by the laser at the threshold can be destabilized by adjacent modes, thus leading the laser emission towards chaotic or regular multimode dynamics. In the latter case our simulations show that the field oscillations are associated to self-confined structures which travel along the laser cavity, bridging mode-locking and solitary wave propagation. In addition, we show how a RF modulation of the bias current leads to active mode-locking yielding high-contrast, picosecond pulses. Our results compare well with recent experiments on broad-band THz-QCLs and may help in the understanding of the conditions for the generation of ultrashort pulses and comb operation in mid-IR and THz spectral regions.

Degradation diagnosis of transformer insulating oils with terahertz time-domain spectroscopy

We report the frequency-dependent complex optical constants, refractive index and absorption, and complex dielectric properties over the frequency range from 0.2 to 3.0 THz for aged power transformer mineral insulating oils. These results have been obtained using terahertz time-domain spectroscopy (THz-TDS) and demonstrate the double-Debye relaxation behavior of the mineral insulating oil. The measured complex optical and dielectric characteristics can be important benchmarks for liquid molecular dynamics and theoretical studies of insulating oils. Due to clear differences in THz responses of aged mineral insulating oils, THz-TDS can be used as a novel on-site diagnostic technique to monitor the insulation condition in aged power transformers and may be valuable alternative to characterize other developing eco-friendly insulating oils and industrial liquids.

Terahertz spectroscopy of anisotropic materials using beams with rotatable polarization

We introduce a polarization-resolved terahertz time-domain spectrometer with a broadband (0.3–2.5 THz), rotatable THz polarization state, and which exhibits minimal change in the electric field amplitude and polarization state upon rotation. This was achieved by rotating an interdigitated photoconductive emitter, and by detecting the orthogonal components of the generated THz pulse via electro-optic sampling. The high precision (<0.1°) and accuracy (<1.0°) of this approach is beneficial for the study of anisotropic materials without rotating the sample, which can be impractical, for instance for samples held in a cryostat. The versatility of this method was demonstrated by studying the anisotropic THz optical properties of uniaxial and biaxial oxide crystals. For uniaxial ZnO and LaAlO3, which have minimal THz absorption across the measurement bandwidth, the orientations of the eigenmodes of propagation were conveniently identified as the orientation angles that produced a transmitted THz pulse with zero ellipticity, and the birefringence was quantified. In CuO, a multiferroic with improper ferroelectricity, the anisotropic THz absorption created by an electromagnon was investigated, mapping its selection rule precisely. For this biaxial crystal, which has phonon and electromagnon absorption, the polarization eigenvectors exhibited chromatic dispersion, as a result of the monoclinic crystal structure and the frequency-dependent complex refractive index.

Thermoelectric properties of [formula omitted] for device applications: A first principle TB-mBJ potential study

Here we report the structural, electronic, optical, and thermoelectric properties of delafossite type 2H-CuGaO2 using first principles calculations. The present calculation predict an indirect band gap of 1.20eV and a direct band gap of 3.48eV. A detailed analysis of the electronic structure is provided based on atom and orbital projected density of states. Frequency dependent dielectric functions, refractive index, and absorption coefficient as a function of photon energy are discussed. The thermoelectric properties with power factor, and the figure of merit are reported as a function of chemical potential in the region ±0.195(μ−EF)eV at constant temperature of 300 and 800K. The thermoelectric properties shows that 2H-CuGaO2 could be potential candidate for engineering devises operating at high temperature for the chemical potential in the range of ±0.055(μ−EF)eV and beyond this range the thermoelectric performance of 2H-CuGaO2 get reduced.

Optimization and Characterization of High-Harmonic Generation for Probing Solid Density Plasmas

The creation of high energy density plasma states produced during laser–solid interaction on a sub-picosecond timescale opens a way to create astrophysical plasmas in the lab to investigate their properties, such as the frequency-dependent refractive index. Available probes to measure absorption and phase-changes given by the complex refractive index of the plasma state are extreme-UV (EUV) and soft X-ray (XUV) ultra-short pulses from high harmonic generation (HHG). For demanding imaging applications such as single-shot measurements of solid density plasmas, the HHG probe has to be optimized in photon number and characterized in intensity and wavefront stability from shot-to-shot. In an experiment, a coherent EUV source based on HHG driven by a compact diode-pumped laser is optimized in photons per pulse for argon and xenon, and the shot-to-shot intensity stability and wavefront changes are characterized. The experimental results are compared to an analytical model estimating the HHG yield, showing good agreement. The obtained values are compared to available data for solid density plasmas to confirm the feasibility of HHG as a probe.

