Linear Dielectric Function Research Articles

Theoretical investigation of optoelectronic properties in PbS/CdS core/shell spherical quantum dots under the effect of the electric field intensity, hydrogenic impurity and geometric parameters

In this paper, we theoretically investigated the combined effects of the external electric field (EF) strength and the geometric parameters on the linear, nonlinear and total dielectric functions as well as the effective dielectric function (DF) coefficients of PbS/CdS spherical core/shell quantum dots (CSQD) in the presence of the hydrogenic impurity located at the center. The subband energy eigenvalues and their corresponding wave functions are obtained by solving the time-independent Schrödinger equation and using the variational method (VM) in the framework of the effective mass approximation (EMA). The linear, nonlinear and total DF as well as the effective DF were discussed and evaluated under the influence of EF intensity, geometric parameters, the change of the number of quantum dots (QDs) per unit volume and optical intensity I based on the compact density matrix (CDM) approach. The obtained results show that the height peaks of the linear, nonlinear and Total DF as well as the effective DF coefficients increase and their resonant peak moves towards lower energies as the EF and geometric factors increase in both cases with and without considering the hydrogenic impurity effect. Furthermore, our findings show that the impact of optical intensity and the number of QDs per unit volume does not change the resonance peaks of imaginary parts of DF and effective DF but decreases their magnitudes. As a result, we believe that numerical results will present important developments and provide great contributions in designing new optoelectronic devices related to CSQD hetero-nanostructure.

Linear and Nonlinear Dielectric Response of Intrinsically Disordered Proteins.

Linear and nonlinear dielectric responses of solutions of intrinsically disordered proteins (IDPs) were analyzed by combining molecular dynamics simulations with formal theories. A large increment of the linear dielectric function over that of the solvent is found and related to large dipole moments of IDPs. The nonlinear dielectric effect (NDE) of the IDP far exceeds that of the bulk electrolyte, offering a route to interrogate protein conformational and rotational statistics and dynamics. Conformational flexibility of the IDP makes the dipole moment statistics consistent with the gamma/log-normal distributions and contributes to the NDE through the dipole moment's non-Gaussian parameter. The intrinsic non-Gaussian parameter of the dipole moment combines with the protein osmotic compressibility in the nonlinear dielectric susceptibility when dipolar correlations are screened by the electrolyte. The NDE is dominated by dipolar correlations when electrolyte screening is reduced.

Optical properties of ZnSe using linear response theory

The electronic structure and optical response of ZnSe are studied in this work. The studies are carried out using first-principles full-potential linearized augmented plane wave method. After settling the crystal structure, the electronic band structure of the ground state of ZnSe is calculated. Linear response theory is applied to study optical response considering bootstrap (BS) and the long range contribution (LRC) kernels for the first time. We also use the random phase and adiabatic local density approximations for comparison. A procedure based on empirical pseudopotential method is developed to find material dependent parameter α required in the LRC kernel. The results are assessed by calculating the real and imaginary parts of linear dielectric function, refractive index, reflectivity, and the absorption coefficient. Results are compared with other calculations and available experimental data. The results of LRC kernel finding α from the proposed scheme are encouraging and at par with the BS kernel.

Investigation of linear and nonlinear optical properties of amorphous carbon nanofilms prepared by electron beam evaporation

Carbon nanofilms are easier to integrate in electronic devices than their two-dimensional counterparts. With proper engineering, they can offer similar optical and electrical properties without any alteration because of material transfer effects. In the current research, amorphous carbon nanofilms with thicknesses of 2, 5 and 20 nm were prepared by electron beam evaporation onto glass substrates. The surface morphology, structural and optical properties were analyzed. Through ellipsometric analysis, the linear dielectric functions were quantified. The results confirm the presence of multiphase amorphous carbon structures in the films with different electronic hybridization. The 2 nm films which had the highest sp2/sp3 ratio exhibited a crossover wavelength at 397 nm related to epsilon near zero localization. The third-order nonlinear behavior close to this wavelength in all samples showed maximum response. Details and discussion of the preformed characterization and relation to potential applications are presented.

