C. Testelin

Hole–spin dephasing time associated with hyperfine interaction in quantum dots

The spin interaction of a hole confined in a quantum dot with the surrounding nuclei is described in terms of an effective magnetic field. We show that, in contrast to the Fermi contact hyperfine interaction for conduction electrons, the dipole-dipole hyperfine interaction is anisotropic for a hole, for both pure or mixed hole states. We evaluate the coupling constants of the hole-nuclear interaction and demonstrate that they are only 1 order of magnitude smaller than the coupling constants of the electron-nuclear interaction. We also study, theoretically, the hole-spin dephasing of an ensemble of quantum dots via the hyperfine interaction in the framework of frozen fluctuations of the nuclear field, in the absence or in the presence of an applied magnetic field. We also discuss experiments which could evidence the dipole-dipole hyperfine interaction and give information on hole mixing.

Anisotropic spin splitting of the electron ground state in InAs quantum dots

Photoinduced circular dichroism experiments in an oblique magnetic field allow measurements of Larmor precession frequencies, and so give a precise determination of the electron Lande g factor and its anisotropy in self-assembled InAs∕GaAs quantum dots emitting at 1.32eV. In good agreement with recent theoretical results, the authors measure ∣g⊥∣=0.397±0.003 and ∣g‖∣=0.18±0.02.

Interdiffusion in annealed CdMnTe/CdTe/CdMgTe quantum wells studied by the Zeeman effect

Thermally induced interdiffusion in asymmetric CdMnTe/CdTe/CdMgTe quantum wells was investigated by a spin-tracing method based on the exciton Zeeman splitting. From numerical simulations of confined exciton energies in a magnetic field the diffusion lengths for all investigated samples were obtained. The results indicate a strain relaxation during the RTP process. The difference between normal and inverted profiles is also discussed.

Optical pumping and reversal of hole spin in InAs/GaAs quantum dots

We have obtained the optical pumping of hole spins, in p-doped InAs/GaAs quantum dots, via the generation of an intermediate trion state by a train of circularly polarized pulses. We show that we can optically control the orientation of the initialized hole spin, independently of the orientation of the intermediate trion state, by choosing the excitation energy of the circularly polarized light. This brings a supplementary degree of freedom for hole-spin manipulations in quantum dots.

Theoretical Model and Experimental Study of Effects of Rapid Thermal Annealing on Self-assembled In(Ga)As/GaAs Quantum Dots

This paper presents a study of rapid thermal annealing (RTA) effects on self-assembled In(Ga)As/GaAs quantum dots (QDs) using Raman scattering and photoluminescence (PL) spectroscopy. The PL dependence on excitation shows the existence of two size distributions of QDs (small and large) after annealing. However, Raman scattering shows one longitudinal optical (LO) phonon peak of the QD. We have presented two simple models (Raman scattering model (RSM) and PL model) to determine both composition and size of QDs. First, in the RSM, we have taken into account the strain effects. Second, in the PL model, we have assumed a conical QD shape, and we have taken into account the electron-hole confinement, strain effect, and the electron-hole Coulombic interaction. A Gaussian distribution has been introduced to describe the random distribution of indium content for different QDs. We initiated our study with the simulation of the Raman spectra by the RSM to determine the value of indium content of In xGa 1-xAs QDs before and after annealing. We have introduced this value into the PL model to determine the QD size. Finally, we found a weak FWHM of composition distribution that shows a homogeneous indium composition in different QDs in both as-grown and annealed QDs.

Fine structure of excitons and electron–hole exchange energy in polymorphic CsPbBr3 single nanocrystals

All inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) belong to the novel class of confined metal-halide perovskites which are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to outstanding properties. A deep knowledge of the band-edge excitonic properties of these materials is thus crucial to further optimize their performances. Here, high-resolution photoluminescence (PL) spectroscopy of single bromide-based NCs reveals the exciton fine structure in the form of sharp peaks that are linearly polarized and grouped in doublets or triplets, which directly mirror the adopted crystalline structure, tetragonal (D4h symmetry) or orthorhombic (D2h symmetry). Intelligible equations are found that show how the fundamental parameters (spin-orbit coupling, ΔSO, crystal field term, T, and electron-hole exchange energy, J) rule the energy spacings in doublets and triplets. From experimental data, fine estimations of each parameter are obtained. The analysis of the absorption spectra of an ensemble of NCs with a quasi-bulk behavior leads to ΔSO = 1.20 ± 0.06 eV and T = -0.34 ± 0.05 eV in CsPbBr3. The study of individual luminescence responses of NCs having sizes comparable to the exciton Bohr diameter, 7 nm, allows us to estimate the value of J to be around ≈3 meV in both tetragonal and orthorhombic phases. This value is already enhanced by confinement.

Excitonic complexes in strain-free and highly symmetric GaAs quantum dots fabricated by filling of self-assembled nanoholes

We perform a theoretical study of the optical transitions for different excitonic complexes in highly symmetric strain-free GaAs quantum dots (QDs) fabricated by epitaxially filling nanoholes (NHs) in an AlGaAs surface. NHs are formed by local droplet etching. As a first step, we propose a QD shape modeling consistent with atomic force microscopy (AFM) profiles and an experimental growth procedure. We investigate the QD height dependence of s- and p- shell exciton recombination energies in the framework of the effective mass approximation with an exact numerical diagonalization method. A comparison between theoretical results and available spectroscopic data is carried out. Systematic evolution of the binding energies of neutral (X), charged excitons (X–, X+) and the biexciton (XX), with QD height, is interpreted in terms of a balance between the Coulomb interactions and charge carrier correlation effects. Our calculations demonstrate the important role of the correlation energies in elucidating the bound...

Optical properties of type-II AlInAs/AlGaAs quantum dots by photoluminescence studies

We report photoluminescence (PL) characterization and model simulation of AlInAs/AlGaAs type-II quantum dots (QDs). A thorough and precise determination of the band parameters for QD and matrix materials is given, focusing on the effects of alloy composition and strain state on the electronic properties. Origins of experimentally observed PL emission peaks are identified through a comparison with the band lineup theoretically determined in this work. We interpret the QD emission as originating from indirect type-II transitions involving electrons in the barrier X valley and heavy holes with S and P symmetry.

The role of heavy-light-hole mixing on the optical initialization of hole spin in InAs quantum dots.

The initialization of a resident hole spin by the absorption of a circularly polarized light at resonance involves the formation of an excited state called a trion state. For a pure heavy hole, this optical initialization is mediated by the hyperfine electron-nuclear coupling in the trion state. We show here that for a mixed-hole spin an additional mechanism for the optical initialization appears, associated to crossed transitions; it becomes dominant and keeps a high level of hole spin polarization when the magnetic field screens the electron-nuclear interaction. Finally, using a simple model, we obtain a good theoretical agreement with pulsed pump-probe experiments.

Low field excitonic Zeeman splittings in gallium nitride

Abstract Reflectivity and Kerr rotation were measured on a GaN layer grown by MOVPE on sapphire substrate. Fine excitonic structures were observed. Energies of A, B, and C free excitons were determined, as well as those of 2s excited states of A and B excitons, thus the exciton binding energies were obtained for A and B excitons, in reasonable agreement with previously reported data. Very small excitonic Zeeman splittings (down to 80xa0μeV) were determined from the analysis of the Kerr rotation data. The splittings, although different from those obtained at high fields on homoepitaxial GaN layers, can be described using an excitonic model with the same band parameters.