We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of 29 microJ cm-2, 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantum-well surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of +/-5 degrees and spatial size around 5 microm.

We report on the current properties of Al1−xInxN (x ≈ 0.18) layers lattice-matched (LM) to GaN and their specific use to realize nearly strain-free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state-of-the-art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of (1–5) × 1018 cm−3 and a large Stokes shift (∼800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified through the properties of GaN/AlInN multiple quantum wells (QWs) suitable for near-infrared intersubband applications. A built-in electric field of 3.64 MV cm−1 solely due to spontaneous polarization is deduced from photoluminescence measurements carried out on strain-free single QW heterostructures, a value in good agreement with that deduced from theoretical calculation. Other potentialities regarding optoelectronics are demonstrated through the successful realization of crack-free highly reflective AlInN/GaN distributed Bragg reflectors (R > 99%) and high quality factor microcavities (Q > 2800) likely to be of high interest for short wavelength vertical light emitting devices and fundamental studies on the strong coupling regime between excitons and cavity photons. In this respect, room temperature (RT) lasing of a LM AlInN/GaN vertical cavity surface emitting laser under optical pumping is reported. A description of the selective lateral oxidation of AlInN layers for current confinement in nitride-based light emitting devices and the selective chemical etching of oxidized AlInN layers is also given. Finally, the characterization of LM AlInN/GaN heterojunctions will reveal the potential of such a system for the fabrication of high electron mobility transistors through the report of a high two-dimensional electron gas sheet carrier density (ns ∼ 2.6 × 1013 cm−2) combined with a RT mobility μe ∼ 1170 cm2 V−1 s−1 and a low sheet resistance, R ∼ 210 Ω/□.

We observe the build up of strong (∼50%) spontaneous vector polarisation in emission from a GaN-based polariton laser excited by short optical pulses at room temperature. The Stokes vector of emitted light changes its orientation randomly from one excitation pulse to another, so that the timeintegrated polarisation remains zero. This behaviour is completely different to any previous laser. We interpret this observation in terms of the spontaneous symmetry breaking in a Bose-Einstein condensate of exciton-polaritons.PACS numbers: 71.36.+c, 03.75.Kk, Polariton lasers are coherent light sources based on emission of light from a coherent ensemble of excitonpolaritons -the mixed light-exciton quasiparticles in semiconductor microcavities. The concept of polariton lasing was first proposed in 1996 [1], followed a few years later by reports of coherent polariton emission in microcavities [2,3,4]. Recently, we reported polariton lasing at room temperature in GaN-based microcavities [5]. Apart from being very promising for applications, the concept of polariton lasing involves several fundamental physics issues. Contrary to conventional lasers, polariton lasers emit coherent and monochromatic light spontaneously. This is achieved when mixed light-matter quasiparticles (exciton-polaritons), Bosecondense inside a semiconductor microcavity. BoseEinstein condensation (BEC) of the polaritons is a subject of intense experimental and theoretical research at present. Several experimental works claiming polariton BEC have appeared recently [6,7,8]. Though polariton BEC implies polariton lasing these two phenomena are not identical. For polariton lasing a macroscopically populated quantum state of exciton polaritons must be created, which can be considered as a polariton condensate. Polariton lasing does not require thermal equilibrium in the system or the spontaneous build-up of the order parameter, which are the main criteria for BEC when understood as a thermodynamic phase transition. Which experimental measurement should be considered as decisive proof for the exciton-polariton BEC is still a subject of debate within the community. Thermalisation of the exciton-polaritons detected by angle-resolved photoluminescence (PL) measurements has been considered one of the key criteria for a long time [6,7]. However, a similar angular dependence of the PL has also been observed in GaAs-based photon lasers [9]. The spatial coherence of polariton emission demonstrated in Ref.[6] is characteristic for conventional lasers as well. Recent theoretical work suggests that observation of the spontaneous buildup of the vector polarisation in emission from polariton lasers would be evidence for the spontaneous symmetry breaking in the system [10,11,12]. In turn, spontaneous symmetry breaking is considered to be a smoking gun for BEC ever since the pioneering work of Goldstone [13,14].Here we report observations of the build up of the spontaneous vector polarization at room temperature in bulk GaN microcavities. Unlike the recent low temperature e...

