Strong light-matter coupling in bulk GaN-microcavities with double dielectric mirrors fabricated by two different methods

Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on (111) Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al0.2Ga0.8N / AlN Bragg mirror followed by a lambda/2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2,mu m thick GaN layer is grown, and progressively thinned to a final thickness of lambda. Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations. (C) 2010 American Institute of Physics. [doi:10.1063/1.3477450

Localized states in 2D semiconductors doped with magnetic impurities in quantizing magnetic field

A theory of magnetic impurities in a 2D electron gas quantized by a strong magnetic field is formulated in terms of Friedel-Anderson theory of resonance impurity scattering. It is shown that this scattering results in an appearance of bound Landau states with zero angular moment between the Landau subbands. The resonance scattering is spin selective, and it results in a strong spin polarization of Landau states, as well as in a noticeable magnetic field dependence of the $g$ factor and the crystal field splitting of the impurity $d$ levels.Comment: 12 pages, 4 figures Submitted to Physical Review B This version is edited and updated in accordance with recent experimental dat

Influence of the mirrors on the strong coupling regime in planar GaN microcavities

The optical properties of bulk $\lambda/2$ GaN microcavities working in the strong light-matter coupling regime are investigated using angle-dependent reflectivity and photoluminescence at 5 K and 300 K. The structures have an Al$_{0.2}$Ga$_{0.8}$N/AlN distributed Bragg reflector as the bottom mirror and either an aluminium mirror or a dielectric Bragg mirror as the top one. First, the influence of the number of pairs of the bottom mirror on the Rabi splitting is studied. The increase of the mirror penetration depth is correlated with a reduction of the Rabi splitting. Second, the emission of the lower polariton branch is observed at low temperature in a microcavity containing two Bragg mirrors and exibiting a quality factor of 190. Our simulations using the transfer-matrix formalism, taking into account the real structure of the samples investigated are in good agreement with experimental results.Comment: published versio

Resonant light delay in GaN with ballistic and diffusive propagation

We report on a strong delay in light propagation through bulk GaN, detected by time-of-flight spectroscopy. The delay increases resonantly as the photon energy approaches the energy of a neutral-donor bound exciton (BX), resulting in a velocity of light as low as 2100 km/s. In the close vicinity of the BX resonance, the transmitted light contains both ballistic and diffusive components. This phenomenon is quantitatively explained in terms of optical dispersion in a medium where resonant light scattering by the BX resonance takes place in addition to the polariton propagation.Comment: 4 pages, 4 figure

Echium oil is not protective against weight loss in head and neck cancer patients undergoing curative radio(chemo)therapy: a randomised-controlled trial

Background: Therapy-induced mucositis and dysphagia puts head and neck (H&N) cancer patients at increased risk for developing cachexia. Omega-3 fatty acids (n-3 FA) have been suggested to protect against cachexia. We aimed to examine if echium oil, a plant source of n-3 FA, could reduce weight loss in H&N cancer patients undergoing radio(chemo)therapy with curative intent. Methods: In a double-blind trial, patients were randomly assigned to echium oil (intervention (I) group; 7.5 ml bis in die (b.i.d.), 235 mg/ml α-linolenic acid (ALA) + 95 mg/ml stearidonic acid (SDA) + 79 mg/ml γ-linolenic acid (GLA)) or n-3 FA deficient sunflower oil high oleic (control (C) group; 7.5 ml b.i.d.) additional to standard nutritional support during treatment. Differences in percentage weight loss between both groups were analysed according to the intention-to-treat principle. Erythrocyte FA profile, body composition, nutritional status and quality of life were collected. Results: Ninety-one eligible patients were randomised, of whom 83 were evaluable. Dietary supplement adherence was comparable in both groups (median, I: 87%, C: 81%). At week 4, the I group showed significantly increased values of erythrocyte n-3 eicosapentanoic acid (EPA, 14% vs −5%) and n-6 GLA (42% vs −20%) compared to the C group, without a significant change in n-6 arachidonic acid (AA, 2% vs −1%). Intention-to-treat analysis could not reveal a significant reduction in weight loss related to echium oil consumption (median weight loss, I: 8.9%, C: 7.6%). Also, no significant improvement was observed in the other evaluated anthropometric parameters. Conclusions: Echium oil effectively increased erythrocyte EPA and GLA FAs in H&N cancer patients. It failed however to protect against weight loss, or improve nutritional parameters. Trial registration: ClinicalTrials.gov Identifier NCT01596933

EMQN best practice guidelines for genetic testing in dystrophinopathies.

