Excitonic Interband Research Articles (Page 1)

It has been found that hetero layers of typical β-ZnSe and atypical α-ZnSe modifications can be obtained by the isovalent substitution method. Isovalent impurities are formed which predetermine the formation of dominant radiation with a quantum yield of η = 12–15% in the short wavelength edge region. Low-temperature studies and λ-modulation techniques allowed us to identify the radiation components. This radiation is generated by interband recombination and exciton annihilation. The high temperature stability of the radiation was confirmed over temperature variations including 77, 300, and 480 K.

Exciton states and interband absorption spectra in quantum wells using a variationally optimized diagonalization method

In this article, we study electron dynamics in HgTe quantum dots with a 1.9 μm gap, a material relevant for infrared sensing and emission, using ultrafast spectroscopy with 35 fs time resolution. Experiments have been carried out at several probing photon energies around the gap, which allows us to follow the relaxation path of the photoexcited electrons. We compare such dynamics in two kind of samples, HgTe quantum dots with long ligands and with short ligands, in order to distinguish the role of the coupling between adjacent quantum dots. Three main dynamics can be observed in the transient reflectivity on both samples, with slightly different relaxation times: two fast decays on the time scale of hundreds of femtoseconds and a few picoseconds, respectively, followed by a slower relaxation back to the unperturbed value over hundreds of picoseconds. The two fast components are associated with intraband relaxation of the photoexcited electrons within the conduction band, while the final relaxation path can be assigned to Auger relaxation mechanisms and to the slower interband exciton recombination.

We present time-resolved Kerr rotation (TRKR) spectra in thin films of CH3NH3PbI3 (MAPI) hybrid perovskite using a unique picosecond microscopy technique at 4 K having a spatial resolution of 2 μm and temporal resolution of 1 ps, subjected to both an in-plane applied magnetic field up to 700 mT and an electric field up to 104 V/cm. We demonstrate that the obtained TRKR dynamics and spectra are substantially inhomogeneous across the MAPI films with prominent resonances at the exciton energy and interband transition of this compound. From the obtained quantum beating response as a function of magnetic field in the Voigt configuration, we also extract the inhomogeneity of the electron and hole Lande g-values and spin coherence time, T2*. We also report the TRKR dependence on both the applied magnetic field and electric field. From the change in the quantum beating dynamics, we found that T2* substantially decreases upon the application of an electric field. At the same time, from the induced spatial TRKR changes, we show that the electric field induced effects are caused by ion migration in the MAPI films.

Optical force exerted on the two dimensional transition-metal dichalcogenide coated dielectric particle by Gaussian beam

Influence of external fields on the exciton binding energy and interband absorption in a double inverse parabolic quantum well

In this study, we investigated the exciton binding energy and interband transition between the electron and heavy-hole for the single and double quantum wells which have different hyperbolic-type potential functions subject to electric, magnetic, and non-resonant intense laser fields. The results obtained show that the geometric shapes of the structure and the applied external fields are very effective on the electronic and optical properties. In the absence of the external fields, the exciton binding energy is a decreasing function of increasing well sizes except for the strong confinement regime. Therefore, for all applied external fields, the increase in the well widths produces a red-shift at the absorption peak positions. The magnetic field causes an increase in the exciton binding energy and provides a blue-shift of the absorption peak positions corresponding to interband transitions. The effect of the electric field is quite pronounced in the weak confinement regime, it causes localization in opposite directions of the quantum wells of the electron and hole, thereby weakening the Coulomb interaction between them, causing a decrease in exciton binding energy, and a red-shift of the peak positions corresponding to the interband transitions. Generally, an intense laser field causes a decrease in the exciton binding energy and produces a red-shift of the peak positions corresponding to interband transitions.

A large size single crystal growth, scientific evaluation, and giant Faraday effect of cadmium manganese telluride

Interband transitions and exciton binding energy in a Razavy quantum well: effects of external fields and Razavy potential parameters

Two-dimensional (2D) Sn-based perovskites are a kind of non-toxic environment-friendly luminescent material. However, the research on the luminescence mechanism of this type of perovskite is still very controversial, which greatly limits the further improvement and application of the luminescence performance. At present, the focus of controversy is defects and phonon scattering rates. In this work, we combine the organic cation control engineering with temperature-dependent transient absorption spectroscopy to systematically study the interband exciton relaxation pathways in layered A2SnI4 (A = PEA+, BA+, HA+, and OA+) structures. It is revealed that exciton-phonon scattering and exciton-defect scattering have different effects on exciton relaxation. Our study further confirms that the deformation potential scattering by charged defects, not by the non-polar optical phonons, dominates the excitons interband relaxation, which is largely different from the Pb-based perovskites. These results enhance the understanding of the origin of the non-radiative pathway in Sn-based perovskite materials.

