Ref HAL: hal-01726096_v1
DOI: 10.1209/0295-5075/120/60006
WoS: 000426262900001
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13 Citations
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We propose a system made of three quantum harmonic oscillators as a compact quantum engine for producing mechanical work. The three oscillators play respectively the role of the hot bath, the working medium and the cold bath. The working medium performs an Otto cycle during which its frequency is changed and it is sequentially coupled to each of the two other oscillators. As the two environments are finite, the lifetime of the machine is finite and after a number of cycles it stops working and needs to be reset. Remarkably, we show that thismachine can extract more than 90% of the available energy during 70 cycles. Differently from usually investigated infinite-reservoir configurations, this machine allows the protection of induced quantum correlations and we analyse the entanglement and quantum discord generated during the strokes. Interestingly, we show that high work generation is always accompanied by large quantum correlations. Our predictions can be useful for energy management at the nanoscale, and can be relevant for experiments with trapped ions and experiments with light in integrated optical circuits.

Ref HAL: hal-01714025_v1
DOI: 10.1103/PhysRevA.97.022509
WoS: 000425489100009
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22 Citations
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We theoretically study the Casimir-Polder force on an atom in an arbitrary initial state in a rather general electromagnetic environment wherein the materials may have a nonreciprocal bianisotropic dispersive response. It is shown that under the Markov approximation the force has resonant and nonresonant contributions. We obtain explicit expressions for the optical force both in terms of the system Green function and of the electromagnetic modes. We apply the theory to the particular case wherein a two-level system interacts with a topological gyrotropic material, showing that the nonreciprocity enables exotic light-matter interactions and the opportunity to sculpt and tune the Casimir-Polder forces on the nanoscale. With a quasistatic approximation, we obtain a simple analytical expression for the optical force and unveil the crucial role of surface plasmons in fluctuation-induced forces. Finally, we derive the Green function for a gyrotropic material half-space in terms of a Sommerfeld integral.

Ref HAL: hal-01712338_v1
DOI: 10.1103/PhysRevMaterials.2.024001
WoS: 000424196500001
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17 Citations
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The giant birefringence of layered h-BN was demonstrated by analyzing the interference patterns in reflectance and transmittance measurements in the mid-infrared to the deep ultraviolet energy range. The refractive index for polarization perpendicular to the c axis is much higher than the refractive index for polarization parallel to the c axis, and it displays a strong increase in the ultraviolet range that is attributed to the huge excitonic effects arising from the unique electronic structure of h-BN. Thus, h-BN is shown to exhibit a giant negative birefringence that ranges from −0.7 in the visible to −2 in the deep ultraviolet close to the band gap. The electronic dielectric constants for polarization perpendicular and parallel to the c axis were determined to be ε∞⊥ = 4.95 and ε∞∥ = 2.86, respectively. The anisotropy we find in high-quality h-BN is significantly larger than proposed in previous experimental studies but in excellent agreement with ab initio calculations.

Ref HAL: hal-01700054_v1
DOI: 10.1016/j.spmi.2017.12.029
WoS: 000425199200016
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5 Citations
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The optical properties of Al0.1Ga0.9N nanostructures grown by molecular beam epitaxy on Al0.5Ga0.5N (0001) templates are investigated. By combining morphological and photo- luminescence (PL) characterizations, we have performed an in-depth analysis of the nanostructures properties as a function of the deposited amount. It is shown that: 1) the nanostructures present an asymmetrical height distribution, and a variation in their shape (i.e. both symmetric and elongated nanostructures are observed); 2) the main PL emission is found in the UV range (above 3.6 eV); and 3) a broad emission in the near UV-blue range is observed. These results allowed to attribute the main PL peak to nanostructures with properties in close agreement to the nominal Al0.1Ga0.9N concentration and deposited amount. Concerning the broad PL band at lower energy, it has been correlated to the formation of an additional type of nanostructures with larger size and lower Al compo- sition compared to the Al0.1Ga0.9N ones.

Ref HAL: hal-01700051_v1
DOI: 10.1016/j.spmi.2017.10.030
WoS: 000425566100013
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3 Citations
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In this work, we investigate the impact of the quantum confined Stark effect and of the carrier localization on the internal quantum efficiency of polarized single or multiple InxGa1-xN/GaN quantum well(s), and semi-polar (11e22) multiple InxGa1-xN/InyGa1-yN quantum well. We find that increasing the influence of the quantum confined Stark effect at constant indium content with increasing the well-width induces a reduction of the internal quantum efficiency onsets for a decreasing value of the photoexcitation density. Similar but no so dramatic trend is reported when increasing the indium content and thus when increasing the localization of carriers to localized fluctuations of the chemical composition of the alloy. In addition, a change of the electric field internal to active layers (quantified by using time-resolved photoluminescence spectroscopy) realized by growing samples along a semi-polar orientation leads to experimental observation of a substantial enhancement of the threshold of photoexcitation density at which onsets the reduction of the internal quantum efficiency. A correlation is found through several orders of magni- tude between the photoexcitation density PT for the onset of the collapse of IQE and the values of the photoluminescence radiative decay time trad. A scaling law is found in the investigated samples: PT ~ trad-n with n 1⁄4 3/2 ± 0.15 which evidences that quantum confined Stark effect is the main origin for the efficiency droop in nitride light-emitting diodes based on InxGa1-xN active layers.

DOI: 10.1103/PhysRevB.97.041301
WoS: WOS:000419471200001
5 Citations
Résumé:

We develop a method of nonperturbative optical control over adiabatic remagnetization of the nuclear spin system and apply it to verify the spin temperature concept in GaAs microcavities. The nuclear spin system is shown to exactly follow the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization. These findings open a way for the deep cooling of nuclear spins in semiconductor structures, with the prospect of realizing nuclear spin-ordered states for high-fidelity spin-photon interfaces.

Ref HAL: hal-01698220_v1
Ref Arxiv: 1708.05238
DOI: 10.1103/PhysRevB.97.035303
WoS: WOS:000419615100007
Ref. & Cit.: NASA ADS
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15 Citations
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We addressed the carrier dynamics in so-called G-centers in silicon (consisting of substitutional-interstitial carbon pairs interacting with interstitial silicons) obtained via ion implantation into a silicon-on-insulator wafer. For this point defect in silicon emitting in the telecommunication wavelength range, we unravel the recombination dynamics by time-resolved photoluminescence spectroscopy. More specifically, we performed detailed photoluminescence experiments as a function of excitation energy, incident power, irradiation fluence and temperature in order to study the impact of radiative and non-radiative recombination channels on the spectrum, yield and lifetime of G-centers. The sharp line emitting at 969 meV ($\sim$1280 nm) and the broad asymmetric sideband developing at lower energy share the same recombination dynamics as shown by time-resolved experiments performed selectively on each spectral component. This feature accounts for the common origin of the two emission bands which are unambiguously attributed to the zero-phonon line and to the corresponding phonon sideband. In the framework of the Huang-Rhys theory with non-perturbative calculations, we reach an estimation of 1.6$\pm$0.1 $\angstrom$ for the spatial extension of the electronic wave function in the G-center. The radiative recombination time measured at low temperature lies in the 6 ns-range. The estimation of both radiative and non-radiative recombination rates as a function of temperature further demonstrate a constant radiative lifetime. Finally, although G-centers are shallow levels in silicon, we find a value of the Debye-Waller factor comparable to deep levels in wide-bandgap materials. Our results point out the potential of G-centers as a solid-state light source to be integrated into opto-electronic devices within a common silicon platform.

Commentaires: . Réf Journal: Phys. Rev. B 97, 035303 (2018)