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Person# Theodoros Koutserimpas

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Related research domains (8)

Electromagnetic radiation

In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. Types of EMR inc

Resonator

A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at othe

Electromagnetic field

An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by moving electric charges. It is the field described by classical electrodynamics (a classical field

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In this thesis, the electromagnetic wave propagation is studied in nonstationary-medium scenarios. The electromagnetic fields under material time-modulation are shown to conserve their momentum but not their energy. The mathematical foundations and analysis to treat wave propagation in time-Floquet media are given additionally to the related parametric amplification phenomena, which are mapped to the stability analysis of the corresponding hypergeometric equations. Assuming a time-variation of permittivity, permeability and conductivity the appropriate time-domain solutions are derived, based on an observation of the fields in the past. The formulation of a time-transitioning state matrix connects the unusual energy transitions of electromagnetic fields in time-varying media with the exceptional point theory, a theory strongly connected with parity-time symmetry. Consequently, the state-matrix approach of this thesis allows the analysis of the electromagnetic waves in terms of parity and time-reversal symmetries and signify parity-time symmetric wave-states without the presence of a spatially symmetric distribution of gain and loss, or any inhomogeneities and material periodicity. The parametric amplification phenomena of time-Floquet media and more precisely those that generate a Mathieu equation at the first momentum gap are theoretically studied and numerically compared with simulations using FDTD and connected with the parity-time scattering conventional characteristics. In the last part of this thesis, studies regarding resonant acoustic and electromagnetic systems are exhibited. The theoretical foundation to treat both acoustic and electromagnetic resonant phenomena is given based on the coupled mode theory and the appropriate Hilbert space. Two examples of interest are shown leveraging the time-dynamics of a temporal resonant system. The first example is related to the design of an artificial resonant acoustic lattice with the appropriate time-modulation leading to an effective zero index of refraction. The second example is related to resonant systems with temporal coupling and the possibility to induce nonreciprocal gain by leveraging the frequency conversion occurring in parametric systems. This thesis enriches the literature and the theoretical bases for dynamical wave systems and provides an insight on the broad capabilities of time-varying systems in electromagnetics, optics and acoustics. It may be used as a guidance to realize wave devices that amplify and actively filter wave signals for many future applications in lasing, sensing, signal amplifying, energy transferring and imaging.

Romain Christophe Rémy Fleury, Theodoros Koutserimpas, Mohammad Sajjad Mirmoosa

Invariance under time translation (or stationarity) is probably one of the most important assumptions made when investigating electromagnetic phenomena. Breaking this assumption is expected to open up novel possibilities and result in exceeding conventional limitations. However, to explore the field of time-varying electromagnetic structures, we primarily need to contemplate the fundamental principles and concepts from a nonstationarity perspective. Here, we revisit one of those key concepts: the polarizability of a small particle, assuming that its properties vary in time. We describe the creation of induced dipole moment by external fields in a nonstationary, causal way, and introduce a complex-valued function, called temporal complex polarizability, for elucidating a nonstationary Hertzian dipole under time-harmonic illumination. This approach can be extended to any subwavelength particle exhibiting electric response. In addition, we also study the classical model of the polarizability of an oscillating electron using the equation of motion whose damping coefficient and natural frequency are changing in time. Next, we theoretically derive the effective permittivity corresponding to time-varying media (comprising free or bound electrons, or dipolar meta-atoms) and explicitly show the differences with the conventional macroscopic Drude–Lorentz model. This paper will hopefully pave the road towards better understanding of nonstationary scattering from small particles and homogenization of time-varying materials, metamaterials, and metasurfaces.

2022,

In this paper, we study the interactions of electromagnetic waves with a non-dispersive dynamic medium that is temporally dependent. Electromagnetic fields under material time-modulation conserve their momentum but not their energy. We assume a time-variation of the permittivity, permeability and conductivity and derive the appropriate time-domain solutions based on the causality state at a past observation time. We formulate a time-transitioning state matrix and connect the unusual energy transitions of electromagnetic fields in time-varying media with the exceptional point theory. This state-matrix approach allows us to analyze further the electromagnetic waves in terms of parity and time-reversal symmetries and signify parity-time symmetric wave-states without the presence of a spatially symmetric distribution of gain and loss, or any inhomogeneities and material periodicity. This paper provides a useful arsenal to study electromagnetic wave phenomena under time-varying media and points out novel physical insights connecting the resulting energy transitions and electromagnetic modes with exceptional point physics and operator symmetries.

2020