Heavy-hole Exciton Research Articles

Нетривиальная зависимость спектральных характеристик экситонов в квантовых ямах от мощности резонансного оптического возбуждения

Basic exciton parameters, the energy of exciton transition and the radiative and nonradiative broadenings, are experimentally studied by means of reflectance spectroscopy for a heterostructure with the 14-nm GaAs/AlGaAs quantum well. Particular attention is paid to the nonradiative broadening which is sensitive to optical creation of free carriers and long-lived nonradiative excitons. A sublinear increase of the broadening of the heavy-hole and light-hole exciton resonances is observed when the light-hole exciton resonance is excited with increasing power. A simple model is developed, which allows one to well reproduce the observed dependence.

Theory of single and double electron spin-flip Raman scattering in semiconductor nanoplatelets

A theory of electron spin-flip Raman scattering (SFRS) is presented that describes the Raman spectral signals shifted by both single and twice the electron Zeeman energy under nearly resonant excitation of the heavy hole excitons in semiconductor nanoplatelets. We analyze the spin structure of photoexcited intermediate states, derive compound matrix elements of the spin-flip scattering and obtain polarization properties of the one- and two-electron SFRS common for all the intermediate states. We show that, in the resonant scattering process under consideration, the complexes "exciton plus localized resident electrons" play the role of main intermediate states rather than tightly bound trion states. It is demonstrated that, in addition to the direct photoexcitation (and similar photorecombination) channel, there is another indirect channel contributing to the SFRS process. In the indirect channel, the photohole forms the exciton state with the resident electron removed from the localization site while the photoelectron becomes localized on this site. The theoretical results are compared with recent experimental findings for ensembles of CdSe nanoplatelets.

Exciton-exciton interaction and cascade relaxation of excitons in colloidal CdSe nanoplatelets

The exciton-exciton interaction and phonon-induced cascade relaxation of free electrons and holes determine the dynamic of stationary resonantly excited excitons in CdSe/CdS nanoplatelets. Along with saturation of the heavy-hole and light-hole exciton transitions explained by the phase space filling effect the exciton-exciton interaction increases and starts to play a significant role in excitons dynamic. The lowest four band structure of CdSe/CdS nanoplatelets at the Γ point of Brillouin zone and its modification with CdS shell thickness changing are experimentally investigated for the first time. Bleaching of the two additional to the heavy- and light-hole exciton transitions with the energies higher than the absorbed photons energy can occur at room temperature due to rapid cascade relaxation of free electrons and holes formed as a result of the exciton-exciton interaction. To analyze the recombination and interaction of excitons in the colloidal nanoplatelets under high excitation densities the rate equations describing the exciton-exciton and exciton-electron interactions are applied.

Difference in the behavior of the photon echo of excitons in InGaAs/GaAs quantum wells from the predictions of the model of two-level system ensemble

In this work, we studied the photon echo from heavy-hole excitons in a thin InGaAs/GaAs quantum well. To analyze the results, we used the model of an ensemble of two-level systems. The model allows us to describe the temporal profile and the moment of arrival of the echo signal, as well as the echo amplitude decay with increasing delay between pulses. In addition, excitation-induced dephasing effect was observed, that was beyond the limits of applicability of the model.

The excitons in infinite potential centered multilayered coaxial quantum wire and the magnetic field effects on their properties

Abstract The exciton properties in a multilayered coaxial cylindrical quantum wire layered as A l A s / G a A s / A l x G a 1 − x A s / G a A s / A l x G a 1 − x A s are investigated. The exciton binding energies are obtained as a functions of the thicknesses of the GaAs well wires and the barrier wires formed by A l x G a 1 − x A s keeping the total radius of the structure constant and considering two by two variation of those. The binding energies has only one minimum for each variation and they all have approximate minimum binding energy values for their certain set of thicknesses. The magnetic field effects on the binding energies are also reported. The external magnetic field produces noticeably different effects on the electronic properties of the heavy hole and light hole excitons. The magnetic field can be shifted smaller strengths to obtain the minimum binding energy if the radius of the insulator wire in the center of the structure is increased.