Optical properties of a-As2Se3 thin films

&nbsp;Chalcogenide glassy semiconductors currently have found widely applications in the opto-and microelectronics devices. On this work we studied the optical properties of amorphous a-As2Se3 thin films which were produced on the quartz substrate by thermal evaporation method with different thickness: 500 nm, 1000 nm, 2000 nm. The transmission spectra were measured by spectrophotometer in the spectral range 250 -1000 nm. It is known that Swanepoel method can be used to determine the frequency dependent refractive index of thin films via interference fringes of transmission spectra. Using the obtained transmission spectrum and Swanepoel’s method we calculated the optical constants of a- As2Se3 thin films in the 700 – 900 nm wavelength range. The found optical constants of a-As2Se3 films were used to theoretically determine the transmission spectrum by the matrix transfer method (TMM). The comparison of theoretical and experimental values of transmittance was made and proved the practical usage of these methods. &nbsp; &nbsp; &nbsp; G M T &nbsp; Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский &nbsp; Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Звуковая функция ограничена 200 символами &nbsp; &nbsp; Настройки : История : Обратная связь : Donate Закрыть

Extended Cauchy equations of congruent LiNbO_3 in the terahertz band and their applications

The refractive indices of ordinary and extraordinary waves (no and ne) of congruent LiNbO3 (CLN) in the terahertz (THz) band are measured at different temperatures. Extended Cauchy equations are proposed to describe the temperature- and frequency-dependent refractive indices. With less than 0.5% deviation, both no and ne can be obtained accurately. The values of no and ne are ~7 and ~5.4, respectively, which are much larger than those in the visible band. The extremely high birefringence (~1.6) may lead to some interesting applications of linear, electro-optic, and nonlinear optical effects of CLN. As an example, true zero-order THz waveplates (WPs) could be obtained with considerable low loss based on thin CLN wafers. The working frequency can be tuned by changing the environmental temperature. Moreover, wide bandwidth achromatic THz WPs were also achievable by stacking four CLN wafers together with precise orientation control.

Combating/eliminating dispersion-induced fading using suppressed carrier link for OFDM signal

ABSTRACTWe investigate the performance of 20-GHz radio over fibre (RoF) system having orthogonal frequency division multiplexed (OFDM) as radio signal using two modulation techniques – balanced detection for intensity modulation direct detection (IMDD) and coherent heterodyne detection suppressed carrier (SC) modulation. Dispersion-induced power fading is seen in conventional IMDD links due to the dependence of dispersion over frequency-dependent refractive index of the fibre. SC link is seen to compensate the power fading by terminating the direct current and even-order harmonics with the suppression of carrier along with balanced detection.

Effect of Si substitution on structural, electronic and optical properties of YNi4Si-type DyNi5−xSix (x=0, 1, 2) compounds

We employed first principle calculations for investigation of structural, electronic and optical properties of YNi4Si-type DyNi5−xSix (x=0, 1, 2) compounds. These properties are studied first time on YNi4Si-type DyNi5−xSix compounds. The exchange and correlation potential is treated by the Coulomb corrected local spin density approximation (LSDA+U) method for better accounting of the correlation between the 4f electrons. The optimized lattice constants and internal cell parameters are in agreement with the available data. Self consistence band structure calculations show that Ni-3d states remains in valance band and dominant below the EF, while Dy-5d and 4f states mainly contributes above Fermi Energy (EF) in DyNi5−xSix (x=0, 1, 2) compounds. We also find that when silicon for nickel substitution takes place (DyNi4Si), there is a gradual hybridization of Ni-3d and Si-3p states results, nickel moments decrease rapidly in agreement with the experiment. Optical spectra shows the main absorption peak around 4eV depends on the substituent concentration and could be due to transition from hybridized band (Ni-3d and Si-3p), below EF to free Dy-4d states. Frequency-dependent refractive index, n(ω), and the extinction coefficient, k(ω), of DyNi5−xSix (x=0, 1, 2) are also calculated for the radiation up to 14eV.

