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.

A central challenge in making 2D material-based devices faster, smaller, and more efficient is to control their charge carrier density at the nanometer scale. Traditional gating techniques based on capacitive coupling through a gate dielectric cannot generate strong and uniform electric fields at this scale due to divergence of the fields in dielectrics. This field divergence limits the gating strength, boundary sharpness, and minimum feature size of local gates, precluding certain device concepts (such as plasmonics and metamaterials based on spatial charge density variation) and resulting in large device footprints. Here we present a nanopatterned electrolyte gating concept that allows locally creating excess charges by combining electrolyte gating with an ion-impenetrable e-beam-defined resist mask. Electrostatic simulations indicate high carrier density variations of \Delta n ~ 1014 cm^2 across a length of only 15nm at the mask boundaries on the surface of a 2D conductor. We implement this technique using cross-linked poly(methyl methacrylate) (PMMA), experimentally prove its ion-impenetrability and demonstrate e-beam patterning of the resist mask down to 30nm half-pitch resolution. The spatial versatility enables us to demonstrate a compact mid-infrared graphene thermopile with a geometry optimized for Gaussian incident radiation. The thermopile has a small footprint despite the number of thermocouples in the device, paving the way for more compact high-speed thermal detectors and cameras.