Effect of nanostructure size and dielectric environment on linear and nonlinear dielectric functions in GaN/AlxGa1-xN core shell quantum dots

In this research, the effective mass approximation (E.M.A) and the compact density matrix approach have been used to compute the energy eigenvalues, linear, nonlinear and total dielectric functions for GaN/AlxGa1-xN core/shell quantum dot (CSQD) encapsulated with three different matrices (PVA, PMMA and SiO2). The influence of the geometrical factor (core to shell radii ratio) on the electron spectrum and transition energy has been analyzed. Our theoretical investigation indicated that with the appropriate dielectric environment, peaks of the real and imaginary parts of the linear, nonlinear and total dielectric functions can be spectacularly enhanced and experience a red or a blue shift. These findings suggest the possibility of tuning the resonant peaks by using the combined effect of the spatial confinement and dielectric discontinuity between the dot and the surrounding medium which can be beneficial in many fields especially for optoelectronic device applications.

Mechanical and optical properties of polymeric nitrogen achieved by compression: DFT study

The recent successful characterization of black phosphorus alike N8 compound at 140 GPa has spurred interest in solid nitrogen and its potential industrial applications. In this work, the previously unexplored mechanical and optical properties of nitrogen at 140 GPa has been systematically studied using first-principles calculations and various physically motivated empirical approaches. Experimentally proven orthorhombic-N8 structure was correctly reproduced. Body-centered cubic-N4 phase of much lower formation enthalpy was also uncovered under similar pressure. Both the superdense, wide bandgap compounds have sufficient mechanical and dynamical stabilities. While their Vickers hardness of 40.29 and 39.45 GPa indicates a softer degree than diamond under similar pressure, analyses of their elastic parameters and linear optical dielectric function reveal strong covalent bonding, anomalously high elastic moduli, fracture toughness (5.86 and 6.26 MPa m1/2) that bear close comparison with those of diamond, large refractive index (> 2.2) over broad energy spectrum and high absorption coefficient (104-105 cm−1) in ultraviolet range. Moreover, cubic-N4 with a charge energy density of 79.93 J cm−3 could be potential material for energy storage devices.

Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites

Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric SrTiO3 thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.Received 14 September 2020Revised 20 January 2021Accepted 12 March 2021DOI:https://doi.org/10.1103/PhysRevX.11.021023Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDielectric propertiesElectric polarizationElectronic excitation & ionizationElectronic transitionsFerroelectricityLight-matter interactionNonlinear opticsPhononsStrong electromagnetic field effectsUltrafast phenomenaTechniquesTerahertz time-domain spectroscopyUltrafast pump-probe spectroscopyCondensed Matter, Materials & Applied PhysicsNonlinear DynamicsAtomic, Molecular & Optical

Comprehensive anisotropic linear optical properties of the Weyl semimetals TaAs and NbAs

TaAs and NbAs are two of the earliest identified Weyl semimetals that possess many intriguing optical properties, such as chirality-dependent optical excitations and giant second harmonic generation (SHG). Linear and nonlinear optics have been employed as tools to probe the Weyl physics in these crystals. Here we extend these studies to address two important points: determining the complete anisotropic dielectric response, and to explore if and how they can reveal essential Weyl physics. For the first time, we determine the complete anisotropic dielectric functions of TaAs and NbAs by combining spectroscopic ellipsometry and density functional theory (DFT). Parameterized Lorentz oscillators are reported from 1.2-6 eV (experiment) and 0-6 eV (DFT), and good agreement is shown between them. Both linear and nonlinear optical properties have been reported to reveal Weyl physics. We suggest that strong optical resonances from trivial bands are the likely origin of the large optical second harmonic generation previously reported at these energies. Furthermore, by comparing the contribution of a small k-space centered around the Weyl cones to the total linear dielectric function, we find that these contributions are highly anisotropic and are <25% of the total dielectric function below 0.5 eV; above 1eV, these contributions are negligible. Thus, the study of Weyl physics using optical techniques requires very low energies and even there, a careful assessment is required in distinguishing the much smaller contributions of the Weyl bands from the dominant contributions of the trivial bands and Drude response to the total dielectric function.

