Ref HAL: hal-02272702_v1
Ref Arxiv: 1902.02974
DOI: 10.1021/acs.nanolett.9b00914
WoS: 000481563800013
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Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of dipolar excitons. We demonstrate the in-plane confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface. This result is a prerequisite for the electrical control of the exciton densities and fluxes, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Ref HAL: hal-02100722_v1
DOI: 10.1364/OE.27.011800
WoS: 000464614400146
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Nanophotonic circuits using group III-nitrides on silicon are still lacking one key component: efficient electrical injection. In this paper we demonstrate an electrical injection scheme using a metal microbridge contact in thin III-nitride on silicon mushroom-type microrings that is compatible with integrated nanophotonic circuits with the goal of achieving electrically injected lasing. Using a central buried n-contact to bypass the insulating buffer layers, we are able to underetch the microring, which is essential for maintaining vertical confinement in a thin disk. We demonstrate direct current room-temperature electroluminescence with 440 mW/cm 2 output power density at 20 mA from such microrings with diameters of 30 to 50 µm. The first steps towards achieving an integrated photonic circuit are demonstrated.

Ref HAL: hal-01952488_v1
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Exciton-photon coupling in a waveguide geometry is attracting more and more interest in the field of polaritonics. Waveguide polaritons are obtained when the exciton and the guided mode of a photonic waveguide are brought in the strong coupling regime, in strong analogy with cavity polaritons in Fabry-Perot resonators. One of the first demonstrations of waveguide polaritons was reported in a waveguide embedding InGaAs/GaAs quantum wells, with a Rabi splitting of 6 meV [Walker et al., APL 102, 012109 (2013) and Nat. Comm. 6, 8317 (2015)], leading to the observation of ultra-low power temporal solitons at T=10K. Nitride materials offer the possibility to investigate polariton physics up to room temperature thanks to the large binding energy and large oscillator strength of the excitons in GaN. The first demonstration of strong coupling in nitrides waveguides was based on InGaN multiple quantum wells, with a Rabi splitting of 63 meV at 4K, and a strong coupling observed up to T=100K [Ciers et al., Phys. Rev. Applied 7, 034019 (2015)]. In the meantime, we have achieved polariton lasing in a waveguide geometry for the first time in another wide bandgap material, ZnO [Jamadi et al., Arxiv:1708.00501], up to room-temperature.Here we report the enhancement of the Rabi splitting in a bulk GaN slab waveguide, compared to the case of multiple quantum wells. The Rabi splitting in a monomode GaN waveguide reaches 140meV, and slightly decreases to 100 meV at T=300K. To measure it, we have fabricated diffraction gratings on the waveguides and performed polarization-resolved Fourier-space imaging of the TE and TM modes dispersion. Besides, through Fourier-space imaging and spectral filtering of the photoluminescence image we have measured the polaritons propagation length as a function of the detuning: at T=10K it amounts to 10µm at zero detuning and 30µm at more negative detuning (= -Rabi); most importantly, similar propagation lengths are observed up to room temperature.The increased Rabi splitting and large propagation lengths pave the way for the demonstration of polariton lasers at room temperature in a GaN waveguide, and for a control of the nonlinear processes in polariton fluids of light.

Ref HAL: hal-01952415_v1
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The integration of nitride semiconductors on a silicon platform is a very promising field for nanophotonic applications in the UV-blue spectral range. Efficient sources are a main topic that can be achieved by compact microlasers based on GaN/AlN and InGaN/GaN multiple quantum wells (MQW) grown on a silicon substrate. The epitaxy of these structures is performed by the controlled-growth of an AlN and GaN buffer layers to prevent from dislocations. To obtain versatile and efficient micro-emitters, the technological development of micro-resonator has been improved for nitride MQW micro-disks on silicon. This unique Nitride-on-Silicon platform has been allowed by releasing the micro-disk membrane with selective under-etching of the silicon substrate. For integrated photonics applications, these micro-disks can be electrically powered but also coupled to optical waveguides to a better extract of the emission. We have already shown that we can reach a few thousand for the quality factor of the microdisks with nitride MQW grown by MBE1 but also in nitride-on-silicon photonic crystal cavities2. The laser emission of the microdisks under optical pumping and at room temperature was obtained over a broad spectral range extending from 275 nm to 470 nm1.Here we investigate the physics of the micro-resonators with InGaN/GaN MQW grown by MOCVD. Photoluminescence (PL) emission is probed under pulsed optical pumping at 266 nm. Laser emission is reached at room temperature at 390 nm. We focus on the gain properties of the InGaN/GaN MQW active layer. We have investigated the temperature dependence of the microlaser threshold vs temperature. We show that the typical T0 for these microlaser is elevated (T0=77K), meaning that the temperature dependence around room temperature is reduced. We also model the spectra to extract the carrier density at threshold. And we will draw up a comparison with InGaN/GaN MQW microlasers grown by MBE.References: 1M. Mexis et al., Optics Letters, vol. 36 p.2203 (2011), Sellés et al., APL 109, 231101 (2016) and Sci. Rep. 6, 21650 (2016), 2F. Tabataba-Vakili et al., APL 2017