Binding energy of excitons in an infinitely deep spherical quantum dot under intense THz laser field

We study the effects of intense THz laser field on the ground-state binding energy of heavy hole excitons confined in GaAs spherical quantum dots. The calculation is performed using the variational method in the framework of the single band effective mass theory. Our results show that (i) the laser electric field lowers the binding energy for all quantum dot radii, making the exciton clustered near the centre of the dot, (ii) the binding energy is mainly due to the dressed potential making the kinetic part insensitive to the field and (iii) the behaviour of the exciton, under the approximations used, can be modelled by a unique set of plots, depending on the material only via its excitonic units.

The strain, energy band and photoluminescence of GaAs0.92Sb0·08/Al0.3Ga0.7As multiple quantum wells grown on GaAs substrate

GaAsSb based materials have become the promising system for infrared semiconductor lasers and detectors. In this article, the strain, energy band structures and photoluminescence (PL) of GaAs0.92Sb0·08/Al0.3Ga0.7As strained quantum wells (QWs) grown with molecular beam epitaxy are systematically analyzed both theoretically and experimentally. The theoretical results are derived by Kane's model and k⋅p method, and the optical properties of a high-quality GaAs0.92Sb0·08/Al0.3Ga0.7As strained QWs sample are thoroughly investigated by excitation- and temperature-dependent PL measurements. The theoretical results show the strain has significant influence on the band structure of QWs. In experimental part, it is found that the light-hole exciton emission coexists with the heavy-hole exciton line in the temperature range of 50 K–300 K. However, the emission of localized excitons, which is caused by the nonuniformity of component in the GaAsSb well layer, takes over the light-hole exciton emission at lower temperatures (<50 K).

Angular momentum transfer from photon polarization to an electron spin in a gate-defined quantum dot

Gate-defined quantum dots (QDs) are such a highly-tunable quantum system in which single spins can be electrically coupled, manipulated, and measured. However, the spins in gate-defined QDs are lacking its interface to free-space photons. Here, we verify that a circularly-polarized single photon can excite a single electron spin via the transfer of angular momentum, measured using Pauli spin blockade (PSB) in a double QD. We monitor the inter-dot charge tunneling which only occur when the photo-electron spin in one QD is anti-parallel to the electron spin in the other. This allows us to detect single photo-electrons in the spin-up/down basis using PSB. The photon polarization dependence of the excited spin state was finally confirmed for the heavy-hole exciton excitation. The angular momentum transfer observed here is a fundamental step providing a route to instant injection of spins, distributing single spin information, and possibly towards extending quantum communication.

Heavy-hole and light-hole excitons in nonlinear absorption spectra of colloidal nanoplatelets

We study the nonlinear optical response of CdSe/CdS nanoplatelets (NPLs) in the vicinity of heavy hole (hh) and light hole (lh) exciton resonances by means of the two color pump-probe technique. The population of hh and lh excitonic states leads to the decrease of the light absorption at both resonances under resonant excitation of one of them. Increase of the pump power leads to the saturation of the absorption. This effect allows us to estimate the exciton total lifetime.

Magnetoexciton in type-II semiconductor nanocone

We study spectral properties of heavy-hole exciton captured by a type-II InP/GaAs nanocone in the effective mass approximation. Assuming that the effective mass of the hole is essentially larger than of the electron, we solve the two-particle Schrödinger equation by using the adiabatic approximation and the Fourier series expansion method. We analyze the alteration of the averaged spatial separation of the electron and the hole, the period of the Aharonov–Bohm oscillations of the energy levels and the magnetic momenta as functions of the external magnetic field, applied along the symmetry axis. Our theoretical analysis reveals a possibility for controlling the electric polarization in type-II semiconductor structure with captured exciton, by means of the applying of an external magnetic field.

Energy spectrum of excitons in square quantum wells

Energies of the ground and excited states of excitons in GaAs/AlGaAs and InGaAs/GaAs finite square quantum wells (QWs) of various widths are calculated. This is achieved by studying the three-dimensional Schrödinger equation for the exciton in a QW and, in particular, by determining the lower energy boundary of the continuous spectrum of the corresponding differential operator. The eigenvalue problem for the Schrödinger equation is solved numerically by the finite-difference method properly taking into account discontinuities of the material parameters at the interfaces of the QW. The calculated bound states of electron-hole pairs are classified based on the types of their dominant in-plane and quantum-confinement one-dimensional functions of the wave function factorized form. A dependence of energy levels on a QW width as a parameter is thoroughly studied for widths up to 100 nm. The accurate radiative decay rates for calculated s-like exciton states are also obtained. Calculated energy spectra are confronted with the experimental reflectance spectra measured for high-quality InGaAs/GaAs heterostructures with QWs. The ground and, at least, a few excited states of the heavy-hole exciton in QW are identified in the experimental spectra.

