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).

Abstract-Biochemical detection applications and on-chip optical interconnects are examples of the useful applications considered for integrated photonic platforms dedicated to the UV and visible spectral range. This achievement requires the realization of efficient and compact microlaser sources that can be coupled to optical waveguides and are compatible with photonic circuitry. We develop a cost-effective practical approach relying on the controlled growth of thin AlN buffer layers on silicon substrates, followed by the growth of multiple quantum wells (MQW) GaN/AlN (for UV operation), or InGaN/GaN MQWs (for violet and blue operation) with a high radiative efficiency up to T=300K. This unique Nitride-on-Silicon platform could be released into a membrane by the selective under-etching of the silicon substrate.We present a series of microdisk lasers operating at room temperature under pulsed optical excitation over a broad spectral range extending from 275nm to 470nm. All microdisks present a Q factor above 1000 and reach 4000 in the best resonators. We are able to investigate the gain threshold of the different active layers. The microlasers operate under pulsed optical excitation, and the lasing threshold is reduced by a factor 10 from deep-UV GaN/AlN microdisks to the violet InGaN/GaN ones. This work allows us to expect a variety of integrated photonics applications, including for example multi-color integrated laser sources: we demonstrate here the versatility of this nitride-on-silicon platform, and the first realization on this platform of efficient active layers for a tunable lasing action over a 200nm broad UV to visible spectral range. [1,2].Acknowledgements: The authors acknowledge support from the projects GANEX (ANR-11-LABX-0014) and QUANONIC (ANR-13-BS10-0010). GANEX belongs to the public funded 'Investissements d'Avenir' program managed by the French ANR agency. This work was also partly supported by the RENATECH network.REFERENCES1.Sellés et al., APL 109, 231101 (2016)2.Sci. Rep. 6, 21650 (2016)

The polariton emission in a microcavity is usually measured from the surface, and little is known about the actual role of guided modes below the light cone in air, that can couple to the exciton transition, collect part of the emission and contribute to polariton losses. In this work we investigate a ZnO planar microcavity with mesas. The emission of the guided modes is observed at the mesa’s edge, together with the vertical emission. The microcavity operates as a polariton laser under pulsed optical pumping [1].The spectra collected under the excitation spot and at the mesa’s edge are very different, as shown on figure 1a. The vertical emission is dominated by the lower polariton branch (LPB) associated to the cavity mode whereas the edge emission forms a broad peak modulated by fringes which spacing decreases as the spot moves away from the edge. While these fringes could indicate Fabry-Perot oscillations of polaritons propagating in-plane, in analogy with radiative polaritons in GaAs microcavity wires [2], a careful analysis shows that they are the signature of beatings between 5 to 10 guided modes, without being necessary to invoke multiple reflections. These modes have a large attenuation length of 27 µm, intermediate between the propagation length of LPB polaritons (a few µm in a ZnO microcavity [3]) and the one of waveguide polaritons (100µm in a GaN microcavity [4]).Under pulsed optical pumping, the guided modes are strongly excited. They form leaky modes for the excitonic reservoir, as well as for the LPB polaritons themselves. Most interestingly, when the vertical microcavity displays polariton lasing, the polariton laser emission feeds the guided modes, which then cascade in energy (Fig. 1b,c).This work evidences therefore that the threshold of the polariton laser in conventional vertical microcavities is increased due to the existence of usually hidden guided modes, that can deplete the exciton reservoir and collect resonantly scattered light from the lasing LPB mode.

The broad-area vertical-cavity surface-emitting lasers (VCSELs) can be brought into a bistable regime by the use of a frequency-selective feedback (as well as saturable absorbers), leading to the formation of cavity solitons (CSs). Such CSs are generated under electrical injection below the lasing threshold of the VCSEL, and appear in the broad area laser as bright spots. They can also be arranged into self-organized arrays, switched on and off with an external laser holding beam, and brought to high-order states [1]. The self-localization can even be obtained in space and time, and self-pulsing spatial solitons have been demonstrated in a mode-locked VCSEL [2]. Here we explore the topological properties of the high-order states of the CS, and we report the spontaneous formation of anti-vortex CSs with a hyperbolic polarization structure. Beams with a spatially non-uniform distribution of polarization are interesting due to their intrinsic beauty, novel functionalities in quantum optics and newly enabled applications [3]. They possess a circular symmetric intensity structure, most often in form of a doughnut, combined with a spatially non-uniform polarization field containing polarization singularities and are referred to as `vector vortex beams'. The engineering of such vectorial vortex beams has been demonstrated in solid-state lasers with a mesa or a meta-surface shaping the spatial pattern [4], whereas they are here spontaneously obtained in the broad-area laser. Vortex beams, including the exotic anti-vortex reported here, have been predicted within the framework of spin-flip models of VCSELs [5], and result from the interplay between the natural anisotropies of the laser and the spin-dependent gain dynamics.

Three-dimensional solitons are highly sought after objects in nonlinear science. In nonlinear photonics they correspond to light bullets, but their experimental observation as long-term stable structures remains elusive in spite of impressive advances [1]. Broad-area vertical-cavity surface-emitting lasers (VCSELs) combine a large Fresnel number with a high nonlinearity and fast time scales. Stationary spatial solitons in 2D as well as mode-locking and temporal solitons without self-localization in space are known to exist. It is intriguing to explore a combination of these two aspects to achieve self-localization of light in three dimensions [2]. Here we report on the observation of mode-locked states of spatial solitons.The experimental system consists of a VCSEL with a 200 micrometer emission aperture emitting around 980 nm. It is coupled to a self-imaging external cavity closed by a volume Bragg grating (VBG) as a frequency-selective element. The round-trip time in the external cavity is about 0.7 ns. By varying the VCSEL current, the detuning between the VCSEL and the VBG resonances is changed and single-mode operation in the external cavity and multi-mode operation can be realized. The latter may correspond to irregular dynamics or fairly regular self-pulsing which represents mode-locked states (Fig. 1). These data are in a range where the self-pulsing spatial soliton is bistable, i.e. it coexists with the non-lasing state as expected for a self-localized state. We obtain self-pulsing at the fundamental and the harmonic round-trip frequency, as well as occasionally at even higher harmonics. The peaks in the RF-spectrum are extremely narrow (<; 100 kHz, close to the resolution limit) indicating a high phase-correlation between the participating modes. The optical spectrum displays three dominant modes. The pulse width (FWHM) is about 110 ps for the harmonic mode-locking and 270 ps for the fundamental mode-locking, roughly consistent with the expectation from spectral bandwidth. The results clearly establish mode-locking of spatial solitons on 3 modes creating temporal structures shorter than the external cavity. Numerical simulations of the model introduced in [4] are in good agreement with the experimental data [3].

Beams with a spatially non-uniform distribution of polarization attracted considerable recent interest due to their intrinsic beauty, novel functionalities in quantum optics and newly enabled applications [1]. Typical realizations investigated possess a circular symmetric intensity structure, most often in form of a doughnut, combined with a spatially non-uniform polarization field containing polarization singularities and are referred to as `vector vortex beams'. Typically, considerable engineering effort is necessary to create these non-trivial polarization configurations. In contrast, we report on the spontaneous formation of vector vortex beams in a broad-area vertical-cavity surface-emitting laser (VCSEL) with frequency-selective feedback [2]. In particular, an anti-vortex state with a hyperbolic polarization structure is observed. The relation to vectorial high-order solitons is discussed.