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Asymptotic Floquet theory for first order ODEs with finite Fourier series perturbation and its applications to Floquet metamaterials

by H. Ammari and E.O. Hiltunen and T. Kosche

(Report number 2021-32)

Abstract
Our aim in this paper is twofold. Firstly, we develop a new asymptotic theory for Floquet exponents. We consider a linear system of differential equations with a time-periodic coefficient matrix. Assuming that the coefficient matrix depends analytically on a small parameter, we derive a full asymptotic expansion of its Floquet exponents. Based on this, we prove that only the constant order Floquet exponents of multiplicity higher than one will be perturbed linearly. The required multiplicity can be achieved via folding of the system through certain choices of the periodicity of the coefficient matrix. Secondly, we apply such an asymptotic theory for the analysis of Floquet metamaterials. We provide a characterization of asymptotic exceptional points for a pair of subwavelength resonators with time-dependent material parameters. We prove that asymptotic exceptional points are obtained if the frequency components of the perturbations fulfill a certain ratio, which is determined by the geometry of the dimer of subwavelength resonators.

Keywords: asymptotic Floquet theory, subwavelength quasifrequency, time-modulation, metamaterial, exceptional point

@Techreport{AHK21_974,
  author = {H. Ammari and E.O. Hiltunen and T. Kosche},
  title = {Asymptotic Floquet theory for first order ODEs with finite Fourier series perturbation  and its applications to Floquet metamaterials},
  institution = {Seminar for Applied Mathematics, ETH Z{\"u}rich},
  number = {2021-32},
  address = {Switzerland},
  url = {https://www.sam.math.ethz.ch/sam_reports/reports_final/reports2021/2021-32.pdf },
  year = {2021}
}

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