Interband Optical Transitions Research Articles

Real‐Time Detection of Coherent Vibrational Dynamics in TiN Films

AbstractTitanium nitride (TiN) has recently gained considerable interest because of its remarkable plasmonic properties and for its strong electron–phonon (e–ph) coupling, leading to extremely fast (<100 fs) electron‐lattice cooling. Here, the generation of coherent phonons in TiN films is reported, along with their real‐time detection by means of broadband transient reflection spectroscopy with sub‐15‐fs temporal resolution. The measurements show damped oscillations, superimposed to excited state electronic decay. A coherent vibrational mode is revealed, with ≈10 THz frequency ascribed to defect‐activated normal modes, consistent with spontaneous Raman scattering data, and a dephasing time of ≈250 fs. Two π‐phase flips are also observed located at photon energies corresponding to interband optical transitions (at 3.2 and 2.5 eV), ascribed to selective coupling of the vibrational mode to these transitions; the energy modulation induced by the vibrational coherence is evaluated. It is shown that the displacive excitation of coherent phonons model describes the coherent response in terms of temporal behavior and of spectral amplitude profile. Overall, a comprehensive and detailed analysis of coherent phonons in TiN films, so far undected, is provided and relevant information on TiN photo‐physical properties, potentially useful for its applications, is given.

Comprehensive analysis: Exploring quaternary Heusler alloys CoFeXGe (X = Hf and Ta) through first‐principles calculations

AbstractThe research focused on the quaternary Heusler alloys CoFeXGe (with X being Hf or Ta) using a first principles approach via density functional theory and the Wien2k code. The alloys demonstrated a stable ferromagnetic ground state and exhibited negative formation energies, suggesting their experimental synthesis feasibility. Investigation of the electronic properties revealed that both materials are half‐metallic ferromagnets. The calculated indirect band gaps of CoFeHfGe and CoFeTaGe are 1.46 and 0.77 eV, respectively. The mechanical stability of the materials was confirmed through elastic constants analysis. To gain insights into the materials optical behavior and their potential applications in photonics and related fields, correlation of interband optical transitions with electronic band structure was carried out. Finally, Boltzmann transport theory was utilized to evaluate the thermoelectric potential of these alloys over a temperature range extending from 100 to 1000 K.

Effects of high-temperature annealing on vacancy complexes and luminescence properties in multilayer periodic structures with elastically strained GeSiSn layers

Effects of postgrowth high-temperature annealing on vacancy complexes and photoluminescence (PL) from GeSiSn/Si multiple quantum wells (MQWs) are studied. The series of PL peaks related to the vacancy-tin complexes was observed for as-grown samples including different structures, such as GeSiSn/Si MQWs, multilayer periodic structure with GeSiSn quantum dots (QDs), GeSn cross-structures upon GeSiSn/Si MQWs, and thick GeSiSn layers. The PL band intensity is significantly reduced after annealing at 700 °C corresponding to the reduction in vacancy density, as demonstrated by the positron annihilation spectroscopy (PAS) data. Such annealing also results in the appearance of the PL signal related to the interband optical transitions in GeSiSn/Si MQWs. However, the high temperature could negatively impact the sharpness of heterointerfaces due to Sn diffusion, thus limiting the PL efficiency. To improve the luminescence properties of GeSiSn/Si structures, we proposed a two-stage technique combining both the annealing and subsequent treatment of samples in a hydrogen plasma at 200 °C. The plasma treatment significantly reduces the PL band of vacancy-related defects, whereas annealing at a moderate temperature of ∼600 °C prevents the blurring of heterointerfaces. As a result, we demonstrate an increase in the relative efficiency of interband PL of type II GeSiSn/Si MQW structures emitting in the range of 1.5–2 μm.

Dielectric function and interband critical points of compressively strained ferroelectric K0.85Na0.15NbO3 thin film with monoclinic and orthorhombic symmetry

The dielectric function and interband critical points of compressively strained ferroelectric K0.85Na0.15NbO3 thin film grown by metal-organic vapor phase epitaxy (MOVPE) are studied in broad spectral and temperature ranges by spectroscopic ellipsometry (SE). The temperature dependence of the measured pseudodielectric functions is strongly affected by a structural phase transition from the monoclinic Mc-phase to the orthorhombic c-phase at about 428 K. Using a parametric optical constant model, the corresponding dielectric functions as well as the interband optical transitions of the film are determined in the spectral range of 0.73–6.00 eV. Standard critical point (SCP) analysis of the 2nd derivatives of the dielectric functions identified three and four critical points for monoclinic and orthorhombic symmetries, respectively. A systematic redshift of the threshold energies with increasing temperatures was observed.

