S. Jaziri

Excitons in parabolic quantum dots in electric and magnetic fields

We report a perturbative-variational calculation on the effect of magnetic and electric fields on excitons in parabolic quantum dots. The effects of electric and magnetic fields on 1 s exciton energy and oscillator strength are investigated. The competition between the confinement and correlation effects on the one hand, and the external field effects (electric and magnetic) on the other hand, is also discussed.

Effects of electric and magnetic fields on excitons in quantum dots

Abstract We have developed a perturvative-variational method to investigate the effects of electric and magnetic fields on excitons in parabolic quantum dots. The application of an electric field induces a quantum confined Stark effect, alters the exciton binding energy and oscillator strength. The application of a magnetic field induces a diamagnetic shift, enhances the exciton binding energy and oscillator strength. The effects of the external fields are discussed for different size of quantum dots.

Quasi-bound states and continuum absorption background of polar Al0.5Ga0.5N/GaN quantum dots

A theoretical interpretation of the photoluminescence excitation spectra of self-organized polar GaN/(Al,Ga)N quantum dots is presented. A numerical method assuming a realistic shape of the dots and including the built-in electric field effects is developed to calculate their energy structure and hence their optical absorption. The electron and hole spectra show the existence of a set of quasi-bound states that does not originate from the wetting layer and plays a crucial role in the observed absorption spectrum of the GaN/(Al,Ga)N dots. Transitions involving these quasi-bound states and wetting layer states give a sufficient explanation for the observed continuum absorption background. The properties of this absorption band, especially its extension, depend strongly on the dots size. Our simulation provides a natural explanation of the experimental luminescence excitation spectra of ensembles of dots of different heights. Our theoretical model can be convenient for future optical studies including syste...

Electric field effect on hybrid exciton states in organic-inorganic quantum wells

Abstract A new type of excitonic state is considered in a composite organic-inorganic semiconductor nanostructure. Our interest is focused on the mixed exciton formation at the interface in organic (anthracene)-inorganic ( ZnSe ZnCdSe ) quantum wells, in electric fields. The electric field induces spatial separation of electrons and holes in the inorganic quantum wells, leading to a reduction of their binding energy and a shift of the resonance region where Frenkel and Wannier-Mott excitons are strongly mixed. In strong fields, a quadratic Stark shift is found. An important effect is the possibility of using the Stark shift to tune the resonance between Frenkel and Wannier excitons. This fact is very important from a practical point of view because it may be difficult to grow such a structure exactly at resonance.

Confined exciton-polaritons in parabolic and Gaussian quantum wells

In order to study the confined polaritons in gradual quantum structures we have calculated the exciton binding energies of parabolic and Gaussian quantum wells using a variational perturbative approach. In the linear regime, we have established, by means of the semi-classical theory, the dispersion modes of a polariton confined in quantum wells, where the confining potentials are parabolic and Gaussian. In fact, we have evaluated the oscillator strength of each quantum well in order to get an idea about the confinement size and we have concluded that the polariton is more confined in a Gaussian quantum well than in a parabolic quantum well. To make a complete description of the confined polariton we have taken into account the electron - hole exchange energy. Numerical results show that the largest effect arising from the exciton - radiation interaction is the ZT and LT splittings, which strongly depend on the quantum confinement.

EXCITONIC STATES OF WEAKLY CONFINING QUANTUM WIRES

We consider theoretically the excitonic levels of weakly one-dimensional quantum wires in the regime where the center of mass of the exciton is confined by the wire potentials for the electron and hole. A general frame based on the effective-mass approximation is presented and the case of harmonic profiles for the electron and hole confining potentials is considered in detail. The breaking of translation symmetry by the wire potentials introduces a series of transitions between the 1S and 2S excitons of the underlying isolated well, which lead to an intrinsic high-energy tail for the wire absorption. We evaluate also the exciton scattering by longitudinal acoustical phonons and by alloy disorder in Ga1−xInxAs-based systems.

Centre-of-mass quantization of excitons in GaAs quantum boxes

By using a variational-perturbative method, the authors have studied the excitons confined in wide quantum boxes, i.e. the size quantization of the exciton centre-of-mass. The exciton energies are significantly shifted to high energy. The oscillator strength is enhanced with decreasing box size.

Frenkel‐Wannier‐Mott Exciton States in Organic–Inorganic Semiconductor Quantum Wells Subjected to a Magnetic Field

The optical properties of nanostructures using composite organic–inorganic semiconductors, are dominated by a new type of excitonic states. These new hybrid excitations can be described as Frenkel-Wannier-Mott excitons. Frenkel excitons have very strong oscillator strength while Wannier-Mott excitons are very sensitive to external perturbations: static electric and magnetic fields. Our interest is centred on mixed exciton formation under magnetic field effects; calculations were performed for a system composed of a monolayer of organic semiconductor (anthracene) weakly adsorbed at a single parabolic quantum well of inorganic semiconductor (ZnSe/ZnCdSe). The application of a magnetic field leads to an additional confinement. With the transverse magnetic field, a changeover of the characteristic length resulting from inorganic well width and the cyclotron length is obtained from the application of the magnetic field. The lower states of the dispersion law for hybrid Frenkel-Wannier exciton possess a minimum near the center of the Brillouin zone. It is deepest with increase in the applied magnetic field.

Measure of Entanglement States of Two Interacting Electrons in Vertically Coupled Quantum Dots Induced by a Time-Dependent Electric Field

We study the dynamics of two electrons located in two vertically tunnel-coupled quantum dots in the presence of an oscillatory electric field. By solving the time-dependent Schrödinger equation, we predict the dynamical generation of entangled electron states, such as the EPR (Einstein, Podolsky, and Rosen) pairs or Bell states. The Schmidt rank and the von Neumann entropy are evaluated to characterize the degree of entanglement of the two electron states.

Electronic coupling of vertically coupled quantum dots: effect of the time-dependent magnetic field

Abstract The fast development of the new classes nanostructures such as self-organized quantum dots leads to the possibility of controlling their shape, their dimensions, the structure of energy levels and the number of confined electrons (from one to 1000). A fascinating idea of great promise is the possibility of utilizing electronic spins in semiconductor quantum dots as quantum bits (qubits) for a new generation of computers. In this work, we consider two vertically Ga 0.68 In 0.32 As coupled-quantum dots embedded in GaAs. We study the dynamical response of two electrons in these two vertically tunnel-coupled quantum dots with a time dependent magnetic field and including the Coulomb interaction between the electrons. We find that, for a certain frequency ranges, a sinusoidal perturbation acts like an attractive effective interaction between electrons. Including the Zeeman splitting on the spins of the electrons we discuss the dynamical evolution of the magnetization.