Ref HAL: hal-02275857_v1
DOI: 10.1038/s41467-019-11487-0
WoS: WOS:000478576500011
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8 Citations
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InSe is a promising material in many aspects where the role of excitons is decisive. Here we report the sequential appearance in its luminescence of the exciton, the biexciton, and the P-band of the exciton-exciton scattering while the excitation power increases. The strict energy and momentum conservation rules of the P-band are used to reexamine the exciton binding energy. The new value >= 20 meV is markedly higher than the currently accepted one (14 meV), being however well consistent with the robustness of the excitons up to room temperature. A peak controlled by the Sommerfeld factor is found near the bandgap (similar to 1.36 eV). Our findings supported by theoretical calculations taking into account the anisotropic material parameters question the pure three-dimensional character of the exciton in InSe, assumed up to now. The refined character and parameters of the exciton are of paramount importance for the successful application of InSe in nanophotonics.

Ref HAL: hal-02272702_v1
Ref Arxiv: 1902.02974
DOI: 10.1021/acs.nanolett.9b00914
WoS: 000481563800013
Ref. & Cit.: NASA ADS
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1 Citation
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Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of dipolar excitons. We demonstrate the in-plane confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface. This result is a prerequisite for the electrical control of the exciton densities and fluxes, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Ref HAL: hal-02267129_v1
DOI: 10.1103/PhysRevB.100.085426
WoS: 000481468900005
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12 Citations
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Metasurfaces, the two-dimensional (2D) counterpart of metamaterials, have recently attracted a great deal ofattention due to their amazing properties, including negative refraction, hyperbolic dispersion, and manipulationof the evanescent spectrum. In this work, a theory model is proposed for the near field radiative heat transfer(NFRHT) between two nanoparticles in the presence of an anisotropic metasurface. Specifically, the metasurfaceis modeled as an array of graphene strips (GS), which is an ideal platform to implement any metasurfacetopology, ranging from isotropic to hyperbolic propagation. The NFRHT between two nanoparticles aresignificantly amplified when they are placed in the proximity of the GS, and regulated over several ordersof magnitude. In this configuration, the anisotropic surface plasmon polaritons (SPPs) supported by the GSare excited and provide a new channel for the near-field energy transport. The dependence of conductancebetween two nanoparticles on the orientation, the structure parameters, the chemical potential of the GS, andthe interparticle or the particle-surface distances are analyzed by clearly identifying the characteristics of theanisotropic SPPs such as dispersion relations, propagation length, and decay length. These results demonstrate apowerful method to regulate the energy transport in particle systems, and create a way to explore the anisotropicoptical properties of the metasurface based on the measured heat transfer properties.

Ref HAL: hal-02192575_v1
DOI: 10.1021/acs.jpcc.9b04582
WoS: 000476693800050
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6 Citations
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We investigate the highly anisotropic behavior of the in-plane and out-of-plane infrared-active phonons of hexagonal boron nitride by means of infrared reflectivity and absorption measurements under high pressure. Infrared reflectivity spectra at normal incidence on high-quality single crystals show strict fulfillment of selection rules and an unusually long E1u[transverse-optic (TO)] phonon lifetime. Accurate values of the dielectric constants at ambient pressure ε⊥0 = 6.96, ε⊥∞ = 4.95, ε∥0 = 3.37, and ε∥∞ = 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A2u phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30° allow us to observe both the transverse- and longitudinal-optic A2u modes. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab initio calculations. While Grüneisen parameters are generally small in this layered crystal, the A2u(TO) displays an exceptionally large and negative Grüneisen parameter that results in widening of the type I hyperbolic region with increasing pressure. Softening of the A2u(TO) mode is induced by dynamical buckling of the flat honeycomb layers.

Ref HAL: hal-02192421_v1
DOI: 10.1103/PhysRevLett.123.017401
WoS: WOS:000473540500009
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3 Citations
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We report on the potential of a new spin noise spectroscopy approach by demonstrating all-optical probing of spatiotemporal spin fluctuations. This is achieved by homodyne mixing of a spatially phase-modulated local oscillator with spin-flip scattered light, from which the frequency and wave vector dependence of the spin noise power is unveiled. As a first application of the method we measure the spatiotemporal spin noise in weakly n-doped CdTe layers, from which the electron spin diffusion constant and spin relaxation rates are determined. The absence of spatial spin correlations is also shown for this particular system.

Ref HAL: hal-02191041_v1
DOI: 10.1038/s41578-019-0124-1
WoS: 000478802800006
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71 Citations
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For more than seven decades, hexagonal boron nitride (hBN) has been employed asan inert, thermally stable engineering ceramic; since 2010, it has also been used as the optimal substrate for graphene in nanoelectronic and optoelectronic devices. Recent research has revealed that hBN exhibits a unique combination of optical properties that enable novel (nano) photonic functionalities. Specifically, hBN is a natural hyperbolic material in the mid-IR range,in which photonic material options are sparse. Furthermore, hBN hosts defects that can be engineered to obtain room-temperature, single-photon emission; exhibits strong second-order nonlinearities with broad implications for practical devices; and is a wide-bandgap semiconductor well suited for deep UV emitters and detectors. Inspired by these promising attributes, research on the properties of hBN and the development of large-area bulk and thin-film growth techniques has dramatically expanded. This Review offers a snapshot of current research exploring the properties underlying the use of hBN for future photonics functionalities and potential applications, and covers some of the remaining obstacles.

Ref HAL: hal-02190897_v1
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We show that the energy-transport efficiency in a chain of two-level emitters can be drastically enhancedby the presence of a photonic topological insulator (PTI). This is obtained by exploiting the peculiarproperties of its nonreciprocal surface plasmon polariton (SPP), which is unidirectional, and immune tobackscattering, and propagates in the bulk band gap. This amplification of transport efficiency can be asmuch as 2 orders of magnitude with respect to reciprocal SPPs. Moreover, we demonstrate that despite thepresence of considerable imperfections at the interface of the PTI, the efficiency of the SPP-assisted energytransport is almost unaffected by discontinuities. We also show that the SPP properties allow energytransport over considerably much larger distances than in the reciprocal case, and we point out aparticularly simple way to tune the transport. Finally, we analyze the specific case of a two-emitter chainand unveil the origin of the efficiency amplification. The efficiency amplification and the practicaladvantages highlighted in this work might be particularly useful in the development of new devicesintended to manage energy at the atomic scale.