We study the inter-well excitonic coupling in a series of In0.17Ga0.83N/GaN Double QWs (DQWs) with varying central barrier width. We observe the switching between indirect IX (inter-well) and direct DX (intra-well) excitons, for thin barriers (2 nm or less), depending on the exciton density. This density is controlled, in cw-PL, by the pumping laser power density (LPD). Above a certain threshold, we observe a sudden change of the PL blue-shift, when switching from IXs (large slope) to DXs (weak slope). In time-resolved PL, the exciton density evolves as the PL intensity decays with time, and the switching occurs from DXs to IXs, after a certain characteristic time. The decay time of IXs is of the order of hundreds of s, whereas the decay time of DXs is shorter by three orders of magnitude. The switching thresholds in both cw- and TR-PL present clear exponential dependences upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching processes. The described effects were studied at T10K but we found that IXs persist up to T=300K pointing out the importance of large exciton biding energy in nitride QWs.

Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. The physics of indirect excitons (IXs) has been extensively studied in GaAs-based heterostructures: they can propagate over large distances, be trapped in gate-controlled electrostatic traps, and form a cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostructures, IXs in GaN QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. The length scale of the excitonic transport is an important figure of merit of the indirect excitons. The excitonic transport was first observed at low temperature in the case of hetero-epitaxy of the GaN QWs on sapphire substrates [1]. The improvement of the sample quality through homo-epitaxy on GaN substrates allowed observing an efficient transport up to room temperature [2]. Under a focused non-resonant excitation of the quantum well, we observe a spatial decay of the exciton density (deduced from the local blue shift of the QW transition), with a length scale of 100µm at 10K and 30µm at T=300K in our most recent homo-epitaxial samples. Such long propagation distances are a pre-requisite to the development of IX-based optoelectronic devices operating on a large temperature range, and to the exploration of exciton collective states. The realization of an excitonic trap is a first step forward, and we recently investigated the exciton transport in electrostatic traps formed when a semi-transparent gate is deposited on top of the QW sample. Trapping of the excitons is observed, with a typical trap depth of 60-90 meV. The excitons can also be generated outside the trap and relax into the trap, so to spatially separate the trap from the generation spot, which is expected to be a source of heat and decoherence for the excitonic cloud. These results are promising for the investigation of trapped collective states of IXs, and their cooling into exotic phases.

In group-III nitride quantum wells, indirect excitons (IXs) are naturally formed because the electron-hole pair is separated along the growth (0001) axis by strong internal electric fields. These IXs therefore exhibit strong permanent dipole moments and extremely long radiative lifetimes (> 10µs). Previous extensive studies of IXs in GaAs-based, biased double quantum wells, at low temperatures, have emphasized promising properties: IXs can propagate over large distances, be controlled in-situ by light and external gate voltage, form cold and dense gases of interacting bosons, and may form collective quantum states of matter. Compared to IXs in arsenide heterostrutures, IXs in nitride QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature and over a range of exciton densities larger by two orders of magnitude. By using spatially- and time-resolved photoluminescence experiments we investigate the transport of dipolar excitons in GaN-(Al,Ga)N QWs, up to room-temperature, over distances of several tens of micrometers. Variations of conditions and comparison with models allow us to discuss the relative impacts on IX transport of important factors, such as disorder and exciton-exciton interaction.

Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong built-in electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. The physics of indirect excitons (IXs) has been extensively studied in GaAs-based heterostructures: they can propagate over large distances, be trapped in gate-controlled electrostatic traps, and form a cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostructures, IXs in GaN QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range.The length scale of the excitonic transport is an important figure of merit of IXs. We have first investigated the excitonic transport in the case of hetero-epitaxy of the GaN/AlGaN QWs on sapphire substrates, at low temperature [1]. The sample quality has then been improved through homo-epitaxy on GaN substrates, and we have evidenced an efficient transport up to room temperature [2]. Under a focused non-resonant excitation of the quantum well, we observe a spatial decay of the exciton density. The transport length now reaches 100µm at 10K and 30µm at T=300K.Such long propagation distances are a pre-requisite to the development of IX-based optoelectronic devices operating on a large temperature range, and to the exploration of excitonic collective states. The realization of an excitonic trap is a first step towards this goals, and we recently investigated the exciton confinement in electrostatic traps of various shapes. The traps are realized via deposition of semi-transparent gates on top of the QW sample. Trapping of the excitons is demonstrated, with a typical trap depth of 60-90 meV. The excitons can also be generated outside the trap and relax into the trap, so to spatially separate the trap from the generation spot, which is expected to be a source of heat and decoherence for the excitonic cloud.These results are promising for the investigation of trapped collective states of IXs, and their cooling into exotic phases. Acknowledgement: This work is supported by the French National Research Agency (ANR-15-CE30-0020-02 OBELIX andANR-11-LABX-0014 GANEX).[1]F. Fedichkin et al, Phys. Rev. B 91, 205424 (2015).[2]F. Fedichkin et al, Phys. Rev. Appl. 6, 014011 (2016).

Excitons complexes in ZnO/(Zn, Mg)O non-polar quantum well have been studied by usingcontinuous-wave photoluminescence according to temperature and to the density of excitation.The spectrum of photoluminescence of the quantum well consists of three different lines. Theratio between the intensities of these lines depends of the temperature but also of the density ofexcitation. The lines are attributed to the free exciton, the negatively charged trion and thebiexciton. The binding energy of trion and biexciton are found to be respectively 19.5 ± 2 meVand 45 ± 5 meV.

The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

Excitons recombination dynamics in ZnMgO alloy have been studied by time-resolved photoluminescence according to temperature. At low temperature, localization effects of the exciton are found to play a significant role. The photoluminescence (PL) decays are bi-exponential. The short lifetime has a constant value, whereas the long lifetime shows a dependency with temperature. For temperature higher than 100 K the declines show an mono-exponential decay. The PL declines are dominated by non-radiative process at temperatures above 150 K. The PL lifetime dependancy with temperature is analysed using a model including localisation effects and non-radiative recombinations.