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Théorie des Champs & Physique Mathématique
(15) Production(s) de l'année 2021


Multiobjective statistical learning optimization of RGB metalens
Auteur(s): Elsawy Mahmoud, Gourdin A., Binois Mickaël, Duvigneau Régis, Felbacq D., Khadir Samira, Genevet Patrice, Lanteri Stéphane
(Article) Publié:
Acs Photonics, vol. 8 p.2498–2508 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03212349_v2
DOI: 10.1021/acsphotonics.1c00753
Exporter : BibTex  endNote
Résumé: Modelling of multiwavelength metasurfaces relies on adjusting the phase of individual nanoresonators at several wavelengths.The traditional procedure neglects thenearfield coupling between the nanoresonators, which dramatically reduces the overall diffraction efficiency, bandwidth, numerical aperture and device diameter.Anotheralternative design strategy is to combine a numerical optimization technique with fullwave simulations to mitigate this problem and optimize the entire metasurface at once.Here, we present a global multiobjective optimization technique that utilizes statisticallearning method to optimize RGB spherical metalenses at the visible wavelengths. Theoptimization procedure, coupled to a highorder fullwave solver, accounts for the nearfield coupling between the resonators. High numerical aperture RGB lenses(NA= 0.47and NA= 0.56) of 8μm and 10μm diameters are optimized with numerical average1focusing efficiencies of 55% and 45%, with an average focusing error smaller than 6%for the RGB colors. The fabricated and experimentally characterized devices present44.16% and 31.5% respective efficiencies. The reported performances represent thehighest focusing efficiencies for highNA >0.5 RGB metalenses obtained so far. Theintegration of multiwavelength metasurfaces in portable and wearable electronic devices requires high performances to offer a variety of applications ranging from classicalimaging to virtual and augmented reality.



Comment on the paper “Improving Poor Man’s KramersKronig analysis and KramersKronig constrained variational analysis”
Auteur(s): Rousseau E., Izard N., Bantignies J.L., Felbacq D.
(Article) Publié:
Spectrochimica Acta Part A Molecular And Biomolecular Spectroscopy [1994...], vol. 259 p.119849 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03228703_v1
DOI: 10.1016/j.saa.2021.119849
Exporter : BibTex  endNote
Résumé: We clarify some analytical expressions existing in the literature, • Within the correct formulae we conclude that there is no need for an ad hoc improvement on the opposite to the title paper, • We highlight the symmetry properties of the function to be integrated in order to agree with the usual assumptions made to derive the KramersKronig relations, • The analytical formula we provide may be used to increase the accuracy of the "Poor Man's KramersKronig analysis" method and the "KramersKronig constrained variational analysis" method.



Heavenly metrics, BPS indices and twistors
Auteur(s): Alexandrov S., Pioline Boris
(Article) Publié:
Letters In Mathematical Physics, vol. 111 p.116 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03206137_v1
Ref Arxiv: 2104.10540
DOI: 10.1007/s11005021014555
Ref. & Cit.: NASA ADS
Exporter : BibTex  endNote
Résumé: Recently T. Bridgeland defined a complex hyperk\"ahler metric on the tangent bundle over the space of stability conditions of a triangulated category, based on a RiemannHilbert problem determined by the DonaldsonThomas invariants. This metric is encoded in a function $W(z,\theta)$ satisfying a heavenly equation, or a potential $F(z,\theta)$ satisfying an isomonodromy equation. After recasting the RH problem into a system of TBAtype equations, we obtain integral expressions for both $W$ and $F$ in terms of solutions of that system. These expressions are recognized as conformal limits of the `instanton generating potential' and `contact potential' appearing in studies of Dinstantons and BPS black holes. By solving the TBA equations iteratively, we reproduce Joyce's original construction of $F$ as a formal series in the rational DT invariants. Furthermore, we produce similar solutions to deformed versions of the heavenly and isomonodromy equations involving a noncommutative starproduct. In the case of a finite uncoupled BPS structure, we rederive the results previously obtained by Bridgeland and obtain the socalled $\tau$ function for arbitrary values of the fiber coordinates $\theta$, in terms of a suitable twovariable generalization of Barnes' $G$ function.