Dielectric effect on the optical properties of exciton quantum dot with Gaussian plus generalized Hulthen potential as Pseudo-Harmonic potential

The dielectric effect on the intersubband optical properties for a core/shell quantum dots system is investigated with the Gaussian plus generalized Hulthen potential as a Pseudo-Harmonic potential model. The analytical solutions of the bound states energy and eigenfunctions were calculated using the Nikiforov–Uvarov (N–U). A detailed investigation of the linear dielectric function, third-order nonlinear dielectric function, total absorptions coefficient, and the transmission coefficient has been examined using the density matrix formalisms. Our result reveals that the dielectric constant affects the core materials and enhances the spectra shift, while the shell materials for both materials examined remains constant at zero photon-energy levels.

Constrained random phase approximation of the effective Coulomb interaction in lattice models of twisted bilayer graphene

Recent experiments on twisted bilayer graphene show the urgent need for establishing a low-energy lattice model for the system. We use the constrained random phase approximation to study the interaction parameters of such models, taking into account screening from the moir\'e bands left outside the model space. Based on an atomic-scale tight-binding model, we numerically compute the polarization function and study its behavior for different twist angles. We discuss an approximation scheme which allows us to compute the screened interaction, in spite of the very large number of atoms in the unit cell. We find that the polarization has three different momentum regimes. For small momenta, the polarization is quadratic, leading to a linear dielectric function expected for a two-dimensional dielectric material. For large momenta, the polarization becomes independent of the twist angle and approaches that of uncoupled graphene layers. In the intermediate-momentum regime, the dependence on the twist angle is strong. Close to the largest magic angle the dielectric function peaks at a momentum of $1/(4\phantom{\rule{4.pt}{0ex}}\text{nm})$, attaining values of 18--25, depending on the exact model, meaning very strong screening at intermediate distances. We also calculate the effective screened Coulomb interaction in real space and give estimates for the on-site and extended interaction terms for the recently developed hexagonal-lattice model. For freestanding twisted bilayer graphene, the effective interaction decays slower than $1/r$ at intermediate distances $r$, while it remains essentially unscreened at large enough $r$.

Comparison of the structure, electronic, and optical behaviors of tin-doped CdO alloys and thin films

Spray pyrolysis technique (SPT) was applied to fabricate CdO films doped with tin at (0–8%) concentrations on glass substrates. Structural, morphological, and optical behaviors were inspected using X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–Vis spectroscopy. Structural, electronic, and optical behaviors of Cd1−xOSnx alloys were studied in pseudo-potential method using density functional theory (DFT) in scissors approximation. The calculated electronic band structure and optical behavior were achieved in this work. Optical bandgap energy was found to increase from 2.581 to 2.943 eV via increasing Sn content, while theoretically, it was found to increase from 2.884 to 3.106 eV with the increasing Sn doping. Theoretical and experimental findings were close to each other. The linear energy-dependent dielectric functions (real part) and optical behavior-like absorbance and transmittance were also studied.

Impact of dielectric environment on the linear and nonlinear optical properties for CdS/ZnS cylindrical core/shell quantum dots

By the mean of the effective mass approximation (E.M.A) and compact density matrix method, the energy eigenvalues, linear and nonlinear dielectric function have been theoretically investigated for CdS/ZnS cylindrical core/shell quantum dot embedded in PVA, PMMA and SiO2 matrices. Simulation results showed that the presence of the dielectric mismatch in the QD-matrix system can lead to a significant enhancement of the nonlinear dielectric function. According to our calculations, peaks of the real and imaginary part of the dielectric function can experience a redshift or a blue shift with the appropriate dielectric environment.