First principle calculation of physical properties of barium based chalcogenides BaM4S7 (M = Ga, Al); a DFT, DFT-D and hybrid functional HSE06 study

The electronic structure, elastic and optical properties have been calculated for the novel nonlinear optical (NLO) crystals BaQ4S7 (Q = Ga, Al) using plane wave pseudo-potential density functional theory (DFT) method as implemented in CASTEP and ABINIT codes. In this study we used both hybrid HSE06 and DFT-D functionals with GGA approximation. These NLO compounds, which belong to the mm2 point group, are particularly interesting because of their transparency in the mid-infrared region and wide energy band gap. We present results for electronic structure, elastic tensor coefficients, refractive indices and second order nonlinear optical susceptibilities. The calculated energy band gap and frequency dependent refractive indices as well as the NLO coefficients of BaGa4S7 are in good agreement with the experimental values. With no reported theoretical or experimental energy band gap and optical properties of BaAl4S7, we present for the first time its electronic structure and above mentioned optical coefficients. This compound has higher direct band gap with 3.74eV, better optical birefringence and second-order NLO coefficients than most NLO compounds. The second-order NLO coefficients for BaAl4S7 have been calculated as d31=3.15pm/V, d31=2.20pm/V, d33=−6.31pm/V.

Acoustic dispersive prism

The optical dispersive prism is a well-studied element, which allows separating white light into its constituent spectral colors, and stands in nature as water droplets. In analogy to this definition, the acoustic dispersive prism should be an acoustic device with capability of splitting a broadband acoustic wave into its constituent Fourier components. However, due to the acoustical nature of materials as well as the design and fabrication difficulties, there is neither any natural acoustic counterpart of the optical prism, nor any artificial design reported so far exhibiting an equivalent acoustic behaviour. Here, based on exotic properties of the acoustic transmission-line metamaterials and exploiting unique physical behaviour of acoustic leaky-wave radiation, we report the first acoustic dispersive prism, effective within the audible frequency range 800 Hz–1300 Hz. The dispersive nature, and consequently the frequency-dependent refractive index of the metamaterial are exploited to split the sound waves towards different and frequency-dependent directions. Meanwhile, the leaky-wave nature of the structure facilitates the sound wave radiation into the ambient medium.

Optical properties of g-C4N3/BN bilayer film: A first-principles study

Using first-principles method, we explored the frequency dependent dielectric function, refractive index, reflectivity, optical conductivity, loss function, and absorption coefficient of g-C4N3/BN bilayer system. Here, the dependences of the optical properties on the electric field polarization were also explored. Because the dielectric function strongly depended on the electric polarization, we obtained anisotropic optical properties. For instance, the dielectric function was frequency independent for perpendicular electric polarization while a frequency dependent dielectric function was observed for parallel electric polarization. A high refractive index larger than 2 was obtained for parallel polarization in the infrared and the visible frequency ranges, whereas a constant refractive index of approximately 1.5 was observed for perpendicular polarization. We found that the electromagnetic wave propagating perpendicular to the film surface had a very small reflectivity at ultraviolet frequencies, this property could be utilized for applications as anti-reflection ultraviolet coatings. In addition, the g-C4N3/BN bilayer may produce no meaningful joule heating for in-plane wave propagation because of its extremely weak optical conductivity. We propose that optical phonons may generated by incident ultraviolet waves. In addition, the bilayer system may be transparent in the visible range. Overall, we found that metal-free, transparent, half-metallic g-C4N3 films could be utilized for spintronics and optical device applications.

