Recently, we determined using spatial modulation spectroscopy, over a broad optical spectral range, the spectrum and amplitude of the absorption cross-section of individual semiconducting single-wall carbon nanotubes. Here, we will present resonant Raman excitation profiles (REP) of radial breathing modes and G modes measured on the same SWNTs. The transition energies and full width at half-maximum (FWHM) obtained using both measurements techniques will be compared to discuss the different models commonly used to fit REP as illustrated on Figure 1. Furthermore, interference effects on Raman intensity will also be evidenced in the case of two energetically close optical transitions. Finally, some recent results obtained on other systems (SWNTs homogeneous dimer and double-walled carbon nanotubes) will be briefly reported.

The art of deriving classical potential consists in finding the correct balance between the complexity of the functional form whose implementation should be as simple as possible, the computational efficiency, and the reliabilty. In this talk, we will present a new methodology which uses ab initio data for parametrizing classical potentials suited for amorphous systems.We have considered a simple and widespread empirical Ansatz, i.e. Born-Mayer-Coulomb, with the goal to investigate large time/size scales. As a first condition, a classical potential should be able to describe the atomic structure of the system under investigation. In order to optimize the parameters of the classical potential we have then used a cost function that involves the partial pair correlation functions extracted from ab initio simulations. The system considered is liquid silica, a liquid which represents a prototype of strong glass formers exibiting a network structure. From a methodological point of view, we will show, through the different comparisons between ab initio and classical MD, that this structural fitting procedure is a very promising approach to obtain new effective potentials suited for describing not only amorphous material but also crystalline properties.We will present simulations carried out using this new potential and showing that CHIK reproduces with more accuracy a number of properties of molten and glass silica, than the well known BKS potential. Some tests performed on a-quartz also showed that the new potential can also be used to simulate crystalline systems with a good accuracy regarding properties such as asymmetric coordinates and elastic constants. Finally, we will discuss molecular dynamics simulations carried out to represent the structures of amorphous dehydroxylated silica surface in liquid and glass states, and show that the new potential gives a also fair description of the surface.

Dans cet exposé, nous présenterons une analyse des propriétés structurales et vibrationnelles de plusieurs verres silicatés. Ces données ont été obtenues soit au moyen de simulations de dynamique moléculaire ab initio soit en combinant des simulations classiques et ab initio. Les simulations ab initio ont été réalisées dans le cadre de la DFT, avec le code VASP. Nous avons considéré des compositions binaires soda-silicatés ainsi que des compositions borosilicatés plutôt complexes contenant 4 ou 5 oxydes. Les échantillons numériques vitreux ainsi obtenus présentent des caractéristiques structurales et vibrationnelles en très bon accord avec les données expérimentales. De plus, nous avons pu identifier des corrélations entre les unités structurales locales et leurs signatures vibrationnelles.

Confined H$_2$O molecules act as local probes for depressurization phenomena during the pressure induced amorphisation of faujasite NaX at which the OH stretching frequency first decreases and then increases almost to its room pressure value upon further compression. Pair distribution function (PDF) analysis provides evidence that amorphisation corresponds to a collapse of the structure around hydrated sodium cations with strong distortion of the secondary building units (double six-membered rings, sodalite cages). Both the use of guest molecules as local probes in far- and mid-infrared spectroscopy, where we correlate intermolecular water H bonding vibrations and internal mode behaviour under confinement, and PDF analysis could be of great use to study the mechanical behaviour of other hydrated materials.

Self-assembled semiconductors QDs are of great interest in fundamental physics as well as inapplied physics. In GaN/AlN heterostructures, the huge band-offset and the strong excitonicbinding lead to an important exciton confinement allowing an emission even at roomtemperature. GaN QDs grown along the (1000) axis are named “polar” due to their stronginternal electic field which lead to the so-called QCSE. The most important modifications due tothis electric field take place in thick QDs emitting around 330nm and more : the luminescence isredshifted and the radiative lifetime is strongly increased [1]. On the other hand, in thin QDs(smaller than 2.3nm), the quantum confinement plays the key role, thus leading to a significantlysmaller homogeneous linewidth and to the resolution of the exciton fine-structure splitting[2]. Concerning the radiative recombination processes, several papers reported measurements at330nm or more either in polar[3] or non-polar[4] nanostructures. Experimental data on excitonicradiative lifetime are also existent in thin QDs but on a limited dynamical range where thepossible influence of QCSE is not discussed [5].In this work, we report time-resolved photoluminescence measurements in GaN/AlN QDsemitting between 280nm and 360nm. For QDs emitting at 300nm we observe a biexponentialdecay dynamics of the photoluminescence signal (Fig.1, left). The fast component (450 ps) isinterpreted as the radiative lifetime of the QDs in the absence of a noticeable redshift of thetransition. Thanks to our better signal-to-noise ratio than in Ref. 5, we are able to extract a slowcomponent (2 ns) which may originate either from feeding from the barrier or other processesinside the QD. Power dependent experiments further confirm this interpretation. We observe thatthe fast component remains the same as a function of excitation power (Fig.1, right) and, moreimportantly, that the intensity ratio keeps the same value. This rules out any contribution fromthe relaxation of multi-carriers complexes such as biexciton or trion in the short time dynamics

ZnO is a wide bandgap semiconductor with strong excitonic properties, in particular a large oscillator strength and a large exciton binding energy. It therefore raises a strong interest for the generation and control of polariton condensates up to room temperature.The realization of planar ZnO microcavities with good quality factors has long been a strong challenge. We first demonstrated in 2011 a ZnO polariton laser operating at zero exciton-photon detuning and a temperature of 120 K [1], based on a hybrid microcavity (AlN/AlGaN DBR, MBE-grown ZnO, SiO2/SiN DBR) with a quality factor Q=450. This value was still too small to allow the observation of polariton lasing up to room temperature. A different approach was implemented based on a ZnO active layer of high crystalline quality, embedded between two dielectric DBR; such a fully-hybrid microcavity exhibits a high quality factor (Q>2000) and a large Rabi splitting (250 meV). We have observed the condensation of polaritons in this bulk ZnO microcavity over an unprecedented range of exciton-photon compositions and of temperatures, up to room temperature [2]. The complete phase diagram of the ZnO polariton laser has been measured, showing that its threshold is only 6 times larger at 300 K than at 8 K. It is in a good qualitative agreement with the simulations of exciton and polariton relaxation in a kinetic model. This tunability represents an important progress compared to our previous demonstration [1], as well as to other recent reports [3]. It also confers a strong advantage to ZnO microcavities compared to GaN [4], since strong excitonic condensates can here be investigated.The cavity with Q>2000 however presents a strong gradient of the cavity thickness hindering the generation of extended condensates. Equivalent cavities without such a gradient have therefore been developed in the last years, based on high quality AlN/AlGaN DBRs on mesas. Our most recent investigations are dedicated to the role of the shape of the exciting laser spot on the condensate generation. Condensate propagation is observed under tightly focused excitation, driven by the interaction with the reservoir as in GaAs microcavities, whereas localization of the polariton condensates is observed for larger excitation areas.