Laser action with low threshold average pump power density ( $ 50 W Á cm À2 ) at room temperature is reported for a crack-free planar vertical cavity surface emitting laser (VCSEL) structure based on a bottom lattice-matched AlInN=GaN distributed Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n-and p-type GaN layers surrounding only three InGaN=GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL. In addition to low threshold, a spontaneous emission coupling factor b $ 2 Â 10 À3 is derived for this ready-to-be-processed laser structure.In recent years III-nitride semiconductor optoelectronic devices have undergone tremendous development with the progressive commercialisation of blue-violet and white light emitting diodes and that of blueviolet edge-emitting laser diodes (LDs) [1]. However, demonstration of an electrically pumped compact laser source such as vertical cavity surface emitting lasers (VCSELs) has not been reported. Recent advances in the growth of high quality epitaxially-grown nitridebased distributed Bragg reflectors (DBRs) have allowed for significant improvement of the quality factor Q( ¼ l=Dl) of planar microcavity (MC) structures [2] with the recent demonstration of Q values up to 2800 in a crack-free empty lattice-matched (LM) AlInN=GaN-based MC [3]. Lasing in planar nitride VCSEL structures under optical pumping has been reported by several groups [2, 4-7] but usually the active region consists of more than ten quantum wells (QWs) to increase modal gain and overcome optical losses. Actually, in electrically pumped LDs there is usually a trade-off between modal gain and threshold current which leads to an optimum number of wells between three and five. So far there is, to the best of our knowledge, only one report of room-temperature lasing action in an optically pumped nitride VCSEL structure containing less than ten QWs, namely in a 4l AlGaN cavity with three InGaN QWs sandwiched between two high reflectivity (R $ 99%) dielectric (SiO 2 =ZrO 2 ) DBRs [8]. However, the realisation of the latter structure requires lengthy and advanced processing steps which are not compatible with an electrical injection scheme.In this Letter we report on the blue-violet lasing action, at $ 422 nm, under optical pumping with a low average threshold power density of 50 W Á cm À2 in a crack-free planar hybrid 5l=2 GaN MC containing three InGaN QWs with a bottom LM AlInN=GaN DBR and a top dielectric (SiO 2 =Si 3 N 4 ) DBR. The cavity region has both n-and p-type regions as well as an AlGaN electron blocking layer on the p side making such a planar design in all points identical to that of a real structure ready to be processed to fabricate mesas and deposit electrical contacts. A spontaneous emission coupling factor b $ 2 Â 10 À3 is derived from the input-output characteristics for this VCSEL structure.The nitride vertical cavity structure was grown by metal organic vapour phase epitaxy (MOVPE) on a 2-inch c-plane sapphire substrate in a...

A bulk scale process is implemented for the production of nanostructured film composites comprising unary or multi-component metal oxide nanoparticles dispersed in a suitable polymer matrix. The as-received nanoparticles, namely Al$$_2$$ 2 O$$_3$$ 3 , SiO$$_2$$ 2 and TiO$$_2$$ 2 and binary combinations, are treated following specific chemical and mechanical processes in order to be suspended at the optimal size and composition. Subsequently, a polymer extrusion technique is employed for the fabrication of each film, while the molten polymer is mixed with the treated metal oxide nanoparticles. Transmission and reflection measurements are performed in order to map the optical properties of the fabricated, nanostructured films in the UV, VIS and IR. The results substantiate the capability of the overall methodology to regulate the optical properties of the films depending on the type of nanoparticle formation which can be adjusted both in size and composition.