Cathodoluminescence in a scanning electron microscope was applied to a semiconductor quantum dot in a nanowire that is able to emit single photons. We show that cathodoluminescence can be used not only for imaging and spectroscopy but also to measure the correlation function and characterize the purity of the single-photon emitter. The electron beam can be manipulated to minimize the collection of parasitic luminescence. At cryogenic temperatures, we observed that the thermal budget, as measured via the phonon sidebands, is close to that of nonresonant microphotoluminescence. This makes cathodoluminescence an efficient tool in the quest for novel single-photon sources.

The emission properties of a localized solid-state emitter are strongly influenced by its environment. The coupling to acoustic phonons impacts the coherence of the emitter and its temperature dependence, and also results in the apparition of phonon sidebands besides the sharp zero-phonon line. Here, we present a method for measuring the absolute temperature of a localized emitter by directly plotting the ratio of the Stokes and anti-Stokes components of the phonon sideband as a function of the shift from the zero-phonon line. This approach requires no calibration and knowledge of the system, making it applicable to a wide range of emitters and materials. We validate the method using a CdSe quantum dot in a ZnSe nanowire. We thus show that the quantum dot is significantly heated under nonresonant excitation when increasing the incident power at low temperature which is ascribed to the drop in thermal conductivity at these temperatures.

We present a promising solid-state system able to emit triggered single-photons in the blue-green range up to room temperature. The active element is a CdSe quantum dot (QD) embedded in a bottom-up core-shell ZnSe nanowire grown by molecular beam epitaxy. The nanowire shell acts as a waveguide and confines the fundamental optical mode HE11, channeling the photons emitted by the QD along the nanowire axis. The nanowires has a base radius of 70-90 nm and a length of 5-6 mu m with a conical ending that allows to adiabatically expands the guided mode and reduces the divergence angle. Photo-correlation measurements show anti-bunching with g((2))(0) values down to 0.3 at room temperature. Charged excitons degrade the emission properties at room temperature and we found that a neutral nanowire-quantum dot would emit single-photons with a brightness of 0.17 photon per pulse.

Single-photon sources are crucial for developing secure telecommunications. However, most systems operate at cryogenic temperatures. Here, we discuss a promising solid-state system emitting single photons at room temperature in the blue-green range, allowing for quantum communications in free space and underwater. The active element is a core-shell ZnSe tapered nanowire embedding a single CdSe quantum dot grown by molecular beam epitaxy. A patterned substrate enables a thorough study of the one and same nanowire by different methods. Our source exhibits anti-bunching with g(2)(0) < 0.3 near the centre of the photoluminescence line and shows high brightness. This work paves the way for developing single-photon sources operating at non-cryogenic temperatures.

Quantum dots acting as single photon emitters in the blue-green range are fabricated and characterized at cryogenic temperature. They consist in CdSe dots inserted in (Zn,Mg)Se nanowires with a thick shell. Photoluminescence spectra, decay curves and autocorrelation functions were measured under nonresonant continuous-wave and pulsed excitation. An analytical approach is applied simultaneously to the decay curves and correlation functions. It allows a quantitative description of how these two quantities are affected by the exciton rise due to biexciton feeding, the bright exciton decay, the effect of the dark exciton, and the re-excitation between two laser pulses. Linewidths at our limit of resolution (200 $\mu$eV) are recorded. The reported correlation counts vary from a full control by re-excitation from traps, to a small contribution of re-excitation by mobile carriers or other QDs, as low as 5%.

Quantum dots inserted inside semiconductor nanowires are extremely promising candidates as building blocks for solid-state based quantum computation and communication. They provide very high crystalline and optical properties and offer a convenient geometry for electrical contacting. Having a complete determination and full control of their emission properties is one of the key goals of nanoscience researchers. Here we use strain as a tool to create in a single magnetic nanowire quantum dot a light-hole exciton, an optically active quasiparticle formed from a single electron bound to a single light hole. In this frame, we provide a general description of the mixing within the hole quadruplet induced by strain or confinement. A multi-instrumental combination of cathodo-luminescence, polarisation-resolved Fourier imaging and magneto-optical spectroscopy, allow us to fully characterize the hole ground state, including its valence band mixing with heavy hole states.

