Ref HAL: hal-01951768_v1
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The recent developments of nitride nanophotonics, based on photonic crystal membrane nanocavities and microdisk resonators, pave the way to a novel nanophotonic platform. Here we present two recent achievements: first we demonstrate the room-temperature operation of a nitride microlaser emitting in the deep UV spectral range (=275 nm) with GaN/AlN multiple quantum wells. Secondly we tailor the second-harmonic generation of a cw near infra-red radiation coupled to a high quality factor photonic crystal cavity.

Ref HAL: hal-01951085_v1
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The spatial dynamics of the formation of a polariton condensate under a tightly focused excitation is imaged through 2D near-field and far-field 2D tomography in a ZnO microcavity, up to room temperature. The modelling exhibits the role of the outwards polariton flux caused by the reservoir repulsion, that leads to a 3 to 10 fold increase of the condensation threshold and is imprinted in the shape of the polariton condensate

Ref HAL: hal-01951054_v1
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Ultra-violet semiconductor lasers have numerous applications for optical storage, biochemistry, sterilization or optical interconnects. However the usual design of deep UV-emitting ridge lasers relies on complex buffer layers or expensive substrates for the growth of the nitride heterostructures [1]. Moreover microlaser and nanophotonic devices with integrated sources require an alternative approach compatible with thin waveguides.In this work we demonstrate a series of nitride-on-silicon microdisk lasers covering a broad spectral range, from the blue (=470 nm) to the deep ultra-violet (=275nm), and operating at room temperature. They rely on the controlled growth of thin AlN buffer layers on silicon substrates, followed by the growth of GaN/AlN or InGaN/GaN multiple quantum wells (MQWs) with a high radiative efficiency up to T=300K. We exploit the possibility to under-etch selectively the silicon substrate in order to form nitride membranes for photonic resonators; this has allowed many demonstrations of microdisk and photonic crystal cavities with quality factors larger than 1000, ranging from the mid-IR [2] to the near-UV spectral range [3,4], now expanding to the UV-C [5].Following the demonstration of a UV-C microlaser operating atat 275 nm and at room temperature [5], the figure 1 presents the emission characteristics of a series of nitride-on-silicon microdisks with different active layers grown on a thin AlN buffer layer. Their whispering gallery modes are probed under continuous wave excitation at 266 nm with a 15 µm laser spot (Figure 1.b); the photoluminescence is collected from the edge of the microdisks. The chosen microdisk diameters in panel (b) are 5-6 µm in order to properly exhibit the modes, except for the 3 µm microdisk emitting at 275 nm. In all cases, the sharpest peaks are observed in the low energy tail of the PL spectrum where the absorption of the active layer is limited. The measured Q factors exceed 1000, and reach 4000 in the best resonators. The lasing operation is obtained under pulsed quasi-resonant optical pumping (266nm, 400ps pulses) with a threshold excitation density ranging from 2 to 40 nJ/pulse (Figure 1.a). The dependence of the lasing threshold will be discussed.In the UV-B and UV-C spectral ranges, these demonstrations underline the interest of simple binary GaN/AlN MQWs, grown on a thin AlN buffer on a silicon substrate, in order to obtain efficient active layers providing optical gain up to room temperature. The microdisk lasing in the visible spectral range, based on more mature InGaN/GaN MQWs, proves the versatility of the nitride-on-silicon photonic platform, that can embed a broad range of integrated sources.

