arXiv subjects

Yakir Hadad

Publications and source records attributed to Yakir Hadad.

At least 19 recordsLinked to original sources

arXiv2026

Mixtenna: A Self-Biased Nonlinear Patch Antenna for Passive Third-Harmonic Radiation

A nonlinear rectangular patch antenna (RPA) is presented in which back-to-back Schottky diodes are embedded at high-field regions to enable passive, bias-free harmonic generation. The self-biased diodes introduce a power-dependent impedance that drives efficient frequency up-conversion and selective third-harmonic radiation. A tailored matching network enhances third-harmonic excitation and coupling while preserving radiation efficiency at the fundamental frequency. Analytical modeling combined with SPICE-assisted full-wave time-domain simulations predicts strong odd-harmonic content, and measurements on RPA prototypes employing SMS7630 diodes confirm these results. Simulated and measured S-parameters and far-field patterns at 925 MHz and 2.775 GHz show excellent agreement. The demonstrated approach establishes nonlinear loading as an effective mechanism for passive harmonic control in compact radiators, enabling frequency-agile and spectrum-efficient antenna systems.

physics.optics↗

arXiv2024

Exact Local-Field Renormalization for Deep-Subwavelength Particles in Rectangular Cavities

We present a rigorous, semi-analytical framework for predicting the eigenfrequencies of a deep-subwavelength particle embedded in a perfectly conducting rectangular cavity. The formulation retains the \emph{full} cavity-mode spectrum and is therefore fully causal, in contrast to Jaynes--Cummings-type models that truncate the spectrum and fail in the strong-coupling regime. A ladder-type Green-function renormalization is introduced: three successive subtractions---cavity minus rectangular waveguide, waveguide minus parallel plate, and parallel plate minus free space---remove the ``$\infty-\infty$'' singularity of the local field. The resulting local dyadic Green function is obtained using a rapidly convergent recursive algorithm whose computational cost scales linearly with the number of spectral terms. Once the local field is known, the cavity-renormalized polarizability \[ \boldsymbol{\alpha}_{\mathrm{eff}}(\omega) = \left[ \boldsymbol{\alpha}^{-1} - \mathbf{G}_{\mathrm{loc}}(\mathbf{r}') \right]^{-1} \] yields the coupled resonances from \[ \det\!\left[ \boldsymbol{\alpha}_{\mathrm{eff}}^{-1}(\omega) \right] = 0. \] Benchmark cases involving isotropic, gyrotropic, and chiral spheres confirm exponential convergence and capture both the weak- and strong-coupling regimes without adjustable parameters. The method is numerically robust, applies to arbitrary material tensors, and can be extended to structured waveguides whose transverse eigenmodes are obtained numerically, providing a practical design tool for cavity--particle systems spanning microwave to terahertz frequencies.

physics.app-ph↗

arXiv2024

Loss vs Magnetization Threshold Phenomenon for Lorentz Nonreciprocity Induced by a Gyrotropic Particle Inside a Cavity

When a plasmonic particle is subject to a static magnetic field, ${B}_{\rm dc}=B_{0} \hat{z}$, its gyrotropic response gives rise to nonreciprocal dynamics of the entire ambient surroundings. This dynamics depends on the particle's excitation which in turn depends on the gyrotropic material damping rate $\Gamma$. Thus intuitively speaking, the heavier the gyrotropic material loss, the weaker the non-reciprocal response. This is indeed the case when the particle is located in free space. In this letter, we quantify nonreciprocity using the defined measure $\cal{R}$ and show that when the gyrotropic particle is placed inside a cavity, the nonreciprocity measure $\cal{R}$ is robust against material loss up to a certain loss threshold, $\Gamma_{th}$ that depends on the magnetic biasing $B_0$

physics.app-ph↗

arXiv2024

Optimal Design of Dallenbach Absorbers Under Broadband Broad-Angle Illumination

The classical scenario where a \emph{single plane-wave} field impinge a Dallenbach absorber is well studied both theoretically and experimentally. However, occasionally a \emph{spectrum of plane-waves} impinges the absorber. Such a scenario occurs for example if an antenna is located adjacent to the absorbing layer. In this paper, for this scenario we obtain the absorbing performance bound and design an \emph{optimized layered absorber} that approaches the bound. In a numerical demonstration, we explore a realistic case where a dipole antenna is placed in the vicinity of a finite, electrically thin, Dallenbach absorber backed by a PEC plane in the 6G frequency range. In the absence of the absorbing layer covering the PEC plane, severe scattering from the plane distorts the radiated fields. These distortions are robustly mitigated by the specifically tailored optimal absorber to yield a more desired radiation pattern. Additionally, we propose a metamaterial realization that emulates the required properties of the absorbing layer for all field polarizations.

