We present a spectroscopic study of nanocavities obtained by small modifications of a W1 waveguide in an AlN photonic crystal membrane. The AlN film containing GaN quantum dots is grown on silicon. The photonic crystal structure is defined by e-beam lithography and etched by inductively coupled plasma reactive ion etching, while the membrane is released by selective etching of the silicon substrate. The room temperature photoluminescence of the embedded quantum dots reveals the existence of even-symmetry and odd-symmetry confined cavity modes and guided modes. Cavity mode quality factors up to 4400 at 395 nm and 2300 at 358 nm are obtained.

III-N materials have become the dominant materials for UV to blue-green semiconductor light sources. Besides these conventional light emitters, III-N materials are also attracting candidates for less conventional emitters and for new applications (integration with electronics, integration wit lab on chip, array of microsources…). In order to better control the light emission, realizing photonic nanostructures with large quality factors in the UV range is especially challenging due to the larger scattering losses compared to similar structures in the visible and IR spectral ranges. In this work we study the spectroscopy of Al(Ga)N photonic resonators embedding GaN quantum dots (QDs). For room temperature operation, nitride QDs are promising candidates due to their large radiative efficiency [1]. We have studied the spectroscopy of single photonic membrane cavities as well as microdisks, by microphotoluminescence. The recent progresses of the fabrication techniques allowed us to reach state of the art quality factors for both photonic structures, i.e. Q=1800 for a L3 nanocavity [2] and Q=7300 for microdisks [3]. In both systems, photonic modes are modelled, allowing to determine the processes limiting the quality factor. These results are compared to state of the art nitride microdisks and photonic crystal cavities.

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, the investigated cavities consist in a ZnO active layer grown by molecular beam epitaxy embedded between an AlGaN/AlN distributed Bragg reflector (DBR) and a SiO2/SiN DBR. Coherent non-linear emission under non-resonant optical pumping is demonstrated in two different regimes as a function of the exciton-cavity detuning. At large negative detuning, from T=80 K to 300 K, the cavity switches at threshold from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process, and that we realized an exciton laser [Guillet et al., APL 98, 211105 (2011)]. At small negative detuning, and at T=120 K, the cavity persists in the strong coupling regime at threshold with negligible blue shift. Angle-resolved µPL experiments demonstrate the spectral narrowing at threshold, as well as a competition between multiple lasing polariton modes. Those results are compared with macroPL experiments. The far-from-equilibrium behavior of the system is typical of a polariton laser in a cavity presenting photonic disorder [To be published].

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 generation and control of polariton condensates up to room temperature. We have recently reported the condensation of polaritons in a bulk ZnO microcavity over an unprecedented range of exciton-photon compositions and of temperatures, up to room temperature [1]. The complete phase diagram of the ZnO polariton laser has been measured (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 (figure 1c). Moreover imaging of the condensate allows to evidence in-plane free propagation of the polariton condensate starting from the excitation spot. This tunability is obtained on a fully-hybrid microcavity with a high quality factor (Q>2000) and a large Rabi splitting (250 meV); it 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 strong excitonic condensates can here be investigated. Figure 1. (a) Detuning dependence of the polariton condensation threshold for a 2.5 polariton branch, under quasi-continuous excitation; (b) Polariton density at the condensation threshold deduced from the kinetic model; (c) Angle-resolved emission below and above threshold (T=300K, almost zero detuning). The authors acknowledge financial support from FP7 program through ITN networks CLERMONT4 (235114) and SPIN-OPTRONICS (237252). References [1] F. Li et al., Phys. Rev. Lett. 110, 196406 (2013) [2] T. Guillet et al., Appl. Phys. Lett. 98, 211105 (2011) [3] H. Franke et al., New J. Phys. 14, 013037 (2012); T.C. Lu et al., Optics Express 20, 5530 (2012) [4] J. Levrat et al., Phys. Rev. B 81, 125305 (2010)

Semiconductor-based microcavities appear as a prolific system for studying light-matter interaction between a spatially-confined photonic mode and an excitonic resonance. The quasiparticles arising from this coupling (microcavity-polaritons) have enabled in the last years the observation of new lasing regimes as well as polariton Bose-Einstein condensates, vortices and lately solitons. In this panorama ZnO appears as an alternative material to more mature ones, such as GaAs or CdTe, with larger oscillator strengths and enhanced exciton stability. These two properties render ZnO very interesting for studies and applications where large particle densities and/or high temperatures are required. However, the fabrication of ZnO-based microcavities is still challenging and it often requires the use of either nitrides or dielectric materials for the DBRs. Indeed, polariton lasing was demonstrated for the first time in a ZnO-based microcavity only in 2011 [1, 2]. In this work we report on the optical study of a fully-hybrid ZnO-based microcavity in which we combine a high quality active region made up of bulk ZnO and a high cavity quality factor, thanks to the use of two dielectric DBRs. Indeed, the Q-factor of the cavity measured by micro-reflectivity exceeds systematically 1000. Furthermore, as the ZnO active region has a wedge shape a large range of detuning between the cavity and the excitonic mode is accessible. Polariton lasing, as characterized by a strong (typically a factor 5) linewidth reduction, a small blueshift compared to the Rabi splitting (typically 30 times smaller), and an increased intensity emission (four orders of magnitude increase) will be reported. The study of the lasing regime as a function of temperature and detuning will be presented and the differences with respect to other materials, arising due to ZnO peculiarities, will be emphasized. [1] T. Guillet et al., Appl. Phys. Lett. 99 (2011) 161104. [2] L. Orosz et al., Phys. Rev. B.(2012) accepted for publication.