Cooling of nuclear spins in doped semiconductors via their dynamic polarization by optical pumping is a powerful method for harnessing ubiquitous fluctuations of nuclear spin, which constitute a well-known decoherence channel for the electronic spins. The idea of spin cooling is based on the hypothesis of spin temperature, which states that nuclear spin system (NSS) reaches an internal thermal equilibrium long before it comes to equilibrium with the external bath (crystal lattice). Although thermodynamic framework has been successfully employed for the description of a variety of the experimental data, a rigorous check of this concept in semiconductors was impossible until recently, in particular at low magnetic field. The reason for that is the lack of experimental techniques allowing nonperturbative optical control over adiabatic transformation of the NSS.We have recently developed such methods, based on off-resonant Faraday rotation and spin noise spectroscopy [1, 2]. Using these techniques, combined with dark-interval photoluminescence spectroscopy, we established a comprehensive picture of the nuclear spin relaxation efficiency, its magnetic field, temperature, and carrier concentration dependence in doped GaAs, a model system in the field of nuclear spin physics in semiconductors [3-5]. We also analyzed the interplay between four relevant relaxation mechanisms: hyperfine interaction, quadrupole interaction, spin diffusion and Korringa mechanisms.Understanding of field dependence of NSS dynamics allowed us to obtain a new insight into the NSS thermodynamics, and verify the spin temperature concept in GaAs bulk material and microcavities [6]. We have demonstrated, that NSS exactly follows the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization in quantum dots [7]. Our results open a way for the deep cooling of nuclear spins in semiconductor structures, with the prospect of realizing nuclear spin-ordered states for high-fidelity spin-photon interfaces.This work is supported by a joint grant of the Russian Foundation for Basic Research (RFBR, Grant No. 16-52-041301-4 and National Center for Scientific Research (CNRS, PRC SPINCOOL No. 148362)References:[1] R. Giri et al, Physical Review Letters, 111, 087603 (2013)[2] I. I. Ryzhov et al, Applied Physics Letters, 106, 242405 (2015)[3] M. Kotur et al, Physical Review B, 94(8), 081201 (2016)[4] M. Vladimirova et al, Physical Review B, 95, 125312 (2017)[5] M. Kotur et al, eprint arXiv:1802.05013 (2018)[6] M. Vladimirova et al, Physical Review B, 97, 041301 (2018)[7] P. Maletinsky et al, Nature Physics, 5, 407 (2009)

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.

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

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.