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Dipolar, or indirect excitons (IXs) 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 such excitons. Indeed, compared to their counterparts in GaAs-based heterostructures, IXs in nitrides possess two key advantages. They have higher binding energies, and they are “naturally” indirect. This means, that they can be engineered and manipulated in as-grown heterostructures, even without application of an external electric bias. In this work we demonstrate (i) the propagation of IXs over hundreds of micrometers in the plane of GaN/(AlGa)N quantum wells, (ii) confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface (iii) the electrical control of the IX fluxes via an applied gate voltage. Once confined, the exciton fluid accumulates in uniform zones. By modeling the emission spectra of these zones, we get insight into the thermodynamic properties of the thermalized exciton fluid. These results constitute a prerequisite for realization of the GaN-based excitonic devices, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Ref HAL: hal-02466858_v1
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Dipolar, or indirect excitons (IXs) 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 such excitons. Indeed, compared to their counterparts in GaAs-based heterostructures, IXs in nitrides possess two key advantages. They have higher binding energies, and they are “naturally” indirect. This means, that they can be engineered and manipulated in as-grown heterostructures UP TO ROOM TEMPERATURES, even without application of an external electric bias. In this work we demonstrate (i) the propagation of IXs over hundreds of micrometers in the plane of GaN/(AlGa)N quantum wells, (ii) confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface (iii) the electrical control of the IX fluxes via an applied gate voltage. These results constitute a prerequisite for realization of the GaN-based excitonic devices, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Ref HAL: hal-02393815_v1
DOI: 10.1038/s41598-019-54416-3
WoS: 000500557100001
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On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

Ref HAL: hal-02272702_v1
Ref Arxiv: 1902.02974
DOI: 10.1021/acs.nanolett.9b00914
WoS: 000481563800013
Ref. & Cit.: NASA ADS
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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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7 Citations
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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