The exciton properties of (Cd,Mn)Se-NrGO (nitrogen doped reduced graphene oxide) hybrid layered nanosheets have been studied in a magnetic field up to 10 T and compared to those of (Cd,Mn)Se nanosheets. The temperature dependent photoluminescence reveals the hybridization of inter-band exciton and intra-center Mn transition with enhancement of the binding energy of exciton-Mn hybridized state (80 meV with respect to 60 meV in (Cd,Mn)Se nanosheets) and increase of exciton—phonon coupling strength to 90 meV (with respect to 55 meV in (Cd,Mn)Se nanosheets). The circularly polarized magneto—photoluminescence at 2 K provides evidence for magnetic field induced exciton spin polarization and the realization of excitonic giant Zeeman splitting with g eff as high as 165.4 ± 10.3, much larger than in the case of (Cd,Mn)Se nanosheets (63.9 ± 6.6), promising for implementation in spin active semiconductor devices.

ABSTRACT The temperature, hydrostatic pressure, and external electric field effects on the confined exciton in cylindrical quantum dots by considering a parabolic confining potential are investigated. The effects of these external perturbations on the binding energy and interband emission energy are calculated numerically by adopting the variational method within the effective mass approximation. Our findings indicate that the exciton binding energy and interband emission energy depend significantly on decreasing electric field, diminishing temperature, and enhancing the hydrostatic pressure. The contribution of the electric field on the binding energy becomes more important for wide axial parabolic quantum well. We have also shown that the Stark effect of exciton diminishes almost linearly with increasing hydrostatic pressure. Furthermore, the electric field and the quantum dot height lead to enhancing the Stark shift. The behaviour of the excitonic Stark shift as a function of the applied electric field proves the existence of a dipole moment. This physical parameter becomes important for the weak confinement regime.

Layered van der Waals (vdW) magnetic materials have attracted significant research interest to date. In this work, we employ the first-principles many-body perturbation theory to calculate excited-state properties of a prototype vdW magnet, chromium trichloride (CrCl3), covering monolayer, bilayer, and bulk structures. Unlike usual non-magnetic vdW semiconductors, in which many-electron interactions and excited states are sensitive to dimensionality, many-electron interactions are always enhanced and dominate quasiparticle energies and optical responses of both two-dimensional and bulk CrCl3. The electron-hole (e-h) binding energy can reach 3 eV in monolayer and remains as high as 2 eV in bulk. Because of the cancellation effect between self-energy corrections and e-h binding energies, the lowest-energy exciton (optical gap) is almost not affected by the change of dimensionality. Besides, for the excitons with similar e-h binding energies, their dipole oscillator strength can differ by a few orders of magnitude.Our analysis shows that such a big difference is from a unique interference effect between complex exciton wavefunctions and interband transitions. Finally, we find that the interlayer stacking sequence and magnetic coupling barely change quasiparticle band gaps and optical absorption spectra of CrCl3. Our calculated low-energy exciton peak positions agree with available measurements. These findings give insight into the understanding of many-electron interactions and the interplay between magnetic orders and optical excitations in vdW magnetic materials.

The optical absorption of exciton interstate transition in Zn1 − xlMgxlO/ZnO/Zn1 − xcMgxcO/ZnO/Zn1 − xrMgxrO asymmetric double quantum wells (ADQWs) with mixed phases of zinc-blende and wurtzite in Zn1 − xMgxO for 0.37 < x < 0.62 is discussed. The mixed phases are taken into account by our weight model of fitting. The states of excitons are obtained by a finite difference method and a variational procedure in consideration of built-in electric fields (BEFs) and the Hartree potential. The optical absorption coefficients (OACs) of exciton interstate transition are obtained by the density matrix method. The results show that Hartree potential bends the conduction and valence bands, whereas a BEF tilts the bands and the combined effect enforces electrons and holes to approach the opposite interfaces to decrease the Coulomb interaction effects between electrons and holes. Furthermore, the OACs indicate a transformation between direct and indirect excitons in zinc-blende ADQWs due to the quantum confinement effects. There are two kinds of peaks corresponding to wurtzite and zinc-blende structures respectively, and the OACs merge together under some special conditions. The computed result of exciton interband emission energy agrees well with a previous experiment. Our conclusions are helpful for further relative theoretical studies, experiments, and design of devices consisting of these quantum well structures.

