Optically pumped InGaN/GaN quantum well vertical-external-cavity surface-emitting laser emitting at 420 nm has been realized. Lasing at external cavity lengths of up to 50 mm is demonstrated, making integration of practical sized intracavity elements possible. Spectral and beam profile measurements indicate best operation conditions in a semiconfocal cavity configuration. Lasing threshold of 20.9 W is achieved for a 49 mm long cavity with output beam quality parameter M2 not exceeding 1.1.

A 3λ/2 (In,Ga)N/GaN resonant cavity, designed for ∼415 nm operation, is grown by molecular beam epitaxy and is sandwiched between a 39.5-period (In,Al)N/GaN distributed Bragg reflector (DBR), grown on c-plane GaN-on-sapphire pseudo-substrate by metal-organic vapor phase epitaxy and an 8-period SiO2/ZrO2 DBR, deposited by electron beam evaporation. Optical characterization reveals an improvement in the cavity emission spectral purity of approximately one order of magnitude due to resonance effects. The combination of spectrophotometric and micro-reflectivity measurements confirms the strong quality (Q)-factor dependence on the excitation spot size. We derive simple analytical formulas to estimate leak and residual absorption losses and propose a simple approach to model the Q-factor and to give a quantitative estimation of the weight of cavity disorder. The model is in good agreement with both transfer-matrix simulation and the experimental findings. We point out that the realization of high Q-factor (In,Ga)N containing microcavities on GaN pseudo-substrates is likely to be limited by the cavity disorder.

III-nitride based microcavities (MCs) appear as one of the most promising candidates for the realization of room temperature (RT) polariton laser diodes. The present work focuses on the properties of low In content InGaN/GaN multiple quantum wells (MQWs) in terms of inhomogeneous broadening, exciton localization energy, and plastic strain relaxation. For a small number of such QWs, an inhomogeneous line broadening of 41 meV is reported, which is compatible with strong coupling regime requirements. By contrast when considering an InGaN/GaN MQW set, a high density of defects is reported, which is ascribed to plastic strain relaxation. From the evolution of the inhomogeneous line broadening as a function of the number of QWs probed by microphotoluminescence measurements, it is concluded that for the realization of polariton light-emitting devices the QW number should not go beyond 30 for MC structures grown on FS-GaN substrate relying on InGaN/GaN QWs with an indium content similar to 12-15% and thicknesses of 1.5-2 nm/3 nm for the wells and the barriers, respectively. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present a comprehensive study of the anisotropic optical properties of nonpolar GaN/AlGaN multiple quantum wells intentionally designed to act as an active region of a planar microcavity operating in the strong-coupling regime. The strain induced by the underlying AlGaN-based Bragg reflector leads to a redistribution of exciton oscillator strength as revealed by photoluminescence and reflectivity measurements. Complementary k . p calculations show an excellent agreement with experiments and emphasize the opportunity to tune the nature of the light-matter coupling in a microcavity by means of strain engineering. Finally, the validity of the developed model is proven by angle-resolved photoluminescence studies carried out on the complete microcavity structure. The recorded eigenmode spectra reveal the coexistence of the weak- and the strong-coupling regime along the two orthogonal polarization planes.

The pressure-induced changes in the electronic band structures of In-containing nitride alloys, InxGa1-xN and InxAl1-xN are examined experimentally as well as by ab initio calculations. It is found that the band gap pressure coefficients, dE(g)/dp, exhibit very large bowing with x, and calculations with simulation of clustered distributions of the In atoms over the cation sites show a strong enhancement of this effect. This relates well to the experimental data obtained from pressure dependent photoluminescence measurements for InxGa1-xN and InxAl1-xN layers, performed in this work, and combined with existing data for InxGa1-xN layers. We discuss possible explanations of the anomalously large magnitude of the dE(g)/dp bowing in these nitride alloys.

A room temperature polariton condensate realized in a microcavity with embedded GaN quantum wells emits linearly polarized light at threshold with the plane of polarization pinned to one of the crystallographic axes. With increasing pumping power, a depinning of the polarization is observed resulting in a progressive decrease of the polarization degree of the emitted light. This depinning is understood in terms of polariton-polariton repulsion competing with the static disorder potential effect. The polarization behavior differs from that of conventional lasers where the polarization degree usually increases as a function of pumping power.