Canonical structure of minimal varying $\Lambda$ theories
Auteur(s): Alexandrov S., Speziale Simone, Zlosnik Tom
(Article) Publié:
Classical And Quantum Gravity, vol. 38 p.175011 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03197323_v1
Ref Arxiv: 2104.03753
DOI: 10.1088/13616382/ac1852
Ref. & Cit.: NASA ADS
Exporter : BibTex  endNote
Résumé: Minimal varying $\Lambda$ theories are defined by an action built from the EinsteinCartanHolst first order action for gravity with the cosmological constant $\Lambda$ as an independent scalar field, and supplemented by the Euler and Pontryagin densities multiplied by $1/\Lambda$. We identify the canonical structure of these theories which turn out to represent an example of irregular systems. We find five degrees of freedom on generic backgrounds and for generic values of parameters, whereas if the parameters satisfy a certain condition (which includes the most commonly considered Euler case) only three degrees of freedom remain. On de Sitterlike backgrounds the canonical structure changes, and due to an emergent conformal symmetry one degree of freedom drops from the spectrum. We also analyze the selfdual case with an holomorphic action depending only on the selfdual part of the connection. In this case we find two (complex) degrees of freedom, and further discuss the Kodama state, the restriction to de Sitter background and the effect of reality conditions.



Negative cosmological constant in the dark sector?
Auteur(s): Calderon R., Gannouji Radouane, L'Huillier Benjamin, Polarski D.
(Article) Publié:
Physical Review D, vol. 103 p.023526 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03184607_v1
DOI: 10.1103/PhysRevD.103.023526
WoS: WOS:000609014700004
Exporter : BibTex  endNote
Résumé: We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed λ. For such models to be viable, the dark sector should contain an additional component responsible for the latetime accelerated expansion rate (X). We explore the departure of the expansion history of these models from the concordanceΛ cold dark matter (ΛCDM) model. For a large class of our models, the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with wX>−1 will eventually contract and we derive an analytical expression for the scale factor a(t) in the neighborhood of its maximal value. We find also the scale factor for models ending in a Big Rip in the regime where dustlike matter density is negligible compared to λ. We address further the viability of such models, in particular when a high H0 is taken into account. While we find no decisive evidence for a nonzero λ, the best models are obtained with a phantom behavior on redshifts z≳1 with a higher evidence for nonzero λ. An observed value for h substantially higher than 0.70 would be a decisive test of their viability.



Weak gravity on a $\Lambda$CDM background
Auteur(s): Gannouji Radouane, Perivolaropoulos Leandros, Polarski D., Skara Foteini
(Article) Publié:
Physical Review D, vol. 103 p.063509 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03022688_v1
Ref Arxiv: 2011.01517
Ref INSPIRE: 1827879
DOI: 10.1103/PhysRevD.103.063509
Ref. & Cit.: NASA ADS
Exporter : BibTex  endNote
Résumé: We consider Horndeski modified gravity models obeying stability, velocity of gravitational waves cT equals c and quasistatic approximation on subhorizon scales. We assume further a Lambda cold dark matter background expansion and a monotonic evolution on the cosmic background of the α functions as αi=αi0as where i=M, B, a is the scale factor and αi0 (αM0,αB0), s are arbitrary parameters. We show that the growth and lensing reduced (dimensionless) gravitational couplings μ≡Ggrowth/G, Σ≡Glensing/G exhibit the following generic properties today: Σ0<1 for all viable parameters, μ0<1 (weak gravity today) is favored for small s while μ0>1 is favored for large s. We establish also the relation μ≥Σ at all times. Taking into account the fσ8 and EG data we constrain the parameter s to satisfy s≲2. Hence these data select essentially the weak gravity regime today (μ0<1) when s<2, while μ0>1 subsists only marginally for s≈2. At least the interval 0.5≲s≲2 would be ruled out in the absence of screening. We consider further the growth index γ(z) and identify the (αM0,αB0,s) parameter region that corresponds to specific signs of the differences γ0γ0ΛCDM, and γ1γ1ΛCDM, where γ0≡γz=0 and γ1≡dγdzz=0. In this way important information is gained on the past evolution of μ. We obtain in particular the signature γ0>γ0ΛCDM for s<2 in the selected weak gravity region.



Refinement and modularity of immortal dyons
Auteur(s): Alexandrov S., Nampuri Suresh
(Article) Publié:
Journal Of High Energy Physics, vol. 2021 p.147 (2021)
Texte intégral en Openaccess :
Ref HAL: hal02930775_v1
Ref Arxiv: 2009.01172
DOI: 10.1007/JHEP01(2021)147
Ref. & Cit.: NASA ADS
Exporter : BibTex  endNote
Résumé: Extending recent results in ${\cal N}=2$ string compactifications, we propose that the holomorphic anomaly equation satisfied by the modular completions of the generating functions of refined BPS indices has a universal structure independent of the number ${\cal N}$ of supersymmetries. We show that this equation allows to recover all known results about modularity (under $SL(2,\mathbb{Z})$ duality group) of BPS states in ${\cal N}=4$ string theory. In particular, we reproduce the holomorphic anomaly characterizing the mock modular behavior of quarterBPS dyons and generalize it to the case of nontrivial torsion invariant.
