An indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character. Current research on transport properties of IXs is opening a pathway to the development of novel optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1-3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly promising because of much larger exciton binding energies and natural dipoles induced by strong built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (µ-PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long distances from the excitation spot (30 < r < 100µm) turned out to be a secondary PL, excited by the light emitted at the central spot, guided along the plane, due to the refractive index contrast between the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance reached by the measurable PL is limited by recombination of excitons at non-radiative defects. Using GaN substrates instead of sapphire should both suppress the secondary emission and the nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures, except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed observe the mere propagation of excitons up to 35 µm away from the excitation spot and up to 250 K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole-dipole repulsion in high-density regions and for disorder along the sample plane.[1]Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A. C. Gossard. Optics Letters 35 (10), 1587 (2010).[2]A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard. Science 321, 229 (2008).[3]A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard. Optics Letters 32, 2466 (2007).[4]F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T. Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre. Phys. Rev. B 91, 205424 (2015).

We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.

We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density,measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons.We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 μm is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the excitonemission pattern on the scale up to 100 μm. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

GaN-Al0.5NGa0.5N quantum dots deposited on (11-22) planes have been grown by combining Molecular Beam Epitaxy and Metal Organic Vapour Phase Epitaxy. This combination is interesting for realization of ultraviolet operation light emitting diodes, lasers andsingle photon sources,… (1,3) The growth of dots was achieved by MBE using ammonia as nitrogen precursor and growth interruption in ammonia less conditions to trigger corrugation of GaN and dot formation (4). The (11-22) GaN oriented peudosubstrate was realized by MOVPE starting from a M-plane oriented sapphire substrate. The orientation of the growth plane dictates in-plane anisotropies which are effectively found leading to a transition from isolated dots to nanochains - oriented along the <1-100> direction as evidenced from Atomic Force Microscopy features or optical properties: polarization rates and temperature dependent measurements of the radiative recombination process for instance(5). We here restrict to small size isolated quantum dots and present innovative optical properties among which are micro-photoluminescence data versus pump power, polarization of the emitted photons at different temperatures. We also analyse the photoluminescence decay times and model our finding in the context of the effective mass approximation. The crystal field splitting is measured in Al 0.5NGa 0.5N lattice-matched to (11-2) oriented GaN by polarized microphotoluminecence under high photo- injection conditions.

Deep ultra-violet semiconductor lasers have numerous applications for optical storage, biochemistry or optical interconnects. UV-emitting ridge lasers usually embed nitride heterostructures grown on complex buffer layers or expensive substrates – an approach that cannot be extended to nano-photonics and microlasers. We demonstrate here the first deep ultra-violet microlaser operating at 275nm at room temperature under optical pumping. It is based on binary GaN/AlN thin quantum wells (QWs) grown on a silicon substrate and embedded in microdisk resonators. Those QWs indeed combine UV-C emission with a good emission efficency even at room temperature thanks to the huge band offset beetwen GaN and AlN. They form the active layer of state-of-the-art microdisk resonators, which electromagnetic modes, the so-called Whispering-Gallery Modes (WGMs), present a low modal volume and a high quality factor (Q=6000). This active layer can form free-standing membranes and is further compatible with future developments of nitride nanophotonic platforms on silicon.

We demonstrate second harmonic generation in a gallium nitride photonic crystal cavity embedded in a two-dimensional free-standing photonic crystal platform on silicon. The photonic crystal nanocavity is optically pumped with a continuous-wave laser at telecom wavelengths in the transparency window of the nitride material. The harmonic generation is evidenced by the spectral range of the emitted signal, the quadratic power dependence vs. input power, and the spectral dependence of second harmonic signal. The harmonic emission pattern is correlated to the harmonic polarizationgenerated by the second-order nonlinear susceptibilities and the electric fields of the fundamental cavity mode.

GaN/AlGaN polar single quantum wells have attracted particular attention due to their large binding energies and the presence of strong internal electric fields that naturally induce long-lived excitons. In this case, the exciton transport is driven by the potential landscape caused by the local alteration of the built-in electric field, which results from the exciton accumulation at the same point. We study by spatially- and time-resolved PL experiments AlGaN/GaN coupled quantum wells (QW). The active zone of studied sample consists of a 30 nm thick Al0.20Ga0.80N barrier, a 4.4 nm (17 atomic monolayers) GaN QW followed by 1 monolayer of Al0.20Ga0.80N and a 1.6 nm (6 atomic monolayers) GaN QW and 30 nm thick Al0.20Ga0.80N cap layer. By solving Schroedinger-Poisson equation, we illustrate the two regimes of strongly screened electric field and the totally un-screened QW system. We show that the screening of the build-in electric field enhances the electron-hole wave-function overlap and increases the optical recombination energy. In other words, dipoledipole repulsion between excitons raises their potential energy and drastically increases their radiative recombination rate. A blue-shift of 0.3 eV under continues wave, localized excitation and a quasi-continuous red-shift of the emission as excitons are drifted away from the excitation spot up to distances of 90 μm are observed experimentally. Strong LO-phonon replica are also noticed, as a result of the strongly dipolar character of excitons [1]. These replicas follow the same position dependence as the zerophonon line. Using our Schroedinger-Poisson equation solver, we can extract from the measured blue-shift the position-dependent exciton density. Last, we present simulations of the observed transport features, namely the exciton densities and luminescence intensities by solving the nonlinear drift-diffusion equation. We establish that the long-range excitonic transport is mainly dominated by drift caused by repulsive dipole-dipole interaction between excitons in combination with their long lifetimes. [1] S. Kalliakos, X. B. Zhang, T. Taliercio, P. Lefebvre, B. Gil, N. Grandjean, B. Damilano, and J. Massies. Applied Physics Letters 80, 428 (2002).