arXiv subjects

Filippo Capolino

Publications and source records attributed to Filippo Capolino.

At least 73 records · Page 4Linked to original sources

arXiv2021

Traveling Wave Tube Eigenmode Solver for Interacting Hot Slow Wave Structure Based on Particle-In-Cell Simulations

A scheme to characterize the dynamics of the electron beam-electromagnetic power exchange along a traveling wave tube (TWT) is proposed. The method is based on defining a state vector at discrete periodic locations along the TWT and determining the transfer matrix of the unit-cell of the "hot" slow-wave structure (SWS) that takes into account the interaction between the electromagnetic guided field and the electron beam via particle-in-cell (PIC) simulations. Once the estimate of the unit-cell transfer matrix is obtained, we show how to find the hybrid, beam-electromagnetic, eigenmodes in the hot SWS, i.e., where the electromagnetic guided field interacts with an electron beam, by using Floquet theory. In particular, we show how do determine the complex-valued wavenumbers of the hybrid modes and the eigenvectors associated to them. The method is applied to find the hot modes with complex wavenumber that can be supported in a TWT amplifier with a helix SWS. We show dispersion relations of the modal complex wavenumbers of the hybrid modes when varying frequency and beam voltage; the results are in agreement with Pierce theory. The method is also applied to find the complex-wavenumber modes in a hot SWS of a millimeter wave TWT amplifier based on a serpentine waveguide. The technique is general and can be applied to any SWS geometry where electromagnetic modes interact with an electron beam.

physics.plasm-ph↗

arXiv2020

Experimental Demonstration of Exceptional Points of Degeneracy in Linear Time Periodic Systems and Exceptional Sensitivity

We present the experimental demonstration of the occurrence of exceptional points of degeneracy (EPDs) in a single resonator by introducing a linear time-periodic variation of one of its components, in contrast to EPDs in parity time (PT)-symmetric systems that require two coupled resonators with precise values of gain and loss. In the proposed scheme, only the tuning of the modulation frequency is required that is easily achieved in electronic systems. The EPD is a point in a system parameters' space at which two or more eigenstates coalesce, and this leads to unique properties not occurring at other non-degenerate operating points. We show theoretically and experimentally the existence of a second order EPD in a time-varying single resonator. Furthermore, we measure the sensitivity of the proposed system to a small structural perturbation and show that the operation of the system at an EPD dramatically boosts its sensitivity performance to very small perturbations. Also, we show experimentally how this unique sensitivity induced by an EPD can be used to devise new exceptionally-sensitive sensors based on a single resonator by simply applying time modulation.

physics.app-ph↗

arXiv2020

Asymmetric surface wave excitation through metasurface-edge diffraction

The selective excitation of localized surface wave modes remains a challenge in the design of both leaky-wave and bound-wave devices. In this Letter, we show how the truncation of a metasurface can play an important role in breaking the spatial inversion symmetry in the excitation of surface waves supported by the structure. This is done by combining a large anisotropy in the dispersion relation and the presence of an edge which also serves as a coupling mechanism between the plane wave excitation and the induced surface waves. By resorting to the exact solution of the scattering problem based on a discrete Wiener-Hopf technique we show that by inverting the component of the impinging wavevector parallel to the truncation, two distinct surface waves are excited.

physics.optics↗

arXiv2020

Ultra-Sensitive Radio Frequency Biosensor at an Exceptional Point of Degeneracy induced by Time Modulation

We exploit the premises of exceptional points of degeneracy (EPDs) induced in linear time-periodic (LTP) systems to achieve extremely sensitive biosensors. The EPD is formed in a single LC resonator where the total capacitance is comprised of a time-varying capacitor in parallel to a biosensing capacitor. We use the time-periodic variation of a system parameter (e.g., capacitance) to achieve a second order EPD aiming at improving the sensitivity of liquid based radio frequency biosensors, leading to an intrinsic ultra sensitivity. We show the emergence of EPDs in such a system and the ultra sensitivity of the degenerate resonance frequency to perturbations compared to conventional RF sensors. Moreover, we investigate the capacitance and conductance variations of an interdigitated biosensing capacitor to the changes in the concentration of a biological material under test (MUT), leading to subsequent large changes in the resonance frequency of the LTP-LC resonator. A comparison with a standard LC resonator demonstrates the ultra-high sensitivity of the proposed LTP-LC based biosensor. In addition, we show the scalability of the biosensor sensitivity across different frequency ranges.