Dielectric environment effect on linear and nonlinear optical properties for CdS/ZnS core/shell quantum dots

The eigenvalues, transition energy, the linear and nonlinear dielectric functions have been numerically investigated for CdS/ZnS spherical core/shell quantum dots embedded in various dielectric matrices. The evaluation was carried out for three commonly used matrices, such as PVA, PMMA and SiO2. Calculations were done under the effective mass approximation and compact density matrix approach. Results revealed that the nonlinear optical property is strongly affected by the nature of the matrix material. By increasing core/shell radii ratio the energy states as well as the transition energy are decreasing. It is also indicated that the presence of the dielectric mismatch in the QD-matrix system can cause significant enhancement on the linear and third order nonlinear dielectric function.

Quasiparticle and excitonic effects in the optical response of KNbO3

The cubic, tetragonal, and orthorhombic phase of potassium niobate (${\mathrm{KNbO}}_{3}$) are studied based on density-functional theory. Starting from the relaxed atomic geometries, we analyze the influence of self-energy corrections on the electronic band structure within the $GW$ approximation. We find that quasiparticle shifts widen the direct (indirect) band gap by 1.21 (1.44), 1.58 (1.55), and 1.67 (1.64) eV for the cubic, tetragonal, and orthorhombic phase, respectively. By solving the Bethe-Salpeter equation, we obtain the linear dielectric function with excitonic and local-field effects, which turn out to be essential for good agreement with experimental data. From our results, we extract an exciton binding energy of 0.6, 0.5, and 0.5 eV for the cubic, tetragonal, and orthorhombic phase, respectively. Furthermore, we investigate the nonlinear second-harmonic generation (SHG) both theoretically and experimentally. The frequency-dependent second-order polarization tensor of orthorhombic ${\mathrm{KNbO}}_{3}$ is measured for incoming photon energies between 1.2 and 1.6 eV. In addition, calculations within the independent-(quasi)particle approximation are performed for the tetragonal and orthorhombic phase. The novel experimental data are in excellent agreement with the quasiparticle calculations and resolve persistent discrepancies between earlier experimental measurements and ab initio results reported in the literature.

Nonlinear optical properties of a slab of CdSe/ZnS quantum dot matrix

In this paper linear and nonlinear optical dielectric function of a slab of CdSe/ZnS quantum dot matrix associated with intersubband transitions is investigated theoretically. To this end, by using effective mass approximation and density matrix, the electronic structure and dielectric function of quantum dots are calculated. Afterwards, the Clausius-Mossotti relation and TKF model is used to calculate the effective dielectric function and susceptibility of slabs of ffc and randomly ordered quantum dots. The dependence of dielectric function, susceptibility, refractive index (RI) and absorption coefficient (AC) of the slab on the size and density of quantum dots have been studied. The results show that for all radii the AC and RI of fcc ordered quantum dots have higher magnitude than randomly ordered ones. It is also found that magnitude of the dielectric function of the slab increases by increasing radius and intensity. At fixed radii the dielectric function, RI and AC increases by increasing number density of quantum dots in the slab.

Symmetry reduction for tunneling defects due to strong couplings to phonons

Tunneling two-level systems (TLSs) are ubiquitous in amorphous solids, and form a major source of noise in systems such as nano-mechanical oscillators, single electron transistors, and superconducting qubits. Occurance of defect tunneling despite their coupling to phonons is viewed as a hallmark of weak defect–phonon coupling. This is since strong coupling to phonons results in significant phonon dressing and suppresses tunneling in two-level tunneling defects effectively. Here we determine the dynamics of a tunneling defect in a crystal strongly coupled to phonons incorporating the full 3D geometry in our description. We find that inversion symmetric tunneling is not dressed by phonons whereas other tunneling pathways are dressed by phonons and, thus, are suppressed by strong defect–phonon coupling. We provide the linear acoustic and dielectric response functions for a tunneling defect in a crystal for strong defect–phonon coupling. This allows direct experimental determination of the defect–phonon coupling. The singling out of inversion-symmetric tunneling states in single tunneling defects is complementary to their dominance of the low energy excitations in strongly disordered solids as a result of inter-defect interactions for large defect concentrations. This suggests that inversion symmetric TLSs play a unique role in the low energy properties of disordered solids.