Saturation of Exciton Absorption in CdSe/CdS Nanoplatelets under Transient Excitation Conditions

Nonlinear absorption of light by colloidal CdSe nanoplatelets with a CdS shell is investigated under the conditions of transient single-photon generation of excitons. By changing the shell thickness, it is possible to implement either resonant excitation of light-hole and heavy-hole excitons or nonresonant excitation. The saturation of absorption by colloidal solutions of nanoplatelets is observed and attributed to the effect of exciton phase space filling. The difference between the saturation intensities for CdSe nanoplatelets with different CdS shell thicknesses is explained by the details of exciton generation and relaxation processes. An increase in absorption observed at the excitation intensity significantly exceeding the saturation intensity is explained by the processes of exciton–exciton interaction and energy exchange between heavy- and light-hole excitons.

Energy reversal of light- and heavy-hole excitons bound to isoelectronic centers

Isoelectronic centers in semiconductor hosts offer convenient optical access to both heavy- and light-hole states, thus providing powerful resources for the ultrafast control of electron or hole spins. From a detailed analysis of the fine structure of excitons bound to isoelectronic defects of ${C}_{s}$ symmetry formed by two nitrogen impurities in GaAs, we demonstrate that the lowest energy states, normally exhibiting a strongly dominant heavy-hole character, can be reversed to a dominant light-hole character by the strain found in the vicinity of the emitter. This reversal suggests that strain could be used to engineer the orbital composition and ordering of excitonic states, allowing enhanced optical control over spin states.

Luminescence of ZnMnTe/ZnMgTe Heterostructures with Monolayer Manganese Inclusions in ZnTe Quantum Wells and Its Behavior in a Magnetic Field

Light emission from ZnMnTe/ZnMgTe structures with the quantum wells ZnTe containing monolayer manganese inclusions and quantum wells Zn0.45Mn0.15Te were investigated under the different excitation conditions. The heavy-hole exciton σ+ and σ– magnetic components show the unusual behavior concerning their energy shifts and intensities. It is possible to explain the Zeeman splitting of the heavy exciton emission band if the following factors are taken into account. Firstly, ZnMnTe/ZnMgTe quantum well structures are of the type II due to the strains as concerns to the electron and heavy hole. Secondly, the electron and hole magnetic sublevels population is far from equilibrium due to the fast energy transfer from the electron- hole system to the 3d-shells of magnetic ions.

Exciton binding energy in a pyramidal quantum dot

The effects of spatially dependent effective mass, non-parabolicity of the conduction band and dielectric screening function on exciton binding energy in a pyramid-shaped quantum dot of GaAs have been investigated by variational method as a function of base width of the pyramid. We have assumed that the pyramid has a square base with area $$a\times a$$ and height of the pyramid $$H=a/2$$ . The trial wave function of the exciton has been chosen according to the even mirror boundary condition, i.e. the wave function of the exciton at the boundary could be non-zero. The results show that (i) the non-parabolicity of the conduction band affects the light hole (lh) and heavy hole (hh) excitons to be more bound than that with parabolicity of the conduction band, (ii) the dielectric screening function (DSF) affects the lh and hh excitons to be more bound than that without the DSF and (iii) the spatially dependent effective mass (SDEM) affects the lh and hh excitons to be less bound than that without the SDEM. The combined effects of DSF and SDEM on exciton binding energy have also been calculated. The results are compared with those available in the literature.

Magnetospatial dispersion of semiconductor quantum wells

Polarization conversion of light reflected from quantum wells governed by both magnetic field and light propagation direction is observed. We demonstrate that the polarization conversion is caused by the magnetospatial dispersion in quantum wells which manifests itself in the reflection coefficient contribution bilinear in the in-plane components of the magnetic field and the light wave vector. The magnetospatial dispersion is shown to arise due to structure inversion asymmetry of the quantum wells. The effect is resonantly enhanced in the vicinity of the heavy-hole exciton. We show that microscopically the magnetospatial dispersion is caused by the mixing of heavy- and light-hole states in the quantum well due to both orbital effect of the magnetic field and the in-plane hole motion. The degree of the structure inversion asymmetry is determined for GaAs/AlGaAs and CdTe quantum wells.