physics.app-ph↗

arXiv2020

Exceptional Point of Degeneracy in Linear-Beam Tubes for High Power Backward-Wave Oscillators

Abstract An exceptional point of degeneracy (EPD) is induced in a system made of an electron beam interacting with an electromagnetic (EM) guided mode. This enables a degenerate synchronous regime in backward wave oscillators (BWOs) where the electron beams provides distributed gain to the EM mode with distributed power extraction. Current particle-in-cell simulation results demonstrate that BWOs operating at an EPD have a starting-oscillation current that scales quadratically to a non-vanishing value for long interaction lengths and therefore have higher power conversion efficiency at arbitrarily higher level of power generation compared to standard BWOs.

physics.app-ph↗

arXiv2020

Distributed Degenerate Band Edge Oscillator

We propose a new class of oscillators by engineering the dispersion of two-coupled periodic waveguides to exhibit a degenerate band edge (DBE). The DBE is an exceptional point of degeneracy (EPD) of order four, i.e., representing the coalescence of four eigenmodes of a waveguide system without loss and gain. We present a distributed DBE oscillator realized in periodic coupled transmission lines with a unique mode selection scheme that leads to a stable single-frequency oscillation, even in the presence of load variation. The DBE oscillator potentially leads to a boost of the efficiency and performance of RF sources, thanks to the unique features associated to the EPD concept. This class of oscillators is promising for improving discrete-distributed coherent sources and can be extended to radiating structures to achieve a new class of active integrated antenna arrays.

physics.app-ph↗

arXiv2020

Frozen Mode in Three-Way Periodic Microstrip Coupled Waveguide

We demonstrate theoretically and experimentally that a periodic three-way microstrip coupled waveguide exhibits a stationary inflection point (SIP). The SIP is a third order exceptional point of degeneracy (EPD) where three eigenmodes of the guiding system coalesce to form a stationary frozen mode with zero group velocity. Here the frozen mode is shown in a reciprocal waveguide; therefore three coupled waveguides (called three-way waveguide) are required, supporting three modes in each direction. We illustrate the occurrence of the frozen mode regime by observing the dispersion diagram and by using the concept of coalescence parameter that is a measure of the separation of the three coalescing eigenvectors (polarization states). Results based on full-wave simulations and measurements demonstrate the coalescence of three modes and how this coalescence is not perfect due to the presence of losses in the microstrip structure at 2 GHz. The coalescence parameter is used to determine how close the three-way system is to the ideal frozen mode condition.

physics.app-ph↗

arXiv2020

Exceptional Point of Sixth Order Degeneracy in a Modified Coupled Resonators Optical Waveguide

We demonstrate for the first time the occurrence of a sixth order exceptional point of degeneracy (EPD) in a realistic multimode optical photonic structure by using a modified periodic coupled resonators optical waveguide (CROW), at the optical wavelength ${λ_e} = 1550 nm$. The 6th order EPD is obtained in a CROW without the need of loss or gain and such an EPD corresponds to a very special band edge of the periodic photonic structure where six eigenmode coalesce, so we refer to it as the 6th order degenerate band edge (6DBE). Moreover, we report a new scaling law of the quality factor Q of an optical cavity made of such a periodic 6DBE-CROW with cavity length as $Q{\propto}N^7$ with $N$ being the number of unit cells in the periodic finite-length CROW. Furthermore, we elaborate on the application of the 6DBE to ultra-low-threshold lasers. We present a novel scaling law of the lasing threshold that scales as $N^{-7}$when operating near the 6DBE. Also, we show the superiority of the threshold scaling of the 6DBE-CROW to the scaling of another CROW with the same size operating near a 4th order EPD that is often referred to as the degenerate band edge (DBE). The lasing threshold scaling of the DBE-CROW laser is shown here for the first time. We also discuss the high sensitivity of the proposed 6DBE-CROW to perturbations, that may find applications in sensors, modulators, optical switches, nonlinear devices, and Q-switching cavities.

physics.optics↗

arXiv2020

Exceptional Points of Degeneracy Directly Induced by Space-Time Modulation of a Single Transmission Line

