Research

2024

1. arXiv

   Subwavelength Localisation in Disordered Systems

   Habib Ammari, Silvio Barandun, and Alexander Uhlmann

   Dec 2024

   Abs DOI

   We consider disordered finite systems of subwavelength resonators where the disorder is in the spatial arrangement of the resonators and their material parameters are perturbed randomly. We elucidate the mechanism of wave localisation in such systems. To do so, we employ the capacitance matrix formalism, make precise the notions of bandgap and localised eigenmodes, study their stabilities with respect to fluctuations in the system, and highlight the importance of defect hybridisation and level repulsion in creating localised eigenmodes for a given arrangement of the resonators by randomly changing their material parameters. We also introduce a method of localised eigenmode prediction based on the discrete Green function of the disordered system of resonators and show the existence of bound eigenmodes in disordered systems where local translation invariance is broken.

2. arXiv

   Truncated Floquet-Bloch Transform for Computing the Spectral Properties of Large Finite Systems of Resonators

   Habib Ammari, Silvio Barandun, and Alexander Uhlmann

   Oct 2024

   Abs DOI

   The truncated Floquet-Bloch transform can be used to characterise the spectral properties of finite periodic and aperiodic large systems of resonators. This paper aims to provide for the first time the mathematical foundations of this transform.

3. arXiv

   Generalised Brillouin Zone for Non-Reciprocal Systems

   Habib Ammari, Silvio Barandun, Ping Liu, and Alexander Uhlmann

   Aug 2024

   Abs DOI

   Recently, it has been observed that the Floquet-Bloch transform with real quasiperiodicities fails to capture the spectral properties of non-reciprocal systems. The aim of this paper is to introduce the notion of a generalised Brillouin zone by allowing the quasiperiodicities to be complex in order to rectify this. It is proved that this shift of the Brillouin zone into the complex plane accounts for the unidirectional spatial decay of the eigenmodes and leads to correct spectral convergence properties. The results in this paper clarify and prove rigorously how the spectral properties of a finite structure are associated with those of the corresponding semi-infinitely or infinitely periodic lattices and give explicit characterisations of how to extend the Hermitian theory to non-reciprocal settings. Based on our theory, we characterise the generalised Brillouin zone for both open boundary conditions and periodic boundary conditions. Our results are consistent with the physical literature and give explicit generalisations to the \k\-Toeplitz matrix cases.

4. arXiv

   Tunable Localisation in Parity-Time-Symmetric Resonator Arrays with Imaginary Gauge Potentials

   Habib Ammari, Silvio Barandun, Ping Liu, and Alexander Uhlmann

   May 2024

   Abs DOI

   The aim of this paper is to illustrate both analytically and numerically the interplay of two fundamentally distinct non-Hermitian mechanisms in a deep subwavelength regime. Considering a parity-time symmetric system of one-dimensional subwavelength resonators equipped with two kinds of non-Hermiticity - an imaginary gauge potential and on-site gain and loss - we prove that all but two eigenmodes of the system decouple when going through an exceptional point. By tuning the gain-to-loss ratio, the system changes from a phase with unbroken parity-time symmetry to a phase with broken parity-time symmetry. At the macroscopic level, this is observed as a transition from symmetrical eigenmodes to condensated eigenmodes at one edge of the structure. Mathematically, it arises from a topological state change. The results of this paper open the door to the justification of a variety of phenomena arising from the interplay between non-Hermitian reciprocal and non-reciprocal mechanisms not only in subwavelength wave physics but also in quantum mechanics where the tight binding model coupled with the nearest neighbour approximation can be analysed with the same tools as those developed here.