Distinguishing quantum dot-like localized states from quantum well-like extended states across the exciton emission line in a quantum well

We have closely examined the emission spectrum at the heavy-hole exciton resonance in a high-quality GaAs multi-quantum well sample using picosecond excitation-correlation photoluminescence (ECPL) spectroscopy. Dynamics of the ECPL signal at low and high energy sides of the excitonic photoluminescence (PL) peak show complementary behavior. The ECPL signal is positive (negative) below (above) the PL peak and it changes sign within a narrow band of energy lying between the excitonic absorption and emission peaks. The energy at which this sign change takes place is interpreted as the excitonic mobility edge as it separates localized excitons in quantum dot-like states from mobile excitons in quantum well-like states.

Luminescence of ZnMnTe/ZnMgTe heterostructures with monolayer manganese inclusions in ZnTe quantum wells and its behavior in a magnetic field

AbstractLight emission from ZnMnTe/ZnMgTe structures with the quantum wells ZnTe containing monolayer manganese inclusions and quantum wells Zn_0.45Mn_0.15Te were investigated under the different excitation conditions. The heavy-hole exciton σ^+ and σ^– magnetic components show the unusual behavior concerning their energy shifts and intensities. It is possible to explain the Zeeman splitting of the heavy exciton emission band if the following factors are taken into account. Firstly, ZnMnTe/ZnMgTe quantum well structures are of the type II due to the strains as concerns to the electron and heavy hole. Secondly, the electron and hole magnetic sublevels population is far from equilibrium due to the fast energy transfer from the electron- hole system to the 3 d -shells of magnetic ions.

Exciton binding energies in CdSe/MgSe quantum well structures

Binding energies of heavy- and light-hole excitons in CdSe/MgSe quantum wells have been determined in terms of quantum well width. Hydrogenic wave functions are assumed to represent exciton wave functions. Contribution of the polarization charges originating from the dielectric mismatch between the well and the barrier materials to the Coulomb interaction is calculated by means of image charge method. The effect of the exciton-phonon interaction on the exciton binding energies are included by means of an effective potential obtained through an exciton-bulk-optical-phonon Hamiltonian. The results indicate that both the modified Coulomb interaction due to the dielectric mismatch and the exciton-phonon interaction considerably change the exciton binding energy. The heavy-hole exciton binding energy in a 10-Å quantum well is seen to be enhanced as much as 87% compared to the value found when only the Coulomb interaction without dielectric mismatch is taken into account. The resulting binding energies of heavy-hole excitons range approximately between 57 and 41 meV for the quantum well width changing between 10 and 35 Å. Compared to the bulk CdSe exciton binding energy 15 meV, the exciton binding energies in CdSe/MgSe quantum wells are increased almost four times.

Fractal dimension method for exciton in cylindrical GaAs/AlxGa1−xAs core-shell-cap nanowires

By use of the fractal dimension method, the binding energies of heavy-hole exciton and light-hole exciton in cylindrical GaAs/Al[Formula: see text]Ga[Formula: see text]As core-shell-cap nanowire are explored. In this study, the exciton is confined in GaAs shell of the GaAs/Al[Formula: see text]Ga[Formula: see text]As core-shell-cap nanowire for a given aluminum concentration of [Formula: see text][Formula: see text]=[Formula: see text]0.3. The numerical results of heavy-hole exciton binding energy, light-hole exciton binding energy and fractal dimension parameter are worked out as functions of shell width and core radius. It has been shown by the calculated results that heavy-hole exciton binding energy and light-hole exciton binding energy firstly increase and then decrease as the shell width increases. When the core radius increases, both the heavy-hole exciton binding energy and light-hole exciton binding energy increase gradually. Exciton problems in GaAs shell of the cylindrical GaAs/Al[Formula: see text]Ga[Formula: see text]As core-shell-cap nanowire are solved in a simple manner to avoid complex and lengthy calculations by using the fractal dimension method.