Temporal aiming

Deflecting and changing the direction of propagation of electromagnetic waves are needed in multiple applications,such as in lens-antenna systems,point-to-point communications and radars.In this realm,metamaterials have been demonstrated to be great candidates for controlling wave propagation and wave-matter interactions by offering manipulation of their electromagnetic properties at will.They have been studied mainly in the frequency domain,but their temporal manipulation has become a topic of great interest during the past few years in the design of spatiotemporally modulated artificial media.In this work,we propose an idea for changing the direction of the energy propagation of electromagnetic waves by using time-dependent metamaterials,the permittivity of which is rapidly changed from isotropic to anisotropic values,an approach that we call temporal aiming.In so doing,here,we show how the direction of the Poynting vector becomes different from that of the wavenumber.Several scenarios are analytically and numerically evaluated,such as plane waves under oblique incidence and Gaussian beams,demonstrating how proper engineering of the isotropic-anisotropic temporal function of ε_(r)(t)can lead to a redirection of waves to different spatial locations in real time.

Momentum considerations inside near-zero index materials

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.

Dispersion coding of ENZ media via multiple photonic dopants

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.

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

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.

Mathematical operations and equation solving with reconfigurable metadevices

Performing analog computations with metastructures is an emerging wave-based paradigm for solving mathematical problems.For such devices,one major challenge is their reconfigurability,especially without the need for a priori mathematical computations or computationally-intensive optimization.Their equation-solving capabilities are applied only to matrices with special spectral(eigenvalue)distribution.Here we report the theory and design of wave-based metastructures using tunable elements capable of solving integral/differential equations in a fully-reconfigurable fashion.We 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 decomposition.As examples,we demonstrate,using system-level simulation tools,the solutions of integral and differential equations.We then expand the matrix inverting capabilities of both architectures toward evaluating the generalized Moore-Penrose matrix inversion.Therefore,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 problems.Finally,a general upper bound of the solution convergence time reveals the rich potential that such metadevices can offer for stationary iterative schemes.