physics.app-ph↗

arXiv2024

Universal radiation dynamics by temporal transitions in optical waveguides

When an excited electromagnetically open optical waveguide goes through a temporal transition of its material properties, it radiates to the ambient surroundings. In this letter, we explore this radiation and reveal, using asymptotic evaluation of path integral in the complex frequency (Laplace) plane, a peculiar space-time dependence of its frequency. Specifically, we derive an exact formula (Eq. (11)) for the instantaneous radiation frequency, which exhibits a chirp behavior with respect to time. This simple formula depends on the ambient properties and on the longitudinal wavenumber βof the guided mode before the temporal transition but not on the specific waveguide structure or materials. In addition, we derive a t^(-3/2) decay rate of the radiative field on time. We verify our analytic results using full-wave simulations of a dispersive and lossy Indium Tin Oxide waveguide that undergoes smooth temporal long transitions over up to ~200 cycles at the initially guided mode frequency. Thus, these theoretical findings offer valuable insights into the behavior of general optical waveguides experiencing temporal transitions and provide a powerful tool for analyzing and designing such THz and optical setups, with potential use in sensing and imaging.

physics.optics↗

arXiv2023

Parasitic Element Time-Modulation for Enhanced Effective Inter-Antenna Coupling: Utilization for Improved Gain-Bandwidth

Time variation has been recently introduced as an additional degree of freedom for wave engineering, that enables going beyond the performances that are expected by linear time-invariant (LTI) systems. In this paper, we introduce the concept of indirect time-modulation of antennas using an add-on time-varying scatterer (parasitic element) that gives rise to an inherent feedback mechanism via the airborne wave system. As opposed to a direct modulated system where a time-dependent element is in contact with the other elements, in an indirect time modulation scheme \emph{no} direct physical contact between the original LTI network and the time-varying add-on scatterer is needed, thus leading to additional flexibility in the design. Using indirect time modulation we demonstrate enhanced effective coupling between remote antenna elements, and the possibility to outperform the gain-bandwidth achieved for the same antenna structure but without time-modulation.

physics.app-ph↗

arXiv2023

Sum Rule Bounds Beyond Rozanov Criterion in Linear and Time-Invariant Thin Absorbers

Dallenbach layer is composed of an absorbing magnetic-dielectric layer attached to a perfect electric conductor (PEC) sheet. Under linearity and time invariance (LTI) assumptions Rozanov has established analytically a sum-rule trade-off between the absorption efficacy over a predefined bandwidth and the thickness of the layer, that is the so-called Rozanov bound. In recent years several proposals have been introduced to bypass this bound by using non-LTI absorbers. However, in practice, their implementation may be challenging. Here, we expose additional hidden assumptions in Rozanov's derivation, and thus we introduce several new sum rules for LTI layer absorbers that are not covered by the original Rozanov's criterion, and give rise to more relaxed constraints on the absorption limit. We then, demonstrate practical LTI designs of absorbing thin layers that provide absorption beyond the Rozanov's bound. These designs are based on the replacement of the original PEC boundary by various types of penetrable impedance sheet.

physics.app-ph↗

arXiv2022

Rayleigh Anomaly Induced Phase Gradients in Finite Nanoparticle Chains

We report on the theoretical study of anomalous phase gradients induced by Rayleigh anomalies in finite nanoparticle chains. These phase gradients, defined with respect to the phase of the applied plane wave, cause a deviation of the diffraction directions from the chain relative to the direction expected from the grating equation for infinite chains. To study the effect theoretically, we use an analytical approach based on the discrete dipole approximation, which reveals the combinatorial nature of the multi-scattering process that governs the chain dynamics. We find an approximate closed-form solution to the particles' dipole moments by describing the single reciprocal system with a successive solution of two non-reciprocal, one-way systems. Within this framework, we obtain the chain excitation by means of interference between different scattering paths. Moreover, we show that the dipole moments along the chain are governed by recursive relations dictated by the generalized Fibonacci series. The presented results provide a new perspective for understanding nanoparticle arrays' dynamics. Specifically, the unique approach for analytically analyzing the spatial excitations of the array inclusions may shed new light on emerging applications of periodic traveling wave antennas in the optical regime, such as LIDARs, topological states analysis and arbitrary beam shaping schemes.

physics.optics↗

arXiv2022

Optimization-free Approach for Analog Filter Design through Spatial and Temporal Soft Switching of the Dielectric Constant