Dystrophinopathies are X-linked diseases, including Duchenne muscular dystrophy and Becker muscular dystrophy, due to DMD gene variants. In recent years, the application of new genetic technologies and the availability of new personalised drugs have influenced diagnostic genetic testing for dystrophinopathies. Therefore, these European best practice guidelines for genetic testing in dystrophinopathies have been produced to update previous guidelines published in 2010.These guidelines summarise current recommended technologies and methodologies for analysis of the DMD gene, including testing for deletions and duplications of one or more exons, small variant detection and RNA analysis. Genetic testing strategies for diagnosis, carrier testing and prenatal diagnosis (including non-invasive prenatal diagnosis) are then outlined. Guidelines for sequence variant annotation and interpretation are provided, followed by recommendations for reporting results of all categories of testing. Finally, atypical findings (such as non-contiguous deletions and dual DMD variants), implications for personalised medicine and clinical trials and incidental findings (identification of DMD gene variants in patients where a clinical diagnosis of dystrophinopathy has not been considered or suspected) are discussed

Studies of 3d4 ions in Al2O3 by cross-relaxation

Measurements are reported of the spin-lattice relaxation time T 1m of the Cr3+ ion in Al2O3 in the presence of either Cr2+ or Mn3+ ions. Both resonant (RCR) and non-resonant (NCR) cross-relaxation processes are identified, the latter in zero magnetic field. By carefully choosing the frequency, RCR to Cr2+ and Mn3+ are clearly visible in a number of samples. Temperature-dependence measurements on the Cr2+-containing crystals give results in agreement with the theoretical calculations detailed here. The theory of NCR is developed and used to extract the energies of the first excited states from the Cr2+ and Mn3+ NCR data. The values obtained are the same as those reported from phonon spectroscopy measurements. Finally, the relation between measurements of T1m and theoretical calculations in any system is discussed in general and discrepancies are accounted for in terms of NCR to either trace impurities or clusters or both.Des mesures du temps de relaxation spin-réseau, T1m, de l'ion Cr3+ dans Al2O3 en présence d'ions Cr2+ ou Mn3+ sont effectuées. Les processus de cross-relaxation résonnante (CRR) et non résonnante (CRN) sont identifiés, les derniers étant étudiés en champ magnétique nul. Si la fréquence est bien choisie, la CRR avec Cr2+ et avec Mn3+ peut être mise clairement en évidence dans différents échantillons. Les mesures de la dépendance en température, dans le cas des cristaux contenant Cr2+ , donnent des résultats en accord avec les calculs théoriques. La théorie de la CRN est développée et utilisée pour déterminer les energies des premiers états excités à partir des courbes expérimentales de CRN avec Cr2+ et Mn3+ . Les valeurs obtenues sont les mêmes que celles données par spectroscopie de phonon. Enfin, la relation entre les mesures de T1m et les calculs théoriques dans un système est examinée dans le cas général et les désaccords sont expliqués par la CRN avec des impuretés ou des groupes d'ions ou avec les deux à la fois

Cross-relaxation in nickel doped rubies

Measurements are reported of the spin-lattice relaxation time T 1m of the Cr3+ ion in Al2O 3 when Ni3+ and Ni2+ ions are also present in very small concentrations. Well-defined reductions in T1 m are correlated with resonant cross-relaxation between Cr 3+ and Ni3+ and between Cr3+ and Ni2+. Non-resonant cross-relaxation (NCR) processes are also studied in zero magnetic field and an exponential dependence on temperature is observed. A detailed analysis enables us to suppose that those Ni3+ ions for which the two lower doublets are split by ~ 6 cm -1 dominate the NCR process. Also reported is the observation of a reduction in T1m when a fast relaxing Ni3+ or Ni2+ ion is resonant with à Cr3+ transition other than that being monitored. Cross-relaxation processes involving three spins — one Cr3+ and two Ni or vice versa — are also discussed. In samples with less than ~ 0.03 at. % Cr, cross-relaxation within the Cr3+ system itself is only important when two or three spins are involved, one of which involves a transition at the monitored frequency. A three-spin Cr3+ process is observed in which all transition frequencies are different.Des mesures du temps de relaxation spin-réseau, T1m de l'ion Cr3+ dans Al2O3 sont rapportées dans le cas où des ions Ni3+ et Ni2+ sont présents en très faibles concentrations. Des réductions bien définies de T1m sont attribuées à la relaxation croisée résonnante entre Cr3+ et Ni3+ et entre Cr3+ et Ni2 +. Les processus de relaxation croisée non résonnante (CRN) sont aussi étudiés en champ magnétique nul et une dépendance exponentielle en température est observée. Une analyse détaillée permet de supposer que les ions Ni3+ pour lesquels les deux doublets les plus bas sont séparés par ~ 6 cm-1 dominent le processus de CRN. Nous montrons aussi que l'on observe une réduction du temps de relaxation, T1 m, mesuré pour Cr3+ dans Al2O3 quand un ion Ni3+ ou Ni2+, qui relaxe vite, résonne avec une transition de Cr3+ différente de celle étudiée. Des processus de relaxation croisée faisant intervenir trois spins — un Cr3+ et deux Ni ou inversement — sont aussi envisagés. Dans les échantillons de concentrations inférieures à 300 x 10-6 ion Cr 3+/ion Al3+ la relaxation croisée à l'intérieur du système Cr3+ lui-même est importante seulement quand deux ou trois spins sont mis en jeu, l'un d'eux faisant intervenir une transition à la fréquence utilisée pour les mesures. Un processus à trois spins pour lequel toutes les fréquences de transition sont différentes est observé

IMPROVED GAINAS GAAS HETEROSTRUCTURES BY HIGH GROWTH-RATE MOLECULAR-BEAM EPITAXY

This work shows that the critical thickness for the two-dimensional-three-dimensional growth mode transition during the growth of Ga0.65In0.35As on GaAs(001) can be significantly increased by increasing the growth rate. This experimental finding is corroborated by a Monte Carlo simulation of this heteroepitaxial growth. Improved quantum well optical properties are demonstrated