We report here on the quantitative 3D reconstruction of core‐shell nanostructures by STEM/EDX using two X‐ray maps acquired at two different tilt angles perpendicular to each other (Rueda et al., 2016; fig. 1). The method is based on the modelling of the NW cross‐section using a series of imbricated ellipses whose dimensions are defined by their major and minor diameters (fig. 2). The number of ellipse depends on the number of chemical phases which are identified from the concentration profiles. The position and orientation of each ellipse are determined by the coordinates of their respective centers and the overall tilt of the nanowire, respectively. More sophisticated models, using hexagons or rectangles instead of ellipses, have been developed in order to take into account the crystal structure of nanowires exhibiting facetted sidewalls. These models are based on the elliptical model, by constructing the tangents to an ellipse, and hence, are defined by the same parameters, which is useful when comparing models. Considering a system of a number of K ellipses with ξ k,j the local concentration of element j for the k th ellipse (k=1 for the largest ellipse), then the average concentration C i,j of element j for the i th pixel along the x‐axis must satisfy the following equations (equations 1): With t 1,i and t k,i , the local thickness at pixel i of the first and k th ellipse, respectively. The local thickness of the first ellipse (= the total thickness of the cross‐section) and the average concentration C i,j of element j present along the beam axis is determined using the zeta‐factor method (Watanabe and Williams, 2006): With: m the total number of element, I b the beam current; ρ the sample density; ζ j the zeta‐factor of element j determined using reference samples of known composition and thickness (Lopez‐Haro et al., 2014); I i,j and A i,j the net X‐ray intensity and the absorption correction term for element j at pixel i, respectively. The absorption correction term is estimated from a simple model that takes into account the direction of the X‐ray emission relative to the position of the detectors, knowing the thickness, density, and mass absorption coefficient of the material through which the radiation travels (Rueda et al., 2016). The method for reconstructing the cross‐section can be divided into three steps: 1) the appropriate cross‐sectional model is selected by comparing the thickness profile calculated from equation [2] with the thickness profile simulated for elliptical, hexagonal, and rectangular cross‐sections (figure 3); 2) the number of ellipses is determined, and their dimensions are evaluated, from the concentration profiles; 3) the local concentrations ξ k,j are determined and the dimensions of the ellipses are adjusted by minimizing the compositional differences between profiles calculated from equation [3] and simulated by equation [1]. This method was applied for reconstructing core‐shell nanostructures on (Mg, Mn, Cd, Zn)(Te,Se) and (Al, Cu)Ge nanowires and (Pt, Co) nanoparticles. Advantages and limitations of the method will be presented and discussed at the conference.

We propose a comprehensive description of the strain configuration induced by the lattice mismatch in a core-shell nanowire with circular cross-section, taking into account the crystal anisotropy and the difference in stiffness constants of the two materials. We use an analytical approach which fully exploits the symmetry properties of the system. Explicit formulae are given for nanowires with the wurtzite structure or the zinc-blende structure with the hexagonal/trigonal axis along the nanowire, and the results are compared to available numerical calculations and experimental data on nanowires made of different III–V and II–VI semiconductors. The method is also applied to multishell nanowires, and to core-shell nanowires grown along the 〈0 0 1〉 axis of cubic semiconductors. It can be extended to other orientations and other crystal structures.

The magnetic state of a single magnetic ion (Mn2+) embedded in an individual quantum dot is optically probed using microspectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn2+ ion (S = 5/2) is analyzed in detail. The exciton-Mn2+ exchange interaction shifts the energy of the exciton depending on the Mn2+ spin component and six emission lines are observed at zero magnetic field. The QD exciton emission is then a direct probe of the Mn spin state. The limiting factors of such detection are then presented: they are mainly due to the influence of geometrical effects such as the anisotropic strain distribution into the QD and its shape anisotropy. In the last part, we show how we can manipulate and understand quantitatively the interaction between the single Mn spin and a confined carrier by controlling the charge state of a quantum dot (0. +/- 1 electron) with an electrostatic (and optical) gate. This opens the way to ail electrical or optical control of the magnetic properties of a single atom. To cite this article: L. Besombes et al., C. R. Physique 9 (2008). (C) 2008 Published by Elsevier Masson SAS on behalf of Academie des sciences.

En introduisant des impuretés magnétiques dans les semi-conducteurs, on obtient des matériaux dont les propriétés électroniques et magnétiques sont fortement couplées. On peut alors utiliser les spécificités des semi-conducteurs et les techniques de la microélectronique pour contrôler une aimantation par un champ électrique ou isoler un spin unique dans une boîte quantique adressable ; inversement, on peut agir sur le spin des porteurs de charge et réaliser ainsi certaines fonctions requises par l’électronique de spin.