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Time-varying materials in the presence of dispersion:plane-wave propagation in a Lorentzian medium with temporal discontinuity

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Photonics Research 2021年第9期9卷 1842-1853页

作者： DIEGO M.SOLíS RAPHAEL KASTNER nader engheta Department of Electrical and Systems Engineering University of PennsylvaniaPhiladelphiaPennsylvania 19104USA Tel Aviv University Tel Aviv 69978Israel

We study the problem of a temporal discontinuity in the permittivity of an unbounded medium with Lorentzian dispersion. More specifically, we tackle the situation in which a monochromatic plane wave forward-traveling in a(generally lossy) Lorentzian-like medium scatters from the temporal interface that results from an instantaneous and homogeneous abrupt temporal change in its plasma frequency(while keeping its resonance frequency constant). In order to achieve momentum preservation across the temporal discontinuity, we show how, unlike in the well-known problem of a nondispersive discontinuity, the second-order nature of the dielectric function now gives rise to two shifted frequencies. As a consequence, whereas in the nondispersive scenario the continuity of the electric displacement D and the magnetic induction B suffices to find the amplitude of the new forward and backward wave, we now need two extra temporal boundary conditions. That is, two forward and two backward plane waves are now instantaneously generated in response to a forward-only plane wave. We also include a transmission-line equivalent with lumped circuit elements that describes the dispersive time-discontinuous scenario under consideration.

关键词： discontinuity plane wave

Mathematical operations and equation solving with reconfigurable metadevices

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Light(Science & Applications) 2022年第10期11卷 2365-2377页

作者： Dimitrios C.Tzarouchis Mario Junior Mencagli Brian Edwards nader engheta University of Pennsylvania Department of Electrical and Systems EngineeringPhiladelphiaPA 19104USA University of North Carolina at Charlotte Department of Electrical and Computer EngineeringCharlotteNC 28223USA

Performing analog computations with metastructures is an emerging wave-based paradigm for solving mathematical *** such devices,one major challenge is their reconfigurability,especially without the need for a priori mathematical computations or computationally-intensive *** equation-solving capabilities are applied only to matrices with special spectral(eigenvalue)*** we report the theory and design of wave-based metastructures using tunable elements capable of solving integral/differential equations in a fully-reconfigurable *** consider two architectures:the Miller architecture,which requires the singular-value decomposition,and an alternative intuitive direct-complex-matrix(DCM)architecture introduced here,which does not require a priori mathematical *** examples,we demonstrate,using system-level simulation tools,the solutions of integral and differential *** then expand the matrix inverting capabilities of both architectures toward evaluating the generalized Moore-Penrose matrix ***,we provide evidence that metadevices can implement generalized matrix inversions and act as the basis for the gradient descent method for solutions to a wide variety of ***,a general upper bound of the solution convergence time reveals the rich potential that such metadevices can offer for stationary iterative schemes.

关键词： equation operations Mathematical

Momentum considerations inside near-zero index materials

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Light(Science & Applications) 2022年第8期11卷 1782-1789页

作者： Michaël Lobet Iñigo Liberal Larissa Vertchenko Andrei VLavrinenko nader engheta Eric Mazur John A.Paulson School of Engineering and Applied Sciences Harvard University9 Oxford StreetCambridgeMA02138USA Department of Physics and Namur Institute of Structured Materials University of NamurRue de Bruxelles 515000NamurBelgium Electrical and Electronic Engineering Department Universidad Pública de NavarraCampus ArrosadíaPamplona31006Spain Department of Photonics Engineering Technical University of DenmarkØrsteds Plads 345ADK-2800 KgsLyngbyDenmark Department of Electrical and Systems Engineering University of PennsylvaniaPhiladelphiaPA19104USA

Near-zero index (NZI) materials, i.e., materials having a phase refractive index close to zero, are known to enhance or inhibit light-matter interactions. Most theoretical derivations of fundamental radiative processes rely on energetic considerations and detailed balance equations, but not on momentum considerations. Because momentum exchange should also be incorporated into theoretical models, we investigate momentum inside the three categories of NZI materials, i.e., inside epsilon-and-mu-near-zero (EMNZ), epsilon-near-zero (ENZ) and mu-near-zero (MNZ) materials. In the context of Abraham–Minkowski debate in dispersive materials, we show that Minkowski-canonical momentum of light is zero inside all categories of NZI materials while Abraham-kinetic momentum of light is zero in ENZ and MNZ materials but nonzero inside EMNZ materials. We theoretically demonstrate that momentum recoil, transfer momentum from the field to the atom and Doppler shift are inhibited in NZI materials. Fundamental radiative processes inhibition is also explained due to those momentum considerations inside three-dimensional NZI materials. Absence of diffraction pattern in slits experiments is seen as a consequence of zero Minkowski momentum. Lastly, consequence on Heisenberg inequality, microscopy applications and on the canonical momentum as generator of translations are discussed. Those findings are appealing for a better understanding of fundamental light-matter interactions at the nanoscale as well as for lasing applications.

关键词： materials. momentum Minkowski

Dispersion coding of ENZ media via multiple photonic dopants

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Light(Science & Applications) 2022年第8期11卷 1865-1875页

作者： Ziheng Zhou Hao Li Wangyu Sun Yijing He Iñigo Liberal nader engheta Zhenghe Feng Yue Li Department of Electronic Engineering Tsinghua University100084BeijingChina Department of Electrical and Electronic Engineering Public University of NavarrePamplona31006Spain Department of Electrical and Systems Engineering University of PennsylvaniaPhiladelphiaPA19104USA

Epsilon-near-zero (ENZ) media are opening up exciting opportunities to observe exotic wave phenomena. In this work, we demonstrate that the ENZ medium comprising multiple dielectric photonic dopants would yield a comb-like dispersion of the effective permeability, with each magnetic resonance dominated by one specific dopant. Furthermore, at multiple frequencies of interest, the resonant supercouplings appearing or not can be controlled discretely via whether corresponding dopants are assigned or not. Importantly, the multiple dopants in the ENZ host at their magnetic resonances are demonstrated to be independent. Based on this platform, the concept of dispersion coding is proposed, where photonic dopants serve as “bits” to program the spectral response of the whole composite medium. As a proof of concept, a compact multi-doped ENZ cavity is fabricated and experimentally characterized, whose transmission spectrum is manifested as a multi-bit reconfigurable frequency comb. The dispersion coding is demonstrated to fuel a batch of innovative applications including dynamically tunable comb-like dispersion profiled filters, radio-frequency identification tags, etc.

关键词： dopant dispersion media