We report on the optical spectroscopy of a single InAs/GaAs quantum dot (QD) doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A 0 whose effective spin J = 1 is significantly perturbed by the QD potential and its associated strain field. The spin interaction with photo-carriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of µeV, but vanishingly small for electrons.PACS numbers: 71.35. Pq, 78.67.Hc,75.75.+a,78.55.Cr The spin state of a single magnetic impurity could be envisaged as a primary building block of a nanoscopic spin-based device [1,2] in particular for the realization of quantum bits [3]. However probing and manipulating such a system require extremely high sensitivity. Several techniques have been successfully developed over the last few years to address a single or few coupled spins: electrical detection [4,5], scanning tunneling microscopy (STM) [6,7,8,9], magnetic resonance force microscopic [10], optical spectroscopy [11]. Recently, Besombes et al. [12,13,14,15] have investigated the spin state of a single Mn +2 ion embedded in a single II-VI self-assembled quantum dot (QD). In this system the magnetic impurity is an isoelectronic center in a 3d 5 configuration with spin S = 5/2. The large exchange interaction between the spin of the photocreated carriers confined inside the dot and the Mn magnetic moment induces strong modifications of the QD photoluminescence (PL) spectrum: 2S + 1 = 6 discrete lines are observed, reflecting the Mn spin state at the instant when the exciton recombines.The case of the Mn ion is different in GaAs, since the impurity is an acceptor in this matrix with a rather large activation energy (113 meV). Two types of Mn centers exist in GaAs, the A 0 and the A − states. In low doped GaAs (below 10 19 cm −2 ), the former is dominant. It corresponds to the 3d 5 + h configuration, where h is a hole bound to the Mn ion with a Bohr radius around 1 nm [16]. When considering a single Mn impurity in InAs QD several issues arise: the impurity configuration, its possible change when photo-carriers are captured, the influence on the binding energy of excitonic complexes, the strength and sign of the effective exchange interaction with each of the carriers (electron or hole) in the QD S-shell. In this Letter, we report the first evidences of a single Mn impurity in an individual InAs QD which enable us to answer most of the above questions. In particular, we find that the formation of excitons, biexciton and trions is weakly perturbed by the impurity center, whereas the effective exchange coupling with the Mn impurity (found in the A 0 configuration) is ferromagnetic for holes (a few 100 µeV's) and almost zero for the electrons.The sample was grown by molecular beam epitaxy on a semi-insulating GaAs [001] substrate. The Mn-doped quantum layer was embedded inbetween an electron reservoir and a Schottky gate. This des...

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International audienceWe use optical orientation technique to monitor the degeneracy control of exciton states in a single InAs/GaAs quantum dot, achieved by applying an in-plane electric field. Under circularly polarized quasiresonant excitation, the exciton photoluminescence shows a pronounced maximum of circular polarization at electric field corresponding to zero fine structure splitting. By analyzing the width of this maximum we are able to determine the homogeneous linewidth of the excitonic transition. This experimental method is shown to be very efficient to test and possibly tune the photonic properties of an individual quantum dot for the emission of entangled photon pairs. (C) 2007 American Institute of Physics

International audienceWe present studies on Coulomb interactions in single self-assembled CdTe quantum dots. We use a field effect structure to tune the charge state of the dot and investigate the impact of the charge state on carrier wave functions. The analysis of the quantum confined Stark shifts of four excitonic complexes allows us to conclude that the hole wave function is softer than electron wave function; i.e., it is subject to stronger modifications upon changing of the dot charge state. These conclusions are corroborated by time-resolved photoluminescence studies of recombination lifetimes of different excitonic complexes. We find that the lifetimes are notably shorter than expected for strong confinement and result from a relatively shallow potential in the valence band. This weak confinement facilitates strong hole wave function redistributions. We analyze spectroscopic shifts of the observed excitonic complexes and find the same sequence of transitions for all studied dots. We conclude that the universality of spectroscopic shifts is due to the role of Coulomb correlations stemming from strong configuration mixing in the valence band

We present results of polarization-resolved micro-photoluminescence (µ-PL) experiments on selforganized InAs/GaAs (QDs). We have investigated how Coulomb interactions are modified by application of an electric field. We observed systematic variation of bright exciton splitting for different electric field configurations (applied in the quantum dot plane or along the sample growth axis). The results show also dependence of direct Coulomb interactions of carriers inside the dots, manifested by changes of PL line position of charged excitons (X + and X -) with respect to the neutral excitonic line.1 Introduction In self-assembled quantum dots (QDs) the Coulomb interaction between carriers is strongly enhanced by the strong confinement on nanometer-scale. In particular, the direct term of Coulomb interaction contributes to the excitation energy of electron-hole pairs (called further excitons) by about 30 meV in comparison to 6 meV for the exciton binding energy in GaAs bulk. Similarly the exchange term manifests by the fine structure (a few hundreds µeV) of the ground state excitonic complex X 0 , which is split between bright states (with angular momentum J z =±1) and dark states (J z =±2). In addition, the bright exciton can be split into a doublet of linearly-polarized lines by the anisotropic electronhole exchange interaction. The latter, called fine structure splitting (FSS) is of the order of a few tens µeV in self-assembled InAs/GaAs QDs. Up to recently, it was one of the main difficulties to overcome for the generation of entangled photon pairs from a single QD [1, 2]. Its fine control is still an important issue to improve the generation of entangled photons, since current successful solutions (high magnetic field above 5 T or post-selection of the emitted photon pairs) are still very demanding [1, 2]. Here, we report on the sensitivity of the QD fine structure splitting and more generally of excitonic complexes binding energies to a static electric field. The quantum confined Stark effect on the single particles is indeed expected to produce an important change of the Coulomb interaction by modifying the overlap and separation of wave functions or possibly by changing the exciton wave function symmetry. As described below two different electric field configurations (vertical or in-plane) have been considered in order to address both natural directions of the strongly anisotropic self-assembled InAs/GaAs QDs. Our results rather point out the robustness of Coulomb interaction against electric field, although a significative change of the FSS can be most of the time achieved supporting this method for a fine tuning of the entanglement degree of emitted photon pairs.