Ref HAL: hal-03736745_v1
DOI: 10.5194/jsss-9-117-2020
WoS: 000522157200001
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Nuclear magnetic resonance imaging (MRI) has became an unavoidable medical tool in spite of its poor sensitivity. This fact motivates the efforts to enhance the nuclear magnetic resonance (NMR) probe performance. Thus, the nuclear spin excitation and detection, classically performed using radio-frequency coils, are required to be highly sensitive and homogeneous. The space harmonics suppression (SHS) method, already demonstrated to construct coil producing homogenous static magnetic field, is used in this work to design radio-frequency coils. The SHS method is used to determine the distribution of the electrical conductive wires which are organized in a saddle-coil-like configuration. The theoretical study of these SHS coils allows one to expect an enhancement of the signal-to-noise ratio with respect to saddle coil. Coils prototypes were constructed and tested to measure 1H NMR signal at a low magnetic field (8 mT) and perform MRI acquired at a high magnetic field (3 T). The signal-to-noise ratios of these SHS coils are compared to the one of saddle coil and birdcage (in the 3 T case) of the same size under the same pulse sequence conditions demonstrating the performance enhancement allowed by the SHS coils.

Ref HAL: hal-02302031_v1
DOI: 10.1002/mrc.4905
WoS: WOS:000481829600022
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Today, in the presence of global warming, understanding how plants respond to drought stress is essential to meet the challenge of developing new cultivars and new irrigation strategies, consistent with the maintenance of crop productivity. In this context, the study of the relation between plants and water is of central interest for modeling their responses to biotic and abiotic constraints. Paradoxically, there are very few direct and noninvasive methods to quantify and measure the level and the flow of water in plants. The present work aims to develop a noninvasive methodology for living plant based on nuclear magnetic resonance (NMR) at low magnetic field and imaging (MRI) to tackle the issue of water quantity in plants. For this purpose, a portable NMR device measuring the signal level at 8 mT was built. This instrument addresses specific challenges such as miniaturization, accessibility, and overheating in order to maintain the plant intact of time over long period. Time dependence of the water content in sorghum plants is reported under abiotic stress as well as the fraction of transpirable soil water and the photosynthesis activity through the leaves. At high magnetic field (9.4 T), T2 maps were acquired on the same sorghum plants at two time points. The combination of these approaches allows us to identify ecophysiological biomarkers of drought stress. One particular interesting result concerns the spatial distribution of water in two anatomically contrasted sorghum genotypes.

Ref HAL: hal-01819285_v1
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Ref HAL: hal-01308230_v1
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In this work we study the spatial distribution of photon modes confined in a 0D cavity and a 1D W1 on AlN-based photonic crystal membranes by spectrally resolved confocal microscopy in the ultra-violet spectral range. The analysis of the influence of the fabrication-induced disorder of the PC on the photon modes show that the cavity modes are robust with respect to disorder, whereas the 1D modes of the W1 waveguide are localized near its cut-off frequency. Modifications of the photon local density of states will be compared to simulations of weakly disordered photonic structures.

Ref HAL: hal-00916331_v1
DOI: 10.1364/OL.38.005059
WoS: 000327815100034
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4 Citations
Résumé:

The spatial distribution of photon modes confined in a 0D cavity and a 1D W1 waveguide is investigated on AlN-based photonic crystal (PC) membranes by spectrally resolved scanning confocal microscopy in the ultra-violet spectral range. The influence of the fabrication-induced disorder of the PC on the photon modes is analyzed. The cavity modes are shown to be robust with respect to disorder, whereas the 1D modes of the W1 waveguide are localized near its cut-off frequency. Those modifications of the lowest energy photonic modes are compared to simulations of weakly disordered photonic structures.

Ref HAL: hal-00389559_v1
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Les boîtes quantiques GaN présentent des propriétés de confinement très attrayantes, jusqu'à 300K. De nombreux développements récents ont montré qu'elles peuvent se comparer favorablement aux boîtes quantiques antérieures et mieux maîtrisées, InAs et CdTe en particulier. Mais l'étude de la structure fine des complexes excitoniques dans les boîtes GaN n'a jusqu'à présent pas été abordée. Nous présentons les propriétés de polarisation, à basse température, de boîtes isolées GaN/AlN. La photoluminescence est fortement polarisée linéairement, jusqu'à 90%, suivant des directions non corrélées aux axes cristallins. Nous n'observons pas de dédoublement du doublet radiatif. Nous interprétons ces résultats dans le cadre d'un modèle matriciel, incluant toutes les symétries pertinentes, qui permet une comparaison entre les systèmes GaN, InAs et CdTe. Nous montrons que la forte polarisation est liée à la proximité des bandes de valence A et B de GaN, et nous proposons que l'absence de doublet soit la signature d'un fort splitting de structure fine, ou bien de l'émission d'excitons chargés.

Commentaires: oral

Ref HAL: hal-00261181_v3
Ref Arxiv: 0803.0899
DOI: 10.1103/PhysRevB.77.235315
WoS: 000257289500082
Ref. & Cit.: NASA ADS
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63 Citations
Résumé:

We report micro-photoluminescence studies of single GaN/AlN quantum dots grown along the (0001) crystal axis by molecular beam epitaxy on Si(111) substrates. The emission lines exhibit a linear polarization along the growth plane, but with varying magnitudes of the polarization degree and with principal polarization axes that do not necessarily correspond to crystallographic directions. Moreover, we could not observe any splitting of polarized emission lines, at least within the spectral resolution of our setup (1 meV). We propose a model based on the joint effects of electron-hole exchange interaction and in-plane anisotropy of strain and/or quantum dot shape, in order to explain the quantitative differences between our observations and those previously reported on, e.g. CdTe- or InAs-based quantum dots.