Ref HAL: hal-01573934_v1
DOI: 10.1063/1.4997608
WoS: 000407742400003
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9 Citations
Résumé:

The variation of the internal quantum efficiency (IQE) of single InGaN quantum well structures emitting from blue to red is studied as a function of the excitation power density and the temperature. By changing the well width, the indium content, and adding a strain compensation AlGaN layer, we could tune the intrinsic radiative recombination rate by changing the quantum confined Stark effect, and we could modify the carrier localization. Strong quantum confined Stark effect and carrier localization induce an increase in the carrier density and then favor Auger non-radiative recombination in the high excitation range. In such high excitation conditions with efficient Auger recombination, the variation of the IQE with the photo-excitation density P is ruled by a universal power law independent of the design: IQE = IQEMAX – a log10P with a close to 1/3. The temperature dependences of the different recombination mechanisms are determined. At low temperature, both quantum confined Stark effect and carrier localization trigger electron-electron repulsions and therefore the onset of the Auger effect. The increase in the value of coefficient C with changing temperature reveals indirect Auger recombination that relates to the interactions of the carriers with other phonons than the longitudinal optical one.

Ref HAL: hal-01570566_v1
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We calculate the radiative heat transfer between two metallic gratings by exploiting the Adaptive Spatial Resolution metod. This technique dramatically improves the rate of convergence allowing to explore smaller separations. The heat flux shows a remarkable amplification of the exchanged energy, due to spoof-plasmon modes. We find a consistent disagreement with some previously obtained results going up to 50% (this disagreement is explained in terms of an incorrect connection between the reflection operators of the two gratings).

Ref HAL: hal-01570553_v1
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Résumé:

The Fourier Modal Method (FMM) is used to obtain the band structure of a 1D graphene based photonic crystal. The structure consists of graphene layers periodically inlayed in a homogeneous dielectric medium. In the model, the graphene sheet is considered as layer with atomic thickness. Under these conditions, we show that it is possible to use the FMM in order to obtain a polynomial eigenvalue problem allowing the computation of the band structure.

Ref HAL: hal-01570545_v1
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The Fourier Modal Method equipped with the concept of Adaptive Spatial Resolution (FMMASR) is derived and presented, in details, in the case of lamellar diffraction gratings under conical mounting. In the present work, we focus on efficiency and reduction of the numerical load.

Ref HAL: hal-01562859_v1
DOI: 10.1103/PhysRevB.95.245209
WoS: WOS:000404020500004
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5 Citations
Résumé:

In III-V semiconductors it was shown theoretically that under optical cooling the nuclear-spin polaron bound to neutral donors would form below some critical nuclear-spin temperature Tc [Merkulov, Phys. Solid State 40, 930 (1998)]. The predicted critical behavior is a direct consequence of the use of the mean-field approximation. It is known however that in any finite-size system a critical behavior must be absent. Here we develop a model of the optically cooled nuclear polaron, which goes beyond the mean-field approximation. An expression of the generalized free energy of the optically cooled nuclear polaron, valid for a finite, albeit large, number of spins, is derived. This model permits us to describe the continuous transition from the fluctuation dominated regime to the collective regime, as the nuclear-spin temperature decreases. It is shown that due to the finite number of nuclear spins involved in the polaron, the critical effects close to Tc are smoothed by the spin fluctuations. Particularly, instead of a divergence, the nuclear-spin fluctuations exhibit a sharp peak at Tc, before being depressed well below Tc. Interestingly, the formation of the nuclear polaron can, in certain conditions, boost the nuclear polarization beyond the value obtained solely by optical pumping. Finally, we suggest that the nuclear polaron could be detected by spin noise spectroscopy or via its superparamagnetic behavior.

Ref HAL: hal-01562694_v1
DOI: 10.1038/ncomms15765
WoS: WOS:000402870600001
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115 Citations
Résumé:

Magnetic skyrmions are quasiparticle-like textures which are topologically different from other states. Their discovery in systems with broken inversion symmetry sparked the search for materials containing such magnetic phase at room temperature. Their topological properties combined with the chirality-related spin–orbit torques make them interestingobjects to control the magnetization at nanoscale. Here we show that a pair of coupled skyrmions of opposite chiralities can be stabilized in a symmetric magnetic bilayer systemby combining Dzyaloshinskii–Moriya interaction (DMI) and dipolar coupling effects. This opens a path for skyrmion stabilization with lower DMI. We demonstrate in a device withasymmetric electrodes that such skyrmions can be independently written and shifted by electric current at large velocities. The skyrmionic nature of the observed quasiparticles is confirmed by the gyrotropic force. These results set the ground for emerging spintronic technologies where issues concerning skyrmion stability, nucleation and propagation are paramount.

Ref HAL: hal-01562609_v1
Ref Arxiv: 1702.02057
DOI: 10.1364/OE.25.014746
WoS: WOS:000404189800077
Ref. & Cit.: NASA ADS
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12 Citations
Résumé:

Radiative heat transfer between uniform plates is bounded by the narrow range and limited contribution of surface waves. Using a combination of analytical calculations and numerical gradient-based optimization, we show that such a limitation can be overcome in complicated multilayer geometries, allowing the scattering and coupling rates of slab resonances to be altered over a broad range of evanescent wavevectors. We conclude that while the radiative flux between two inhomogeneous slabs can only be weakly enhanced, the flux between a dipolar particle and an inhomogeneous slab—proportional to the local density of states—can be orders of magnitude larger, albeit at the expense of increased frequency selectivity. A brief discussion of hyperbolic metamaterials shows that they provide far less enhancement than optimized inho- mogeneous slabs.