Strain-tuned optical properties of bilayer silicon at midinfrared wavelengths

Optical properties of two-dimensional bilayer silicon have been explored at midinfrared wavelengths using density functional theory. In this work, progressive atomic structural deformation and the resultant variations in the optical properties of the bilayer silicon films were investigated under external in-plane strain. A phase transformation of the atomic structure has been observed at an applied in-plane tensile strain of 5.17%, at which the atomic lattice is changed from a low buckled to a buckle-free honeycomb structure. Evaluations of the optical properties were carried out by taking into account the inter- and intraband transitions. An abrupt change in the optical refraction index was observed at the phase transition. In addition, the buckle-free honeycomb structure presents a strain-resistive absorption edge pinned at 1.14 μm wavelength. Exceeding a strain threshold of 12.26% results in the development of both direct- and indirect-energy bandgap openings. The direct bandgap induced interband optical transitions, resulting in absorption peaks at midinfrared wavelengths and a drastic increase in the refraction index. Moreover, by adjusting the strain, the optical absorptions can be tuned in a wide range of wavelength at midinfrared from 1.5 to 11.5 μm.

Interband second-order nonlinear optical susceptibility of asymmetric coupled quantum wells

Asymmetric molecular bonds possess a microscopic second-order nonlinear optical polarizability p(2). Crystals built from them possess a macroscopic second-order nonlinear optical susceptibility, χ(2), if their structure lacks centrosymmetry. χ(2) can be enhanced by introducing additional asymmetry at the meta-structural level. Here, we use a dipole matrix formalism to calculate χ(2) of asymmetric GaAs/AlGaAs coupled quantum well structures at telecommunication frequencies, for which interband (rather than previously considered intersubband) optical transitions govern optical nonlinearities. Using unit cell and envelope wavefunctions and considering all possible transitions between two bound electron and two bound hole states, we predict tenfold enhancement in χ(2) in previously underexplored ranges of quantum well asymmetry and coupling barrier thickness. This work paves the way toward enhanced, tailorable second-order optical nonlinearities for semiconductor digital alloy and superlattice structures.

Optimized structural, optical, and electrical properties of spin-coated Pt(II) octaethylporphyrin thin films for optoelectronic applications

Octaethylporphyrin (PtOEP) is an organic molecule that has unique optical properties, such as high fluorescence quantum yields, strong light absorption, etc. Here we investigate the structural, optical, and electrical characteristics of spin-coated PtOEP thin film. X-ray diffraction (XRD) patterns indicated an increase in the crystallite of the annealed film. Field emission-scanning electron microscope (FESEM) images clearly showed agglomeration and an increase in the nanoparticle size of the annealed film. The Raman spectra of the thin films revealed symmetrical peak positions and improved peak strength for the annealed film. UV-Vis-NIR and photoluminescence (PL) spectroscopy were used to examine optical characteristics. The findings demonstrated that energy gap and PL peak emission were significantly influenced by the grown crystallite size as well as a decrease in lattice defect concentration at the grain boundary during annealing. The optical constant and interband transition strength spectra (Jcv) were estimated and addressed. Based on the linear refractive index and optical gap, third-order nonlinear susceptibility χ(3), nonlinear refractive index n2, and two-photon absorption coefficient, βc of spin-coated PtOEP thin films were calculated and discussed. Finally, the dark current-voltage (J-V) curves were utilized to assess the diode parameters, specifically the zero-bias barrier height (ΦB) and the ideality factor (n), for the ITO/PtOEP/Al diode using thermionic emission theory. Additionally, an investigation was conducted to examine the conduction mechanism of the ITO/PtOEP/Al diode under both forward and reverse bias voltage conditions. On the basis of these findings, spin-coated PtOEP thin films were concluded to be helpful in producing cutting-edge optoelectronic devices.

