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)].

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.

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.

Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal 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 the indirect excitons. The excitonic transport was first observed at low temperature in the case of hetero-epitaxy of the GaN QWs on sapphire substrates [1]. The improvement of the sample quality through homo-epitaxy on GaN substrates allowed observing 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 (deduced from the local blue shift of the QW transition), with a length scale of 100µm at 10K and 30µm at T=300K in our most recent homo-epitaxial samples. 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 exciton collective states. The realization of an excitonic trap is a first step forward, and we recently investigated the exciton transport in electrostatic traps formed when a semi-transparent gate is deposited on top of the QW sample (figure 1). Trapping of the excitons is observed, 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.

The main interest of group-III-nitride nanophotonic circuits is the integration of active structures and laser sources. A photonic platform of group-III-nitride microdisk lasers integrated on silicon and emitting in the blue spectral range is demonstrated. The active microdisks are side-coupled to suspended bus waveguides, and the coupled emission is guided and outcoupled to free space using grating couplers. A small gap size of less than 100 nm between the disk and the waveguide is required in the blue spectral range for optimal evanescent coupling. To avoid reabsorption of the microdisk emission in the waveguide, the quantum wells are etched away from the waveguide. Under continuous-wave excitation, loaded quality factors greater than 2000 are observed for the whispering gallery modes for devices with small gaps and large waveguide bending angles. Under pulsed excitation conditions, lasing is evidenced for 3 μm diameter microdisks integrated in a full photonic circuit. We thus present a first demonstration of a III-nitride microlaser coupled to a nanophotonic circuit.

Excitons in wide nitride Quantum Wells (QWs) are naturally indirect due to the strong internal electric field: within such excitons the electron and the hole are spatially separated, resulting in strong dipole moments and long radiative lifetimes. These properties offer the possibility to explore the collective exitonic phases with complex phase diagrams predicted theoretically in the temperature-density-dipole moment parameter space. Here we study 8 nm-wide GaN/(AlGa)N QWs presenting 1MV/cm built-in electric field in the growth direction, microsecond-long radiative lifetimes, 8 meV exciton emission linewidth and 100 µm transport lengths at cryogenic temperatures. To address the quasi-equilibrium properties of the strongly dipolar excitons (i.e. whose electric dipoles are twice as long as their Bohr radii d = 2aB), we confine them in the QW’s plane using electrostatic traps with a semi-transparent Schottly electrode. These traps consist in semitransparent electrodes deposited on the sample’s surface. We make micro-photoluminescence (µPL) measurements (less than 1µm- resolution) upon point-like optical excitation. We thus obtain the spectrally resolved information on the emission of the excitons as a function of power and density. Our experiments demonstrate an efficient trapping of excitons in the plane of the QW, with a trapping potential of an order of 50 meV. We demonstrate that it is possible to uniformly fill areas up to hundreds of microns in length with excitons and/or electron hole pairs in the QW’s plane. At T=4K, the emission from these areas exhibits a peculiar behavior. At low densities, such that there is no measurable screening of the built-in electric field, the emission from the trap is weak and spatially homogeneous. However, for weak built-in electric field screenings (as low as 10 meV, ~1011 cm-2 pair density), the PL brightens up and becomes spatially inhomogeneous – droplet like. We tentatively interpret this fragmentation as the coexistence of the gas and liquid phases of the excitons. As expected, the fragmented character of the emission disappears when either power and/or temperature is further increased, due to either exciton heating (gas phase), ionization or dissociation (Mott transition). Our results suggest that excitons in GaN QWs are promising candidates for the study of collective phenomena in cold dipolar bosonic systems. The advantages of the nitrides over arsenides include both higher critical temperatures for the onset of collective phenomena, and the ability to increase the dipolarity of systems based on their small exciton Bohr radius.

Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal 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 the indirect excitons. The excitonic transport was first observed at low temperature in the case of hetero-epitaxy of the GaN QWs on sapphire substrates [1]. The improvement of the sample quality through homo-epitaxy on GaN substrates allowed observing 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 (deduced from the local blue shift of the QW transition), with a length scale of 100µm at 10K and 30µm at T=300K in our most recent homo-epitaxial samples. 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 exciton collective states. The realization of an excitonic trap is a first step forward, and we recently investigated the exciton transport in electrostatic traps formed when a semi-transparent gate is deposited on top of the QW sample. Trapping of the excitons is observed, 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.