Elastic, Electronic, Optical and Thermal Properties of Na2Po: An Ab Initio Study

The structural, elastic, electronic, optical and thermodynamic properties of the sodium polonide Na2Po compound have been studied through the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) and tight-binding linear muffin-tin orbital (TB-LMTO) methods. The exchange–correlation potential was treated within the local density approximation for the TB-LMTO calculations and within the generalized gradient approximation for the FP-LAPW + lo calculations. In addition, Tran and Blaha-modified Becke–Johnson (TB-mBJ) potential and Engel–Vosko generalized gradient approximation were used for the electronic and optical properties. Ground state properties such as the equilibrium lattice constant, bulk modulus and its pressure derivative were calculated and compared with available data. The single-crystal and polycrystalline elastic constants of the considered compound were calculated via the total energy versus strain in the framework of the FP-LAPW + lo approach. The calculated electronic structure reveals that Na2Po is a direct band gap semiconductor. The frequency-dependent dielectric function, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for a wide energy range. The variations of the lattice constant, bulk modulus, heat capacity, volume expansion coefficient and Debye temperature with temperature and pressure were calculated successfully using the FP-LAPW + lo method in combination with the quasi-harmonic Debye model.

Structural, elastic, electronic and optical properties of KAlQ2 (Q = Se, Te): A DFT study

First-principles calculations in the framework of density functional theory have been conducted to explore the structural, elastic, electronic and optical properties of two layered ternary compounds chalcogenides of aluminum KAlSe2 and KAlTe2. We have calculated all of the equilibrium structural parameters; the lattice parameters (a, b and c), angle β and twenty three internal atomic coordinates. The obtained results are in excellent agreement with the available experimental data. We have predicted the single-crystal elastic constants Cij of the title materials using stress-strain approach and then derived the elastic moduli of the polycrystalline aggregates and related properties via the Voigt-Reuss-Hill approximations. The band structure and density of states diagrams have been calculated and analyzed. Both compounds demonstrate semiconducting behavior with direct band gap. The linear optical properties, namely the frequency-dependent dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity and energy-loss function, have been calculated and analyzed in a wide energy range up to 20 eV.

Terahertz dielectric properties of (ZnS and ZnO)/Si nano-structures

The application of non-empirical effective medium theory (extended Maxwell-Garnett) to describe composite mixtures of spherical inclusions within a host continuum has been presented. The structures are composites of two different kinds of non-overlapping spheres, one made from a high refractive index non-plasmonic material and the other from a Drude-like material. Frequency-dependent complex dielectric function and refractive index of ZnS/Si and ZnO/Si composites have been calculated in the terahertz regime. The mentioned nano-structured composites also exhibit unique optical properties due to the phenomenon known as surface plasmon resonance (SPR).

Study of systematic trends in electronic and optical properties within ZnM2O4 (M = Co, Rh, Ir) family by FPLAPW method with PBE and TB-mBJ potentials

The systematic trends for electronic and optical properties for the family of spinel oxides ZnM2O4 depending on the type of M element (M=Co, Rh, Ir) have been investigated using full potential linearized augmented plane wave (FPLAPW) method based on the density functional theory. Calculations have been performed by alternative form of generalized gradient approximation proposed by Perdew, Berke and Erzzehof (GGA-PBE) and by orbital independent Tran–Blaha modified Becke-Johnson (TB-mBJ) potentials as coupled with GGA. The optimized unit cell length, a0, and internal parameter, u, calculated are in agreement with experimental data. TB-mBJ scheme shows a significant improvement in bandgaps over PBE value and are closer to the experimental data. By analyzing density of states (DOS), we find that more pronounced splitting of M-d states splitting is responsible for larger band gap in TB-mBJ scheme for ZnM2O4 compounds. It also shows valence band dispersion is reduced in the TB-mBJ scheme compared to the PBE. Optical properties have been calculated for the energy range 0–14eV. The values of calculated reflectivity stays low till 3.5eV which is consistent with energy gap. The results are analyzed on the basis of band to band transitions. We also present the analysis of frequency dependent refractive index, n (ω), and extinction coefficient, k (ω), of ZnM2O4 using PBE and TB-mBJ schemes.

Terahertz homodyne self-mixing transmission spectroscopy

A compact homodyne self-mixing terahertz spectroscopy concept is experimentally investigated and confirmed by calculations. This method provides amplitude and phase information of the terahertz radiation emitted by a photoconductive antenna in a transmission experiment where a rotating chopper wheel serves as a feedback mirror. As a proof-of-principle experiment the frequency-dependent refractive index of Teflon is measured.