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).

We present experimental studies of terahertz radiation detection by InP double heterojunction based transistors. We analyze the relation between their static characteristics and the experimentally determined voltage and current responsivities, showing importance of internal device parasitic capacitances and the external circuit loading effects. Finally, we demonstrate the use of these transistors for terahertz radiation computed tomography leading to 3D visualization of concealed objects. Our results pave the way towards wide use of heterojunction based transistors for terahertz imaging.

We report measurements and theoretical predictions on the effect of an aligning magnetic field on the orientational disorder of a nematic liquid crystal in contact with isotropic solid substrates. Different types of substrates present a similar disorder and a similar dependency on the magnetic field amplitude, i.e. a decreasing of the angular distribution widths and spatial correlation lengths. Measurements are in qualitative agreement with a theory where the orientational disorder emerges from the competition between the aligning magnetic torque and the disorienting thermal fluctuations during the adsorption of the liquid crystal molecules on the substrate.