Periodic and disordered structures for the absorption of electromagnetic waves

NEWS

THESE : Structures périodiques et désordonnées pour l’absorption des ondes électromagnétiques

Thesis defence

Nicolas FERNEZ

Friday 21 December 2018 at 2.00 pm
Amphitheatre IEMN - Avenue Poincaré - Cité Scientifique - Villeneuve d'Ascq

Jury:

Didier LIPPENS (University of Lille, thesis supervisor),
Valérie VIGNERAS (University of Bordeaux, Rapporteur)
Xavier BéGAUD (Telecom ParisTech, Rapporteur)
Geneviève MAZé-MERCEUR (CEA-Cesta, Examiner)
Vincent LAUR (Université de Bretagne Occidentale, Examiner)
Éric LHEURETTE (University of Lille, thesis co-supervisor)
Philippe POULIGUEN (DGA, Examiner)

Summary:

The absorption of electromagnetic waves is an objective that is attracting a great deal of interest these days, particularly for stealth applications in the military field, but also in the civilian sector to limit exposure to wireless communication signals and to preserve the integrity of the information exchanged. The design of absorbers for the low-frequency range remains a challenge insofar as the thickness of the objects is proportional to the wavelength, which can result in significant weight and bulk. This characteristic is a limiting factor, particularly for the protection of on-board equipment. This is why the aim of this thesis is to design electromagnetic absorbers that are both broadband and as thin as possible for low frequencies (typically between 1 and 10 GHz).
Firstly, we sought to understand the role of material parameters (permittivity and complex permeability) in wave absorption. We then described the absorption efficiencies within a resonant structure using the quality factor.
Based on these design principles, we have proposed several types of absorbent structures. The first is based on structuring a ferromagnetic composite material by adding a metal pattern or by routing. Fractal patterns (Moore curves) have enabled us to obtain relative bandwidths of the order of 130 %, at 90 % power absorption, around the frequency of 7 GHz, with a thickness that is small compared with the longest absorbed wavelength. The other absorbers studied in this thesis have a MIM (metal/insulator/metal) structure with randomly distributed resonators. The random distribution of the first absorber, dimensioned for frequencies around 10 GHz, follows the Poisson probability distribution where the resonators can overlap. We used mathematical tools to describe the topology of the distribution in order to link the geometry of the absorber to the absorption of the electromagnetic wave. Two other random structures, dimensioned for millimetre wavelengths, distribute the resonators with a non-contact condition. We show that an increase in the number of resonators makes it possible to obtain an absorption greater than 90 % with bandwidth broadening. Finally, we conducted a prospective study to investigate the response of a random metasurface to electromagnetic and acoustic waves in the infrared range.

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.