Soutenance de thèse

Nicolas FERNEZ

Vendredi 21 décembre 2018 à 14h00
Amphithéâtre IEMN – Avenue Poincaré – Cité Scientifique – Villeneuve d’Ascq

Jury:

• Didier LIPPENS (Université de Lille, Directeur de thèse),
• Valérie VIGNERAS (Université de Bordeaux, Rapporteur)
• Xavier BéGAUD (Telecom ParisTech, Rapporteur)
• Geneviève MAZé-MERCEUR (CEA-Cesta, Examinateur)
• Vincent LAUR (Université de Bretagne Occidentale, Examinateur)
• Éric LHEURETTE (Université de Lille, CoDirecteur de thèse)
• Philippe POULIGUEN (DGA, Examinateur)

Résumé :

Abstract :

Absorption of electromagnetic power arouses a lot of interest not solely for stealth applications in military domain, but also in civil life to reduce the exposure to wireless communication signals and to preserve the totality of exchanged information. Absorbers designing for low frequency domain remains a challenge since the object’s thickness has to be proportional to the working wavelength, which leads to significant mass and size. This characteristic is a limiting factor, especially for the on-board equipment protection. That is why the main objective of this thesis is to design a low-profile electromagnetic absorber specified for broadband operation at low frequency (typically between 1 and 10 GHz).
First, we tried to deeply understand the role of materials’ constitutive parameters (complex permittivity and permeability) in the power absorption. Next we described the absorption efficiency by a resonant structure in terms of quality factor, thus introducing a balance condition.
From this design rules, we proposed several types of absorbing structures. The first one is based on a ferromagnetic composite material structuration either by addition of metallic pattern or by etching technique. Fractals patterns (Moore’s curve) enabled to obtain a relative frequency bandwidth in the range of 130 %, for 90 % power absorption, around a frequency of 7 GHz, for a thickness which is a fraction absorbed wavelength. The other absorbers studied during this thesis display a MIM (metal/ isolator/ metal) structuration with randomly distributed resonators. The random distribution of the first absorber, sized for operating frequencies around 10 GHz, obeys the probability law of Poisson in which overlapping between resonators is allowed. We used some mathematical tools to describe the random distribution’s topology in order to link the absorber’s geometry to the electromagnetic power absorption characteristics. Two other random structures, dimensioned for millimeter wavelengths, distribute the resonators with a no contact condition. We showed that by increasing of density of the resonators, one can obtain an absorbance higher than 90 % with a bandwidth enhancement. Finally, we carried out a prospective study in order regarding a random metasurface which can behaves as a common platform for electromagnetic and acoustic waves in the infrared domain.