Ref HAL: hal-02003644_v1
Ref Arxiv: 1801.03304
DOI: 10.1103/PhysRevApplied.9.064031
WoS: WOS:000435552500005
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34 Citations
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We report on the conversion to telecom wavelength of single photons emitted by a nitrogen-vacancy (NV) defect in diamond. By means of difference frequency generation, we convert spin-selective photons at 637 nm, associated with the coherent NV zero-phonon-line, to the target wavelength of 1588 nm in the L-telecom band. The successful conversion is evidenced by time-resolved detection revealing a telecom photon lifetime identical to that of the original 637 nm photon. Furthermore, we show by second-order correlation measurements that the single-photon statistics are preserved. The overall efficiency of this one-step conversion reaches 17\% in our current setup, along with a signal-to-noise ratio of $\approx$7 despite the low probability $(< 10^{-3})$ of an incident 637 nm photon. This result shows the potential for efficient telecom photon - NV center interfaces and marks an important step towards future long-range entanglement-based quantum networks.

Commentaires: . Réf Journal: Phys. Rev. Applied 9, 064031 (2018)

Ref HAL: hal-01891681_v1
DOI: 10.1103/PhysRevB.98.165302
WoS: 000446400600002
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4 Citations
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The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

Ref HAL: hal-01885407_v1
DOI: 10.1103/PhysRevB.98.144101
WoS: 000446135500002
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3 Citations
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Photonic topological insulator plates violate Lorentz reciprocity, which leads to a directionality of surfaceguided modes. This in-plane directionality can be imprinted via an applied magnetic field. On the basis of macroscopic quantum electrodynamics in nonreciprocal media, we show that two photonic topological insulator surfaces are subject to a tunable, magnetic-field-dependent Casimir torque. Due to the directionality, this torque exhibits a unique 2π periodicity, in contradistinction to the Casimir torques encountered for reciprocal uniaxial birefringent media or corrugated surfaces which are π periodic. Remarkably, the torque direction and strength can be externally driven in situ by simply applying a magnetic field on the system, and we show that this can be exploited to induce a control of the rotation of small objects. Our predictions are relevant for nanooptomechanical experiments and devices.

Ref HAL: hal-01883367_v1
DOI: 10.1103/PhysRevB.98.125146
WoS: 000445969100002
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8 Citations
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We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagnetic environment described by the most general linear constitutive relations. The atom is modeled as a two-level system prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and under the Markov approximation, we show that the optical torque has a resonant and nonresonant part, associated, respectively, with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum, which can both be written explicitly in terms of the system Green function. Our formulation is valid for any three-dimensional inhomogeneous, dissipative, dispersive, nonreciprocal, and bianisotropic structure. We apply this general theory to a scenario in which the atom interacts with a material characterized by strong nonreciprocity and modal unidirectionality. In this case, the main decay channel of the atom energy is represented by the unidirectional surface waves launched at the nonreciprocal material-vacuum interface. To provide relevant physical insight into the role of these unidirectional surface waves in the emergence of nontrivial optical torque, we derive closed-form expressions for the induced torque under the quasistatic approximation. Finally, we investigate the equilibrium states of the atom polarization, along which the atom spontaneously tends to align due to the action of the torque. Our theoretical predictions may be experimentally tested with cold Rydberg atoms and superconducting qubits near a nonreciprocal material. We believe that our general theory may find broad application in the context of nanomechanical and biomechanical systems.

Ref HAL: hal-01871322_v1
DOI: 10.1063/1.5047792
WoS: WOS:000444037000006
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27 Citations
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We study the optical properties of MoS2 nanotubes (NTs) with walls comprising dozens of monolayers. We reveal strong peaks in micro-photoluminescence (l-PL) spectra when detecting the light polarized along the NT axis. We develop a model describing the optical properties of the nanotubes acting as optical resonators which support the quantization of whispering gallery modes inside the NT wall. The experimental observation of the resonances in l-PL allows one to use them as a contactless method of the estimation of the wall width. Our findings open a way to use such NTs as polarization-sensitive components of nanophotonic devices.

Ref HAL: hal-01864295_v1
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The Casimir force arises from the quantum fluctuations of the electromagnetic field. It leads to an attraction between electrically neutral bodies with a vacuum gap that be- comes measureable at nanoscale separations. Under the trend of miniaturization, such quantum electrodynamical effects are expected to play an important role in nanomechanical devices. One remarkable property of the Casimir force is its non-trivial dependence on the shape of the in- teracting bodies. Experiments using the corrugated surface of gratings have demonstrated the deviation of the Casimir force from the proximity force approximation. In these experiments, it was necessary to choose one of the bodies to be a sphere to circumvent alignment difficulties.Here, we present measurement of the Casimir force gradient between two microfabricated silicon beams, both of which contain rectangular corrugations. One of the beams acts as the forcesensing element. As it vibrates in a perpendicular magnetic field, a back electromotive force is generated and the corresponding change in the current is measured. The force gradient exerted on this beam is measured from the resonance frequency shift. The distance to the other beam is controlled using a comb actuator integrated on the same substrate, where electrostatic forces push the second beam towards the force-sensing beam. By using lithography to define the structures, they are aligned to allow the two gratings to interpenetrate when the separation between them is reduced. Our data shows a number of novel features, including strong deviations of the force gradient from the proximity force approximation and a non-zero, distance-independent Casimir force over certain range of displacement.We will also discuss the design of a bridge to measure the difference in Casimir forces on two types of surfaces. By fabricating an additional sensing beam next to the original one and measuring their resonant frequency shifts simultaneously in the same experimental run, the difference in the Casimir force gradient of two different geometries can be compared.

Ref HAL: hal-01863901_v1
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Metasurfaces are planar metamaterials that consist of a single or a few stack of subwavelength thickness metal-dielectric layers. They could be periodically structured or not with subwavelength scale patterns according to the transverse directions. The strong interac- tion between an electromagnetic field components and these surfaces, exhibits some properties that could not be found in nature. These artificial properties strongly depend on the shape and arrangement of the elementary patterns and they are often linked to a plasmon resonance phe- nomenon. Metasurfaces working in the visible range generally consist in periodical arrangments of plasmonic resonators. These resonators are inherently muti-scale, as their responses relie on the excitation of resonances in very small gaps, like in the case of gap-plasmon resonators [1] or for bow-tire antennas. The simulation of their electromagnetic response can be very challeng- ing and may be successfully treated thanks to a modal method holding the complexity of the patterns shape. Here we present a polynomial modal method [2–5] that is particularly suited for the simulation of metallic structures. Advanced coordinates transformation such as matched coordinates and tilted coordinates are included in order to hold efficiently the complexity of the geometry without any approximation. Such a tool even offers the possibility to control the way the resonators are periodically arranged.