Ref HAL: hal-01952385_v1
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Microlasers covering the UV to blue spectral range are important building blocks for biochemical detection applications. This work presents a series of microdisk lasers realized within the same GaN-on-Si photonic platform scheme, and operating at room temperature under pulsed optical pumping over a broad spectral range extending from 275 nm to 470 nm. The III-nitride microdisks embed either binary GaN/AlN multiple quantum wells (MQWs) for UV operation, or ternary InGaN/GaN MQWs for violet and blue operation. The fabrication process is based on e-beam lithography and the release of the nitride membrane after selective under-etching of the silicon susbtrate, and has led to high quality factor resonators, microdisks as well as photonic crystal cavities, within the same platform. A detailed comparison of these microlasers leads to the conclusion that for our Q>1000 microdisk resonators, the pumping threshold is determined by the type of optical excitation, resonant with the barrier or with the MQW excited states, and the emission efficiency of the MQWs [Sellés et al., APL 109, 231101 (2016) and Sci. Rep. 6, 21650 (2016)].

Ref HAL: hal-01951866_v1
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The slab waveguide geometry is attracting more and more interest in the field of polaritonics. Waveguide polaritons are obtained when an excitonic transition and a slab guided mode are brought in the strong coupling regime, in strong analogy with cavity polaritons in Fabry-Perot resonators. Waveguide polaritons have been first demonstrated in GaAs waveguide embedding InGaAs quantum wells, with a Rabi splitting of 6 meV [Walker et al., Nat. Comm. 6, 8317 (2015)]. There, at 10K, the polariton fluid of light presents strong optical nonlinearities, solitons are generated at very low power densities. Wide bandgap materials offer the possibility to combine stronc coupling, non-linearities and polariton lasing at 300K, as recently demonstrated for the first time in a ZnO slab waveguide [Jamadi et al., Arxiv:1708.00501]. In nitride materials, a first demonstration of strong coupling was reached in the waveguide geometry with InGaN multiple quantum wells, with a Rabi splitting of 63 meV [Ciers et al., Phys. Rev. Applied 7, 034019 (2015)].In this work we demonstrate the strong coupling in a bulk GaN waveguide grown on Silicon. The epitaxy of thick (500 nm) AlN buffer at high temperature (1000°C) followed by an AlGaN optical cladding layer allows for a good GaN quality. A thin SiO2 top layer is processed into gratings for the extraction of the guided polaritons. The dispersions of the polariton eigenmodes are measured through the diffraction at the gratings and exhibit a characteristic anti-crossing between the guided photons and the excitonic transition. A Rabi splitting Ω=150 ±30 meV for the TE0 polariton mode is deduced from the modeling of the dispersions.Further, the propagation of the guided polaritons is probed at T=10K under non-resonant excitation: the polaritons are generated at a focused excitation spot, and the attenuation of each polariton branch is measured through spatial filtering combined to far-field imaging. At negative detuning (-1.5Rabi) in the photonic regime, the propagation length is of the order of 200±80 µm (i.e. a attenuation of 50 cm-1). It decreases to 12±1µm (i.e. 840 cm-1) at zero detuning. Such waveguide polaritons have the potentiality for introducing non-linear polariton fluids into the realm of integrated photonics operating at room temperature.

Ref HAL: hal-01951849_v1
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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; these can couple to the exciton transition and give rise to additional polaritons branches, creating thereby a polaritons leak channel, and can further collect part of the “vertical” polaritons emission. Overall, for a vertical microcavity they can be considered as a source of polariton losses. On the other hand, the physics of guided polaritons in waveguides has recently attracted some interest as a new class of fluids of light supporting bright temporal solitons, as well as a new type of polariton lasers [1-3].In this work we investigate the propagation and relaxation properties of guided polaritons in vertical microcavities based on ZnO active layers and we evidence their role in the dynamics of the polariton laser.Figure 1 illustrates the scheme of the study of the guided modes in a ZnO vertical microcavity with mesas, in the strong coupling regime. The excitonic reservoir is pumped non-resonantly under a focused spot, and feeds both the polaritons near k=0 and the guided polaritonic modes. 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 [4]. The guided modes are observed at energies below the minimum of the lower polariton branch. Interestingly, their spectrum presents fringes that are the signature of beatings between multiple guided modes, which are coherently pumped under the excitation spot. This observation, for distances up to 65µm, demonstrate the long ballistic propagation of the guided polaritons, and their efficient feeding by the excitonic reservoir. This last process competes with the relaxation towards the LPB.Under strong pulse excitation, the guided polaritonic modes influence the operation of the polariton laser. In the absence of cracks in the mesas, the polariton laser operates near k=0 and the corresponding peaks appear in the spectrum of the guided modes, indicating an efficient feeding of those modes beyond threshold. If cracks are present in the mesa, then an intricate dynamics can be observed, resulting from a competition between vertical (out-of-plane) and horizontal (in-plane) polariton lasing.