We demonstrate how exceptional points of degeneracy (EPDs) are induced in a single transmission line (TL) directly by applying periodic space-time modulation to the per-unit-length distributed capacitance. In such space-time modulated (STM)-TL, two eigenmodes coalesce into a single degenerate one, in their eigenvalues (wavenumbers) and eigenvectors (voltage-current states) when the system approaches the EPD condition. The EPD condition is achieved by tuning a parameter in the space-time modulation, such as spatial or temporal modulation frequency, or the modulation depth. We unequivocally demonstrate the occurrence of the EPD by showing that the bifurcation of the wavenumber around the EPD is described by the Puiseux fractional power series expansion. We show that the first order expansion is sufficient to approximate well the dispersion diagram, and how this "exceptional" sensitivity of an STM-TL to tiny changes of any TL or modulation parameter enables a possible application as a highly sensitive TL sensor when operating at an EPD.

physics.app-ph↗

arXiv2020

Helicity Density Maximization in a Planar Array of Achiral High-Density Dielectric Nanoparticles

We investigate how a periodic array composed of achiral isotropic high-refractive index dielectric nanospheres generates nearfield over the array surface reaching helicity density very close to its upper bound. The required condition for an array of nanospheres to generate optimally chiral nearfield, which represents the upper bound of helicity density, is derived in terms of array effective electric and magnetic polarizabilities that almost satisfy the effective Kerker condition for arrays. The discussed concepts find applications in improving chirality detection based on circular dichroism (CD) at surface level instead of in the bulk. Importantly the array would not contribute to the generated CD signal when used as a substrate for detecting chirality of a thin layer of chiral molecules. This eliminates the need to separate the CD signal generated by the array from that of the chiral sample.

physics.app-ph↗

arXiv2020

General Conditions to Realize Exceptional Points of Degeneracy in Two Uniform Coupled Transmission Lines

We present the general conditions to realize a fourth order exceptional point of degeneracy (EPD) in two uniform (i.e., invariant along z) lossless and gainless coupled transmission lines (CTLs), namely, a degenerate band edge (DBE). Until now the DBE has been shown only in periodic structures. In contrast, the CTLs considered here are uniform and subdivided into four cases where the two TLs support combinations of forward propagation, backward propagation and evanescent modes (when neglecting the mutual coupling). We demonstrate for the first time that a DBE is supported in uniform CTLs when there is proper coupling between: (i) propagating modes and evanescent modes, (ii) forward and backward propagating modes, or (iii) four evanescent modes (two in each direction). We also show that the loaded quality factor of uniform CTLs exhibiting a fourth order EPD at k=0 is robust to series losses due to the fact that the degenerate modes do not advance in phase. We also provide a microstrip possible implementation of a uniform CTL exhibiting a DBE using periodic series capacitors with very sub-wavelength unit-cell length. Finally, we show an experimental verification of the existence DBE for a microstrip implementation of a CTL supporting coupled propagating and evanescent modes.

physics.app-ph↗

arXiv2020

Exceptional Point of Degeneracy in Backward-Wave Oscillator with Distributed Power Extraction

We show how an exceptional point of degeneracy (EPD) is formed in a system composed of an electron beam interacting with an electromagnetic mode guided in a slow wave structure (SWS) with distributed power extraction from the interaction zone. Based on this kind of EPD, a new regime of operation is devised for backward wave oscillators (BWOs) as a synchronous and degenerate regime between a backward electromagnetic mode and the charge wave modulating the electron beam. Degenerate synchronization under this EPD condition means that two complex modes of the interactive system do not share just the wavenumber, but they rather coalesce in both their wavenumbers and eigenvectors (polarization states). In principle this new condition guarantees full synchronization between the electromagnetic wave and the beam's charge wave for any amount of output power extracted from the beam, setting the threshold of this EPD-BWO to any arbitrary, desired, value. Indeed, we show that the presence of distributed radiation in the SWS results in having high-threshold electron-beam current to start oscillations which implies higher power generation. These findings have the potential to lead to highly efficient BWOs with very high output power and excellent spectral purity.

physics.app-ph↗

arXiv2019

Illusion Mechanisms with Cylindrical Metasurfaces: A General Synthesis Approach

We explore the use of cylindrical metasurfaces in providing several illusion mechanisms including scattering cancellation and creating fictitious line sources. We present the general synthesis approach that leads to such phenomena by modeling the metasurface with effective polarizability tensors and by applying boundary conditions to connect the tangential components of the desired fields to the required surface polarization current densities that generate such fields. We then use these required surface polarizations to obtain the effective polarizabilities for the synthesis of the metasurface. We demonstrate the use of this general method for the synthesis of metasurfaces that lead to scattering cancellation and illusion effects, and discuss practical scenarios by using loaded dipole antennas to realize the discretized polarization current densities. This study is the first fundamental step that may lead to interesting electromagnetic applications, like stealth technology, antenna synthesis, wireless power transfer, sensors, cylindrical absorbers, etc.