In monolayer transition metal dichalcogenides helicity-dependent charge and spin photocurrents can emerge, even without applying any electrical bias, due to circular photogalvanic and photon drag effects. Exploiting such circular photocurrents (CPCs) in devices, however, requires better understanding of their behavior and physical origin. Here, we present symmetry, spectral, and electrical characteristics of CPC from excitonic interband transitions in a MoSe2 monolayer. The dependence on bias and gate voltages reveals two different CPC contributions, dominant at different voltages and with different dependence on illumination wavelength and incidence angles. We theoretically analyze symmetry requirements for effects that can yield CPC and compare these with the observed angular dependence and symmetries that occur for our device geometry. This reveals that the observed CPC effects require a reduced device symmetry, and that effects due to Berry curvature of the electronic states do not give a significant contribution.

The influence of the pressure transmission medium (PTM) on the excitonic interband transitions in monolayer tungsten diselenide (WSe2) is investigated using photoluminescence (PL) spectra under hydrostatic pressure up to 5GPa. Three kinds of PTMs, condensed argon (Ar), 1:1 n-pentane and isopentane mixture (PM), and 4:1 methanol and ethanol mixture (MEM, a PTM with polarity), are used. It is found that when either Ar or PM is used as the PTM, the PL peak of exciton related to the direct K–K interband transition shows a pressure-induced blue-shift at a rate of 32±4 or 32±1 meV/GPa, while it turns to be 50±9 meV/GPa when MEM is used as the PTM. The indirect Λ–K interband transition presents almost no shift with increasing pressure up to approximately 5GPa when Ar and PM are used as the PTM, while it shows a red-shift at the rate of −17±7 meV/GPa by using MEM as the PTM. These results reveal that the optical interband transitions of monolayer WSe2 are very sensitive to the polarity of the PTM. The anomalous pressure coefficient obtained using the polar PTM of MEM is ascribed to the existence of hydrogen-like bonds between hydroxyl in MEM and Se atoms under hydrostatic pressure.

The polarized excitons and optical activity of single-wall carbon nanotubes (SWNTs) are studied theoretically by $\ensuremath{\pi}$-electron Hamiltonian and helical-rotational symmetry. By taking advantage of the symmetrization, the single-particle energy and properties of a SWNT are characterized with the corresponding helical band structure. The dipole-moment matrix elements, magnetic-moment matrix elements, and the selection rules can also be derived. Based on different selection rules, the optical transitions can be assigned as the parallel-polarized, left-handed circularly-polarized, and right-handed circularly-polarized transitions, where the combination of the last two gives the cross-polarized transition. The absorption and circular dichroism (CD) spectra are simulated by exciton calculation. The calculated results are well comparable with the reported measurements. Built on the foundation, magnetic-field effects on the polarized excitons and optical activity of SWNTs are studied. Dark-bright exciton splitting and interband Faraday effect in the CD spectrum of SWNTs under an axial magnetic field are predicted. The Faraday rotation dispersion can be analyzed according to the selection rules of circular polarizations and the helical band structure.

We investigated the interaction between an intense terahertz (THz) pulse and excitons in bulk GaAs by using THz pump near-infrared (NIR) optical probe spectroscopy. We observed a clear spectral oscillation in the NIR transient absorption spectra at low temperature, which is interpreted as the THz pump-induced perturbed free induction decay (PFID) of the excitonic interband polarization. We performed a numerical simulation based on a microscopic theory and identified that the observed PFID signal originates from the THz field-induced ionization of excitons. Using a real-space representation of the excitonic wave function, we visualized how the ionization of an exciton proceeds under the intense single-cycle THz electric field. We also calculated the nonlinear susceptibility with the lowest-order perturbation theory assuming a weak THz pump, which showed a similar spectral feature with that obtained by the full treatment to field-induced ionization process. This coincidence is attributed to the fact that 1s-excitonic interband polarization is modified predominantly through interactions with the p-wave component of the excitonic wave function. A simple phenomenological expression of the PFID signal is presented to discuss effects of the THz pump pulse duration on the spectral oscillation.

A technique for modifying the saturable absorption (SA) properties of 2D nanofilms and a pulse width-dependent theoretical model of SA considering interband exciton recombination have been successfully demonstrated.

We dynamically modulate strong light–matter coupling in a GaAs/AlGaAs microcavity using intense ultrashort laser pulses tuned below the interband exciton energy, which induce a transient Stark shift of the cavity polaritons. For 225-fs pulses, shorter than the cavity Rabi cycle period of 1000 fs, this shift decouples excitons and cavity photons for the duration of the pulse, interrupting the periodic energy exchange between photonic and electronic states. For 1500-fs pulses, longer than the Rabi cycle period, however, the Stark shift does not affect the strong coupling. The two regimes are marked by distinctly different line shapes in ultrafast reflectivity measurements—regardless of the Stark field intensity. The crossover marks the transition from adiabatic to diabatic switching of strong light–matter coupling.