Effects of an external electric field on one-phonon resonant and electron Raman scattering in a semiconductor quantum wire

In this work, the influence of an external electric field is studied in two cases: one-phonon resonant Raman scattering and one-phonon electron Raman scattering, processes that occur in a semiconductor quantum wire with cylindrical symmetry and finite potential barriers. Where we have considered that the electric field is homogeneous and transversal to the system axis. To carry out this study, we obtain a mathematical expression for the differential cross-section for both Raman processes, where for one-phonon resonant Raman scattering, intra-band and inter-band optical transitions are considered, while for one-phonon electron Raman scattering, only intra-band optical transitions are considered. Therefore, to determine the electronic states, we use a valid model when the electric field is weak with respect to confinement. In the case of the Fröhlich electron–phonon interaction, we use a model in which the oscillation modes are completely confined, a model that was developed within the framework of a macroscopic continuum model. Then, the singularities present in the Raman spectra and the effect of the electric field on their position and intensity are analyzed. Finally, how the electric field affects the electron–phonon interaction and the selection rules for optical transitions in a semiconductor quantum wire with cylindrical symmetry are shown.

Linear and nonlinear spin current response of anisotropic spin-orbit coupled systems

We calculate the linear and the second harmonic (SH) spin current response of two anisotropic systems with spin–orbit (SO) interaction. General expressions of wide applicability for the these response functions are first derived for a generic two-band Hamiltonian. The first system is a two-dimensional (2D) electron gas in the presence of Rashba and k-linear Dresselhaus SO couplings. The calculations show how narrow or wide the response spectra can be, what is their overall shape and size, and frequency shiftings, depending on which crystal orientation is selected. The quantitative knowing of this makes possible a comparative study for several orientations, which would allow to select a spectrum with particular characteristic. We find that vanishing linear and second order response tensors are achievable under SU(2) symmetry conditions, characterized by a collinear SO vector field. Additional conditions under which specific tensor components vanish are possible, without having such collinearity. Thus, a proper choice of the growth direction and SO strengths allows to select the polarization of the linear and SH spin currents according to the direction of flowing. The second system is an anisotropic 2D free electron gas with anisotropic Rashba interaction, which has been employed to study the optical conductivity of 2D puckered structures with anisotropic energy bands. The presence of mass anisotropy and an energy gap open several distinct scenarios for the allowed optical interband transitions, which manifest in the linear and SH response contrastingly. The linear response displays only out-of-plane spin polarized currents, while the SH spin currents flow with spin orientation lying parallel to the plane of the system strictly. The models illustrate the possibility of the nonlinear spin Hall effect in systems with SO interaction, under the presence or absence of time-reversal symmetry. The results suggest different ways to manipulate the linear and nonlinear optical generation of spin currents which could find spintronic applications.

Optical properties of two-dimensional quantum ring with diluted magnetic semiconductors structure

In this paper, we study the optical properties of a two-dimensional quantum ring with a diluted magnetic semiconductors structure. The Volcano model was chosen as the confinement potential of a quantum system. For interband optical transitions, the behavior of the absorption coefficient of a quantum ring as a function of the incident photon energy is numerically studied as a function of various values of such parameters as temperature, magnetic field, ring radius, and inner ring radius. In addition, the change in the threshold frequency as a function of the magnetic field and temperature for an interband optical transition is numerically studied. According to the results obtained, the considered parameters change the optical properties of the quantum ring.

Strain-tuned optical conductivity of monolayer PbBiI

In this paper, we investigate the optical response of the PbBiI single-layer by developing a strain-induced Kane–Mele model from Peierls substitution and by employing the Kubo formula at low temperatures. We address three different regimes of uniform and non-uniform classes created by tuning the strength of the strain. From a detailed analysis of the electronic band structure, we find that the Rashba spin splitting gap is destroyed with strain, while the bulk gap slightly changes. We also find that interband optical transitions exhibit a blueshift spectrum with strain. Interestingly, all these findings are independent of the regime and class of strain. However, our simulations show that only the non-uniform class of strain leads to anisotropic optical conductivity. These results enhance optoelectronic applications of low-dimensional materials.