The inverse-scattering problem of an inhomogeneous material has been of interest for many years, and was generally addressed with various optimization techniques. In this paper, we suggest an optimization-free method for solving the inverse-scattering problem of a one-dimensional inhomogeneous medium and use this to demonstrate the design of desired reflection frequency response. In addition, we derive the governing equation of an analog problem - a time-dependent homogeneous medium and use the same technique to design a temporal switching profile for the design of frequency response in k-space.

physics.app-ph↗

arXiv2021

One-Way Acoustic Guiding under transverse mean flow

In a moving acoustic medium, sound waves travel differently with and against the fluid flow. This well-established acoustic effect is backed by the intuition that the fluid velocity bias imparts momentum on the propagating acoustic waves, thus violating reciprocity. Based on this conception, fluid flow that is transverse to the wave direction of propagation will not break reciprocity. In this letter we contrast this common wisdom and theoretically show that the interplay between transverse mean flow and transverse structural gliding-asymmetry can yield strong nonreciprocity and even, surprisingly, one-way waveguiding which is rare in acoustics.

physics.flu-dyn↗

arXiv2021

Absorption and scattering by a temporally switched lossy layer: Going beyond the Rozanov bound

In this paper we study the electromagnetic scattering, absorption, and performance bounds for short time modulated pulses that impinge on a time-varying lossy layer that is sandwiched between vacuum and a perfect electric conductor. The electric characteristics of the layer, namely, the conductivity, permittivity, and permeability are assumed to change abruptly or gradually in time. We demonstrate numerically that a time-varying absorbing layer that undergoes temporal switching of its permittivity and conductance can absorb the power of a modulated, ultra-wideband, as well as a quasi-monochromatic, pulsed wave beyond what is dictated by the time invariant Rozanov bound when integrating over the whole frequency spectrum. We suggest and simulate a practical metamaterial realization that is constructed as a three-dimensional array of resistor loaded dipole. By switching only the dipole's load resistance, desired effective media properties are obtained. Furthermore, we show that Rozanov's bound can be bypassed with abrupt and a more practical gradual, soft, switching thus overcoming some possible causality issue in abrupt switching.

physics.class-ph↗

arXiv2021

On the use of the Padé-Fourier approximation in fast evaluation of the Green's function of layered media

Efficient Green's function evaluation in layered media is a holy-grail of wave theory in general and for electromagnetics in particular. While there is a very large amount of knowledge in this context with vast literature, there are yet challenging cases such as the Green's function in thick lossy media and the Green's function at thick media with negative parameters. Here we propose a technique that can nicely tackle these issues. Our approach is based on a rational function approximation of the spectra using the Fourier-Padé approximation that is carried out in a conformal mapped spectral plane. We show that this approach can be used in challenging scenarios such as very thick and lossy layers, materials with negative parameters such as in plasmonics, and even to approximate a dominant branch-cut contribution far from the source.

physics.comp-ph↗

arXiv2021

Wave Analysis and Homogenization of Spatiotemporally Modulated Wire Medium

In this paper we develop homogenization theory for spatiotemporally modulated wire medium. We first solve for the modal waves that are supported by this composite medium, we show peculiar properties such as extraordinary waves that propagate at frequencies below the cut-off frequency of the corresponding stationary medium. We explain how these unique solutions give rise to an extreme Fresnel drag that exists already with weak and slow spatiotemporal modulation. Next, we turn to derive the effective material permittivity that corresponds to each of the first few supported modes, and write the average fields and Poynting's vector. Nonlocality, nonreciprocity, and anisotropy due to the spatiotemporal modulation direction, are three inherent properties of this medium, and are clearly seen in the effective material parameters. As a figure of merit, we also derive the effective permittivity of a plasma medium with spatiotemporally modulated plasma frequency. This comparison is interesting since the plasma medium can be considered as the effective medium that is obtained by a stationary wire medium. We validate that homogenization and spatiotemporal variation are not necessarily interchangeable operations. And indeed, in certain parameter regimes the homogenization should be performed directly on spatiotemporally modulated composite medium, rather than first homogenize the stationary medium and then phenomenologically introduce the effect of the space-time modulation.

physics.app-ph↗

arXiv2019

Space-Time Modulated Loaded-Wire Metagratings for Magnetless Nonreciprocity and Near-Complete Frequency Conversion

