A series of H-bonded supramolecular architectures were built from monofunctional M-C≡C-R and bifunctional R-C≡C-M-C≡C-R trans-alkynylbis(1,2-bis(diphenylphosphino)ethane)ruthenium(II) complexes and π-conjugated modules containing 2,5-dialkoxy-p-phenylene. Incorporation on each partner of a cyanuric end and of the complementary Hamilton receptor provided the necessary means to keep the constituents together via strong hydrogen bonding. Characterization of all architectures has been performed on the basis of NMR and photophysical methods. In particular, the formation of a Hamilton receptor/cyanuric acid complex has been exemplified by an X-ray single-crystal structure determination. Both self-assembly and accurate modification of the complementary blocks were ensured in such a way that the resulting materials maintain the responsiveness of the electron-rich 2,5-dialkoxy-p-phenylene spacers toward nitroaromatics.

Properties of self-assembled quantum dots (QDs), often referred to as artificial atoms in solid state, are very interesting in order to study physical effects specific to single quantum systems such as the single-photon emission. Thanks to their huge vertical quantum confinement, GaN QDs are emitting even at room temperature and on a large UV range. This peculiarity makes GaN QDs good candidates for fundamental physics as well as conception of devices such as UV-LEDs. In so-called thick QDs (height larger than 2.3 nm), the energies of the confined electronic states are determined by the quantum confined Stark effect. On the contrary, in thin QDs (height smaller than 2.3 nm) the quantum confinement plays the major role, thus leading to a significantly smaller homogeneous linewidth and to the resolution of the exciton fine-structure splitting [Kindel, PRB81, 241309]. As far as time-resolved measurements are concerned, several papers reported experiments in thick QDs emitting at 350nm or more, either in polar [Bretagnon, PRB73, 113304] or non-polar nanostructures [Founta, APL86, 171901]. We report photoluminescence decay time measurements in thin GaN QDs emitting between 280nm and 360nm. For QDs emitting at 290nm we observe a monoexponential decay and the abscence of any red shift for the luminescence spectrum shows that we are in the regime of a single electronhole pair per dot. In this case, we measure the intrinsic radiative recombination time, without any perturbation due to the Stark effect. We find a radiative lifetime of about 350 ps in good agreement with our theoritical model.

In the field of polariton physics, ZnO has attracted a large attention due to it large exciton binding energy and oscillator strength, that enable to observe strong coupling at room temperature. We have recently demonstrated polariton condensation in such a cavity presenting a large quality factor (>2000) [1]. In the present work, we study the time-resolved spectroscopy of the polariton emission in this cavity, in order to assess the critical parameters of the polariton dynamics and their temperature dependance. We also study the theoretical model for the dynamics of microcavity polaritons. Figure 1 presents the experimental study of the photoluminescence dynamics of cavity polaritons. The measured exciton lifetime is about 50ps at 80K and at exciton densities close to the condensation threshold. It is rather independent of the temperature. It also governs the decay of each polariton branch fed by the exciton reservoir. Our modeling clarifies the time-constants involved in the exciton and polariton dynamics. In particular, we address the problem of the relative efficiencies of exciton formation assisted by either optical longitudinal phonons (LO) or acoustic phonons, and the following relaxation towards the polariton branches. We resolve the classical Boltzmann equation [2] which describes the relaxation kinetics of bosonic polariton. The energy relaxation rates of heated excitons due to emission and absorption of phonons have been calculated as a function of the lattice temperature. References [1] F. Li et al., Phys. Rev. Lett. 110, 196406 (2013) [2] G.Malpuech et al., Phys. Rev. B. 65,153310 (2002)

Spatial observation and optical manipulation of polariton quantum liquids are raising a large interest in the studies of polariton condensates inmicrocavities [1]. In this work, we report the study of polariton propagation in a full hybrid ZnO microcavity where the condensation of polaritons up to room temperature has recently been achieved [2]. One particularity of this cavity is its thickness gradient, allowing the access to a large range of exciton-photon detunings. This thickness gradient is also interesting because it creates a dragging force for polaritons and polariton condensates. It is therefore important to measure the spatial distribution of the condensate with respect to the exciting laser spot in order to identify the condensation regimes. Using tomography, we have measured the 2D spatial image of the polariton emission below and above threshold attemperatures ranging from 10K to 300K. We are able to study the diffusion and propagation of a polaritons depending on the exciton density as well as on the microcavity thickness gradient. Figure 1 presents the photoluminescence spectra and the spectrally resolved images, and evidences an asymetric polariton propagation above threshold. This is a first step towards the optical manipulation of macroscopic quantum fluids in ZnO at room temperature. The authors acknowledge financial support from FP7 program through ITN networks CLERMONT4 (235114). References [1] E. Wertz et al., Phys. Rev. Lett. 109, 216404 (2012) [2] F. Li et al., Phys. Rev. Lett. 110, 196406 (2013)

The effect of thermal annealing on In0.25Ga0.75N/GaN quantum wells grown by molecular beam epitaxy at 550°C is investigated. A strong increase in the 300 K photoluminescence (PL) intensity is observed for samples annealed at 880°C, while degradation occurs for higher temperatures. The improvement of the optical properties is ascribed to higher internal quantum efficiency (IQE), as indicated by temperature-dependent and time-resolved PL experiments. The effect of carrier localization due to possible quantum dot formation via indium clustering is ruled out based on high-resolution transmission electron microscopy imaging. IQE improvement is thus attributed to a reduction of point defects upon annealing.

Time-resolved photoluminescence of high-aluminum-content (Al,Ga)N quantum wells and epilayers emitting in the 215-255 nm range.