Résumé:

Excitons in quantum wells (QWs) based on group-III nitrides or ZnO are naturally indirect in real space, due to the strong internal electric field that exists along the growth axis. Within such excitons, the electron and hole are spatially separated, leading to (1) strong dipole moments and (2) long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown that a combination of these two features results in many interesting properties of IXs : they can propagate over large distances, they can be controlled in-situ by light or by external gate voltage, they can cool down to the lattice temperature before recombination, and form cold and dense gases of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in GaN- or ZnO-based 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. Using spatially- and time-resolved photoluminescence experiments, we investigate the exciton transport in several QW samples based on GaN and ZnO. The IX emission is imaged, with spectral and temporal resolutions, along the sample plane. Therefore the corresponding spatial density profiles are obtained and monitored as a function of (i) temperature, (ii) the exciton density at the excitation spot, (iii) the substrate material and (iv) the excitation regime (continuous vs pulsed excitation). We provide a comprehensive analysis of the data combined with numerical modeling, and we show that the efficient propagation of IXs takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole-dipole interaction. Under these latter conditions, exciton mobility is almost temperature-independent from 10K up to room temperature. This suggests that exciton-exciton interaction is by far the dominant scattering mechanism, compared to scattering by the interface disorder. However, exciton transport is maintained up to 300K only in the GaN/AlGaN QW that was epitaxially grown on a GaN substrate, since nonradiative processes dominate the transport in the case of a sapphire substrate, for which high densities of threading dislocations are present. We provide a detailed understanding of the physical mechanisms of IX transport and its temperature and density dependence, and we show that IXs in wide band-gap QWs constitute a promising system for the formation of collective bosonic states in semiconductors. Financial support : projects "INDEX" (FP7 PITN-GA-2011-289968) and "OBELIX" (ANR-15-CE30-0020-02).

Ref HAL: hal-01469387_v1
DOI: 10.1063/1.4971357
WoS: WOS:000390677700001
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23 Citations
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We present 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 over λ = 275 nm–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. This demonstrates the versatility of this nitride-on-silicon platform, and the realization on this platform of efficient active layers for lasing action over a 200 nm broad UV to visible spectral range. We probe the lasing threshold carrier density over the whole spectral range and found that it is similar whatever the emission wavelength for these Q > 1000 microdisk resonators with a constant material quality until quantum confined Stark effect takes over. The threshold is also found independent of microdisk diameters from 3 to 12 μm, with a β factor intermediate between the one of vertical cavity lasers and the one of small modal volume “thresholdless” lasers.

Ref HAL: hal-01398059_v1
DOI: 10.1021/acsphotonics.6b00236
WoS: WOS:000380297200015
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11 Citations
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We demonstrate high-spatial resolution imaging of localized cavity modes through third-harmonic frequency conversion. The experiments are performed with a III-nitrideon-silicon photonic platform. The resonant cavities are formed within suspended two-dimensional photonic crystals and are excited with a continuous-wave excitation around 1550 nm. The III-nitride materials (GaN and AlN) are transparent for both pump and harmonics. The third-harmonic nonlinear process allows one to indirectly observe the fundamental confined cavity mode with spatial profiles equivalent to those usually obtained by local probe microscopy techniques such as scanning near-field optical microscopy. An excellent agreement is obtained between the measured polarization-resolved third harmonic emission patterns and those calculated through the third-order nonlinear polarization. We show that the spatial profiles of the radiated patterns are strongly dependent on defocusing, thus highlighting the strong sensitivity of the imaging.

Ref HAL: hal-01398056_v1
Ref Arxiv: 1603.05093
DOI: 10.1016/j.crhy.2016.07.002
WoS: WOS:000384759600010
Ref. & Cit.: NASA ADS
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13 Citations
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We review the recent developments of the polariton physics in microcavities featuring the exciton-photon strong coupling at room-temperature, and leading to the achievement of room-temperature polariton condensates. Such cavities embed active layers with robust excitons that present a large binding energy and a large oscillator strength, i.e. wide bandgap inorganic or organic semiconductors, or organic molecules. These various systems are compared, in terms of figures of merit and of common features related to their strong oscillator strength. The various demonstrations of polariton laser are compared, as well as their condensation phase diagrams. The room-temperature operation indeed allows a detailed investigation of the thermodynamic and out-of-equilibrium regimes of the condensation process. The crucial role of the spatial dynamics of the condensate formation is discussed, as well as the debated issue of the mechanism of stimulated relaxation from the reservoir to the condensate under non-resonant excitation. Finally the prospects of polariton devices are presented.