In recent years a significant progress has been made in the development of magnet-less nonreciprocity using space-time modulation, both in electromagnetics and acoustics. This approach has so far resulted in a plethora of non-reciprocal devices, such as isolators and circulators, over different parts of the spectrum, for guided waves. On the other hand, very little work has been performed on non-reciprocal devices for waves propagating in free space, which can also have many practical applications. For example, it was shown theoretically that non-reciprocal scattering by a metasurface can be obtained if the surface-impedance operator is continuously modulated in space and time. However, the main challenge in the realization of such a metasurface is due to the high complexity required to modulate in space and time many sub-wavelength unit-cells of which the metasurface consists. In this paper we show that spatiotemporally modulated metagratings can lead to strong nonreciprocal responses, despite the fact that they are based on electrically-large unit cells. We specifically focus on wire metagratings loaded with time-modulated capacitances. We use the discrete-dipole-approximation and an ad-hoc generalization of the theory of polarizability for time-modulated particles, and demonstrate an effective nonreciprocal anomalous reflection (diffraction) with an efficient frequency conversion. Thus, our work opens a venue towards a practical design and implementation of highly non-reciprocal magnet-less metasurfaces in electromagnetics and acoustics.

physics.app-ph↗

arXiv2019

Soft Temporal Switching of TL Parameters: Wave-field, Energy Balance, Applications

Time-varying guiding structures introduce an additional degree of freedom, besides spatial-variation, that enables better control over the guided wave in a device. Periodically time-modulated structures which are usually considered enable wave control over narrowband signals. However, for ultrawideband short-pulse signals, time-variation in the form of temporal discontinuities is required. Such a setup has recently been proposed as a mean to overcome the Bode-Fano bound on impedance matching. While hard (abrupt) temporal discontinuities are relatively simple to analyze by employing continuity of magnetic flux and electric charge, soft (gradual) temporal switching of the guiding structure parameters is more challenging. This work explores the case of a short-pulse dynamics in a one-dimensional, metamaterial TL, medium with general smooth time-variation of its parameters. In this time-varying TL, wave-field solutions are obtained by a WKB approach which is more common in the context of gradual spatial variations. Using this methodology a leading order transmitted and reflected waves due to the time-variation are derived, followed by a discussion of the energy balance in such switched media. A canonical example of capacitor discharge into a long time-varied TL is given. These results may be used as analysis/synthesis tools for time-varying wave devices in electromagnetics and acoustics.

physics.app-ph↗

arXiv2018

Beyond the Bode-Fano Bound: Wideband Impedance Matching for Short-Pulses using Temporal Switching of Transmission-Line Parameters

Impedance matching is one of the most important practice in wave engineering as it enables to maximize the power transfer from the signal source to the load in the wave system. Unfortunately, it is bounded by the Bode-Fano criterion that states, for any passive, linear and time-invariant matching network, a stringent tradeoff between the matching-bandwidth and efficiency; implying severe constraints on various electromagnetic and acoustic wave systems. Here, we propose a matching paradigm that overcome this issue by using a temporal switching of the parameters of a metamaterial-based transmission-line, thus revoking the time-invariance assumption underlying the Bode-Fano criterion. Using this scheme we show theoretically that an efficient wideband matching, beyond Bode-Fano bound, can be achieved for short-time pulses in challenging cases of very high contrast between the load and the generator impedances, and with significant load dispersion; situations common in e.g., small antennas matching, cloaking, with applications for ultra-wideband communication, high resolution imaging, and more.

physics.app-ph↗

arXiv2017

Scattering theory from artificial piezoelectric-like meta-atoms and molecules

Inspired by the natural piezoelectric effect, we introduce hybrid-wave electromechanical meta-atoms and meta-molecules that consist of coupled electrical and mechanical oscillators with similar resonance frequencies. We propose an analytical model for the linearized electromechanical scattering process, and explore its properties based on first principles. We demonstrate that by exploiting the linearized hybrid-wave interaction, one may enable functionalities that are forbidden otherwise, going beyond the limits of today's metamaterials. As an example we show an electrically deep sub-wavelength dimer of meta-atoms with extremely sensitive response to the direction-of-arrival of an impinging electromagnetic wave. This scheme of meta-atoms and molecules may open ways for metamaterials with a plethora of exciting dynamics and phenomena that have not been studied before with potential technological implications in radio-frequencies and acoustics.

physics.app-ph↗

arXiv2017

Clustering in particle chains - summation techniques for the periodic Green's function

1D lattice summations of the 3D Green's function are needed in many applications such as photonic crystals, antenna arrays, and so on. Such summations are usually divided into two cases, depending on the location of the observer: Out of the summation axis, or on the summation axis. Here, as a service for the community, we present and summarize the summation formulas for both cases. On the summation axis, we use polylogarithmic functions to express the summation, and Away from the summation axis we use Poisson summation (equivalent to the expansion of the field to cylindrical harmonics)

physics.optics↗