We create polariton clouds and polariton condensates to characterize their propagation at different temperatures. Our experimental results show that in a thickness gradient microcavity, polariton propagation is mainly ballistic and does not depend on the microcavity gradient. Above threshold, we compare the experimental results to a theoretical model [1] to understand the peculiar k-space images of the polariton condensate. Our results indicate that these images are due to the repulsion of the condensate from the center of excitation by the exciton reservoir created by a small excitation spot (2~3 µm).

A platform for fabricating polariton lasers operating at room temperature is introduced: nitride-based distributed Bragg reflectors epitaxially grown on patterned silicon substrates. The patterning allows for an enhanced strain relaxation, thereby enabling to stack a large number of crack-free AlN/AlGaN pairs and achieve cavity quality factors of several thousands with a large spatial homogeneity. GaN and ZnO active regions are epitaxially grown thereon, and the cavities are completed with top dielectric Bragg reflectors. The two structures display strong-coupling and polariton lasing at room temperature and constitute an intermediate step in the way towardsintegrated polariton devices.

We demonstrate a two-dimensional free-standing gallium nitride photonic crystal platform operating around 1550 nm and fabricated on a silicon substrate. Width-modulated waveguide cavities are integrated and exhibit loaded quality factors up to 34 000 at 1575 nm. We show the resonance tunability by varying the ratio of air hole radius to periodicity, and cavity hole displacement. We deduce a 7.9 dB/cm linear absorption loss for the suspended nitride structure from the power dependence of the cavity in-plane transmission.

Light-emitting devices based on high-Al content (Al,Ga)N suffer from high densities of dislocations or other point defects, counterbalanced by the carrier localization arising from disorder in the ternary alloy. One way to improve the internal quantum efficiency is the controlled Si-doping of AlxGa1-xN/AlyGa1-yN multi-quantum wells, which was assigned to the reduction of point defects by reduction of internal strains, for some ideal concentration of Si. Time-resolved photoluminescence (TR-PL) can be used [S. F. Chichibu et al., Appl. Phys. Lett. 99, 051902 (2011)] to try and correlate the observed PL time-decays with the density of defects, in relation with the nonradiative recombination probability of photoexcited carriers. We present TR-PL studies of MOVPE-grown Si-doped AlxGa1-xN/AlyGa1-yN multi-quantum wells with typical values of x=0.6 and y=0.7 and of AlxGa1-xN epilayers with x up to 0.86. High-Al content MQWs and epilayers exhibit similar bi-exponential PL decay dynamics, with the slower component rapidly quenched when T is increased. The fast decay component remains in the nanosecond range at all temperatures and the PL intensity loss is limited by carrier localization.

In the field of polariton physics, ZnO has attracted a large attention due to it large exciton binding energy and oscillator strength, that enable to observe strong coupling at room temperature. We have recently demonstrated polariton condensation in such a cavity presenting a large quality factor (>2000) [1]. In the present work, we study the time-resolved spectroscopy of the polariton emission in this cavity, in order to assess the critical parameters of the polariton dynamics and their temperature dependance. We also study the theoretical model for the dynamics of microcavity polaritons. Figure 1 presents the experimental study of the photoluminescence dynamics of cavity polaritons. The measured exciton lifetime is about 50ps at 80K and at exciton densities close to the condensation threshold. It is rather independent of the temperature. It also governs the decay of each polariton branch fed by the exciton reservoir. Our modeling clarifies the time-constants involved in the exciton and polariton dynamics. In particular, we address the problem of the relative efficiencies of exciton formation assisted by either optical longitudinal phonons (LO) or acoustic phonons, and the following relaxation towards the polariton branches. We resolve the classical Boltzmann equation [2] which describes the relaxation kinetics of bosonic polariton. The energy relaxation rates of heated excitons due to emission and absorption of phonons have been calculated as a function of the lattice temperature. References [1] F. Li et al., Phys. Rev. Lett. 110, 196406 (2013) [2] G.Malpuech et al., Phys. Rev. B. 65,153310 (2002)

Spatial observation and optical manipulation of polariton quantum liquids are raising a large interest in the studies of polariton condensates inmicrocavities [1]. In this work, we report the study of polariton propagation in a full hybrid ZnO microcavity where the condensation of polaritons up to room temperature has recently been achieved [2]. One particularity of this cavity is its thickness gradient, allowing the access to a large range of exciton-photon detunings. This thickness gradient is also interesting because it creates a dragging force for polaritons and polariton condensates. It is therefore important to measure the spatial distribution of the condensate with respect to the exciting laser spot in order to identify the condensation regimes. Using tomography, we have measured the 2D spatial image of the polariton emission below and above threshold attemperatures ranging from 10K to 300K. We are able to study the diffusion and propagation of a polaritons depending on the exciton density as well as on the microcavity thickness gradient. Figure 1 presents the photoluminescence spectra and the spectrally resolved images, and evidences an asymetric polariton propagation above threshold. This is a first step towards the optical manipulation of macroscopic quantum fluids in ZnO at room temperature. The authors acknowledge financial support from FP7 program through ITN networks CLERMONT4 (235114). References [1] E. Wertz et al., Phys. Rev. Lett. 109, 216404 (2012) [2] F. Li et al., Phys. Rev. Lett. 110, 196406 (2013)

ZnO is a wide bandgap semiconductor with strong excitonic properties, in particular a large oscillator strength and a large exciton binding energy. It therefore raises a strong interest for the demonstration of polariton lasing up to room temperature with microcavities in the strong exciton-photon coupling regime. The strong coupling regime has recently been demonstrated for planar ZnO microcavities fabricated by various approaches. In this work, we report polariton condensation over a broad range of temperatures, up to room temperature, and of exciton-photon composition for the polariton condensate. This is the outcome of the improvement beyond Q=2000 of the microcavity quality factor, one of the key parameters for polariton condensation.Our fully-hybrid bulk ZnO microcavity combines a high quality active region, made up of bulk ZnO from a thinned substrate, and a cavity with a high quality factor, enabled by two dielectric mirrors. The large variation of the cavity thickness allows to analyze polariton condensation processes in thin () as well as thick (2 to 3 ) cavities. The Rabi splitting approaches 250 meV. Polariton condensation is obtained systematically [1], over a wide range of exciton-photon detuning and of temperature up to 300K. The complete phase diagram of the ZnO polariton laser is drawn (cf figure 1a), showing that its threshold is only 6 times larger at 300 K than at 8 K. It is in a good qualitative agreement with the simulations of exciton and polariton relaxation in a kinetic model (figure 1b). Strongly excitonic (96% exciton fraction) as well as strongly photonic condensates (83% photon fraction) are realized, that pave the way to promising advances of the polariton physics in ZnO microcavities. This tunability represents an important progress compared to our previous demonstration of a ZnO polariton laser in a Q=450 microcavity [2], as well as to other recent reports [3]. It also confers a strong advantage to ZnO microcavities compared to GaN [4], since thanks to their very large Rabi splitting the optimal detuning for condensation is positive, instead of negative, at moderate temperatures, and close to zero at room temperature. One important consequence is that more excitonic condensates (i.e. with stronger interactions, and therefore nonlinearities) can be generated and investigated.