Spontaneous (two-pulse) photon echo in the nanoscale structure of self-organized quantum dots

We consider the coherent response of a medium with quasi-zero-dimensional structural units in the form of a photon echo upon resonant selective pulsed excitation of interband optical transitions, the inhomogeneous broadening of which is largely determined by the polydisperse composition of the nanocomposite. The process of formation of a photon echo is analyzed, when optical excitation leads to the formation of exciton complexes by resident electrons, which are localized in nanosized microcrystals in the limiting cases of strong and weak size quantization. By studying the coherent dynamics of the components of the pseudospin electric dipole vector, an expression is derived for the macroscopic specific polarization at the moment of emission of the photon echo signal. The consideration is carried out for a model nanocomposite medium on the example of a dielectric matrix with a system of semiconducting spherical inclusions (spherical quantum dots), the distribution over the radii of which is described by the Lifshitz-Slyozov formula.

One-way light flow by spatio-temporal modulation.

The unidirectional flow of electrons that takes place in a conventional electronic diode has been a cornerstone in the development of the field of electronics. Achieving an equivalent one-way flow for light has been a long-standing problem. While a number of concepts have been suggested recently, attaining a unidirectional flow of light in a two-port system (e.g., a waveguiding configuration) is still challenging. Here, we present what we believe to be a novel approach for breaking reciprocity and achieving one-way flow of light. Taking a nanoplasmonic waveguide as an example, we show that a combination of time-dependent interband optical transitions, when in systems exhibiting a backward wave flow, can yield light transmission strictly in one direction. In our configuration, the energy flow is unidirectional: light is fully reflected in one direction of propagation, and is unperturbed in the other. The concept can find use in a range of applications including communications, smart windows, thermal radiation management, and solar energy harvesting.

Remarkable enhancement of photoluminescence and photoresponse due to photonic crystal structures based on GeSiSn/Si multiple quantum wells

The GeSiSn/Si structures surface with multiple quantum wells (MQWs) was modified by developing photonic crystals (PCs) to increase the photoluminescence and photoresponse near 2 μm. The structural parameters of GeSiSn/Si MQWs obtained by molecular-beam epitaxy were optimized, and PCs were formed. They consisted of a periodic cylindrical hole array. Numerical simulation methods were used to optimize the hole depth, which makes it possible to effectively excite quasi-guided modes and not affect the active layer including GeSiSn/Si MQWs. The increase of the photoluminescence (PL) signal related to vacancy complexes was observed for all samples with the PC structure. The maximum PL amplification of about 5 times at the wavelength near 2 μm was demonstrated. The annealing of PC structures with GeSiSn/Si MQWs resulted in the appearance of PL associated with interband optical transitions. The PL enhancement was observed by almost an order of magnitude in the narrow wavelength range with the PL maximum near 1.8 μm. Based on Ge0.84Si0·076Sn0·084/Si MQWs p-i-n photodiodes with the PC, the photoresponse increase was shown in the wide wavelength range up to 1.8 μm.

Carrier confinement and interband optical transitions in lead chalcogenide quantum wells, nanosheets, and nanoplatelets.

Analytic equation for energy dispersion of electronic states in lead chalcogenide nanosheets is derived within an effective mass model. Selection rules for interband optical transitions are analyzed and expressions for interband optical matrix elements are obtained. It is shown that the main effect of the lateral confinement in nanoplatelets can be accounted for in terms of the quantized in-plane wave vector.

Rheological and optical response of hydroxypropyl methylcellulose under variable temperatures for optical switching based on thermo‐optical effect

AbstractThis work is devoted to investigate novel aspects of the rheological and optical response of hydroxypropyl methylcellulose (HPMC) as a result of temperature variations from 308.15 to 338.15 K. First, new insights on correlation between the solution flow behavior and the film thickness uniformity, under distinct temperatures, are established. Then, the HPMC film is examined from the point of view of some basic surface parameters describing the surface morphology, such as root mean square roughness, surface area ratio, texture direction index, kurtosis, skewness and surface bearing index. The refraction properties of HPMC are reported for the first time as a function of wavelength and temperature. The changes in the optical dispersion parameters as a function of temperature increase reveal the reduction of strength of interband optical transitions from 15.147 to 14.086 eV and a smaller band gap from 6.475 to 6.137 eV. The latter is caused by temperature‐induced dilatation of the polymer lattice and its effect on the electron–lattice interactions. The thermo‐optic coefficient of HPMC is found to be larger than that of other polymers, while its dependence on the wavelength is small, varying from −2.03 × 10−4 K−1 at 489 nm to −2.31 × 10−4 K−1. The resulted properties are corresponding to the demands imposed for materials used in optical switching based on thermo‐optical effect.