Tuning the band gap and optical spectra of monolayer penta-graphene under in-plane biaxial strains

Using first principles calculations, structural, electronic and optical properties of monolayer penta-graphene (MPG) have been studied under different types of in-plane biaxial strains at different magnitudes. The results exhibit that MPG structure under small magnitude of biaxial compressive and tensile strains are more stable than those with large magnitude of strains. It is found that free- strained MPG is an indirect band-gap semiconductor with a band gap of 2.22eV using PBE functional, approximately equal to those of GaN and ZnO. Appling tensil strain slightly increases the bang gap of MPG, while exerting compresive tensile reduces it. The linear photon energy-dependent dielectric functions and related optical properties including refraction index, real part of conductivity and electron energy-loss spectrum (EELS) were also computed and discussed. The optical calculations illustrate that applying tensile and compressive strains on MPG result in blue and red shift in optical spectra. The results obtained from our calculations are beneficial to practical applications of these strained MPG structures in optoelectronics and electronics.

Collisional effects on interaction potential in complex plasma in presence of magnetic field

Interaction potential in complex plasma with streaming ions is derived analytically in presence of ion-neutral collision and magnetic field. The linear dielectric response function obtained describes the behavior of charged micron sized dust particles in strong collisional limit. A new type of repulsive potential is found to be operative among the dust grains apart from the normal Debye–Hückel potential. The amplitude and shielding length involved in the potential are substantially affected by the parameters describing ion cyclotron frequency, collision frequency among ions and neutrals, and ion streaming. It is also observed that the usual mechanism of ion focusing surrounding the grain is inhibited due to collision. As a result, the attractive wake potential structure is destroyed in the ion flow direction. The horizontal interaction involves only Debye–Hückel potential.

Electronic and optical properties of novel carbon allotropes

The vibrational properties, electronic structures and optical properties of novel carbon allotropes, such as monolayer penta-graphene (PG), double-layer PG and T12-carbon, were studied by first-principles calculations. Results of phonon calculations demonstrate that these exotic carbon allotropes are dynamically stable. The bulk T12 phase is an indirect-gap semiconductor having a quasiparticle (QP) bandgap of ∼5.19 eV. When the bulk material transforms to a two-dimensional (2D) phase, the monolayer and double-layer PG become quasi-direct gap semiconductors with smaller QP bandgaps of ∼4.48 eV and ∼3.67 eV, respectively. Furthermore, the partial charge density analysis indicates that the 2D phases retain part of the electronic characteristics of the T12 phase. The linear photon energy-dependent dielectric functions and related optical properties including refractive index, extinction coefficient, absorption spectrum, reflectivity, and energy-loss spectrum were also computed and discussed. Additionally, the chemical stability of monolayer PG and the electronic and optical properties of double-side hydrogenated monolayer PG were also investigated. The results obtained from our calculations are beneficial to practical applications of these exotic carbon allotropes in optoelectronics and electronics.

Energy exchange in strongly coupled plasmas with electron drift

In this paper, the generalized viscoelastic collisional quantum hydrodynamic model is employed in order to investigate the linear dielectric response of a quantum plasma in the presence of strong electron-beam plasma interactions. The generalized Chandrasekhar's relativistic degeneracy pressure together with the electron-exchange and Coulomb interaction effects are taken into account in order to extend current research to a wide range of plasma number density relevant to big planetary cores and astrophysical compact objects. The previously calculated shear viscosity and the electron-ion collision frequencies are used for strongly coupled ion fluid. The effect of the electron-beam velocity on complex linear dielectric function is found to be profound. This effect is clearly interpreted in terms of the wave-particle interactions and their energy-exchange according to the sign of the imaginary dielectric function, which is closely related to the wave attenuation coefficient in plasmas. Such kinetic effect is also shown to be in close connection with the stopping power of a charged-particle beam in a quantum plasma. The effect of many independent plasma parameters, such as the ion charge-state, electron beam-velocity, and relativistic degeneracy, is shown to be significant on the growing/damping of plasma instability or energy loss/gain of the electron-beam.