The exciton spectrum of the cylindrical quantum dot - quantum ring semiconductor nanostructure in an electric field

In the model of effective masses and rectangular potentials for an electron and a hole, the influence of a uniform electric field on the energy spectrum and wave functions of the exciton and the oscillator strengths of interband quantum transitions in the semiconductor (GaAs/AlxGa1-xAs) quantum dot-quantum ring nanostructure is theoretically investigated. The stationary Schrödinger equations for noninteracting quasiparticles in the presence of an electric field cannot be solved analytically. For their approximate solution, the unknown wave functions are sought in the form of an expansion over the complete set of cylindrically symmetric wave functions, and the electron energy is found by solving the corresponding secular equation. The exciton binding energy is found using perturbation theory.
 The dependences of the energy spectra, the wave functions of an electron, hole, and exciton, and the intensity of interband optical quantum transitions on the magnitude of the electric field strength are analyzed.

Hydrostatic deformation potentials of narrow-gap HgCdTe

Close to the semimetal-to-semiconductor topological phase transition, the band structure of HgCdTe is represented by massive Kane fermions that, in a semirelativistic approach, are characterized by two parameters: rest mass and velocity. Using terahertz magneto-optical spectroscopy, we explore the band structure evolution of HgCdTe films with hydrostatic pressure in the vicinity of the band-gap collapse. By analyzing the energies of interband optical transitions as a function of magnetic field, we have determined the rest mass of Kane fermions at different hydrostatic pressures. The pressure dependence of the rest mass allows us to obtain the hydrostatic deformation potential ${a}_{c}\ensuremath{-}{a}_{v}$ at low temperature, where ${a}_{c}$ and ${a}_{v}$ are the deformation potentials of the conduction and valence bands, respectively.

Optical redshift and blueshift spectra in monolayer β12 -borophene: Inversion symmetry breaking effects

We have calculated the optical conductivity of monolayer ${\ensuremath{\beta}}_{12}$-borophene in the presence of an inversion symmetry breaking (ISB) field. We theoretically formulate such a field to effectively be induced by a substrate through an orbital hybridization effect in the experimental growth process. In the electronic band structure, the effect of the ISB field on the anisotropic dispersion appears as a perturbed manipulation of the energy scale. Different orientations of dispersion of triplet and Dirac fermions in ${\ensuremath{\beta}}_{12}$-borophene cause anisotropic optical interband transitions. Simulated ISB-induced optical conductivity using the Kubo formula produces various optical redshift and blueshift spectra. The longitudinal (Hall) component of optical conductivity presents a pure redshift (blueshift) spectrum with the ISB field, whereas the coexistence of both redshift and blueshift spectra emerge for the transverse component. This paper is a handful for practical applications in two-dimensional-based optoelectronic devices.

Effect of C3v symmetry breaking on the noncentrosymmetric quantum spin Hall insulating phase and optical characteristics of monolayer PbBiI

Due to the rise of applications in several optoelectronic and spintronic platforms, theoretically engineering the propagation of light in low-dimensional systems dealing with both charge and spin degrees of freedom has recently triggered considerable interest. By breaking the ${C}_{3v}$ symmetry through external exchange fields, we engineer the propagation of an incident circularly polarized light in a noncentrosymmetric quantum spin Hall insulator, monolayer PbBiI, that is active in the near-infrared region of the electromagnetic spectrum. The Kubo formalism is ideal for optical properties. We endeavor to thoroughly demonstrate that a critical-breaking regime of three types of fields leads to various anisotropic electronic phases and optical interband transitions. We find various near-to-far infrared shifts for the optical activity of the system when the ${C}_{3v}$ symmetry breaking fields are turned on. This is proven in the ensuing two ways: The spectrum of optical conductivity components and the intensities of scattered/absorbed light. We further find the criteria under which the system is transparent. Finally, depending on the exchange fields, we engineer the eccentricities for the reflected and transmitted lights in monolayer PbBiI to see how the polarization of incident circular light becomes linear and elliptical. The applicability of results in industry and medicine is also briefly discussed.