physics.app-ph↗

arXiv2019

Simultaneous Perfect Bending and Polarization Rotation of Electromagnetic Wavefront using Chiral Gradient Metasurfaces

We introduce chiral gradient metasurfaces that allow perfect transmission of all the incident wave into a desired direction and simultaneous perfect rotation of the polarization of the refracted wave with respect to the incident one. Besides using gradient polarization densities which provide bending of the refracted wave with respect to the incident one, using metasurface inclusions that are chiral allows the polarization of the refracted wave to be rotated. We suggest a possible realization of the proposed device by discretizing the required equivalent surface polarization densities realized by proper helical inclusions at each discretization point. By only using a single optically thin layer of chiral inclusions, we are able to unprecedentedly deflect a normal incident plane wave to a refracted plane wave at $45^{\circ}$ with $72\%$ power efficiency which is accompanied by a $90^{\circ}$ polarization rotation. The proposed concepts and design method may find practical applications in polarization rotation devices at microwaves as well as in optics, especially when the incident power is required to be deflected.

physics.app-ph↗

arXiv2019

Exceptional Points of Degeneracy in Periodic Coupled Waveguides and the Interplay of Gain and Radiation Loss: Theoretical and Experimental Demonstration

We present a novel paradigm for dispersion engineering in coupled transmission lines (CTLs) based on exceptional points of degeneracy (EPDs). We develop a theory for fourth-order EPDs consisting of four Floquet-Bloch eigenmodes coalescing into one degenerate eigenmode. We present unique wave propagation properties associated to the EPD and develop a figure of merit to assess the practical occurrence of fourth-order EPDs in CTLs with tolerances and losses. We experimentally verify for the first time the existence of a fourth EPD (the degenerate band edge), through dispersion and transmission measurements in microstrip-based CTLs at microwave frequencies. In addition, we report that based on experimental observation and the developed figure of merit, the EPD features are still observable in structures that radiate (leak energy away), even in the presence of fabrication tolerances and dissipative losses. We investigate the gain and loss balance regime in CTLs as a mean of recovering an EPD in the presence of radiation and/or dissipative losses, without necessarily resorting to Parity-Time (PT)-symmetry regimes. The versatile EPD concept is promising in applications such as high intensity and power-efficiency oscillators, spatial power combiners, or low-threshold oscillators and opens new frontiers for boosting the performance of large coherent sources.

physics.app-ph↗

arXiv2018

Applications and Potentials of Reciprocal Bianisotropic Metasurfaces

We discuss different applications and potentials of reciprocal bianisotropic metasurfaces including uniform and gradient metasurfaces, in particular, with anisotropic, chiral, and omega properties. Based on an analytic analysis, we discuss the necessary conditions to observe asymmetric co- and cross- polarization reflection and/or transmission, polarization conversion and rotation, asymmetric absorption, etc. We consider two kinds of incident wave scenarios based on linear and circular polarization.

physics.optics↗

arXiv2018

Optimally Chiral Electromagnetic Fields: Helicity Density and Interaction of Structured Light with Nanoscale Matter

We propose the concept of helicity maximization applicable to structured light and obtain a universal rela-tion for the maximum of helicity density at a given field energy density. We further demonstrate that us-ing structured light with maximized helicity density eliminates the need of the specific knowledge of en-ergy and helicity densities in determining the chirality of a nanoparticle. The helicity maximization con-cept generalizes the use of the dissymmetry factor in chirality detection to any chiral structure light il-luminating nanoparticles.

physics.app-ph↗

arXiv2018

Helicity Maximization of Structured Light to Empower Nanoscale Chiral Matter Interaction

Structured light enables the characterization of chirality of optically small nanoparticles by taking advantage of the helicity maximization concept recently introduced in[1]. By referring to fields with nonzero helicity density as chiral fields, we first investigate the properties of two chiral optical beams in obtaining helicity density localization and maximization requirements. The investigated beams include circularly polarized Gaussian beams and also an optical beam properly composed by a combination of a radially and an azi-muthally polarized beam. To acquire further enhancement and localization of helicity density beyond the diffraction limit, we also study chiral fields at the vicinity of a spherical dielectric nanoantenna and demon-strate that the helicity density around such a nanoantenna is a superposition of helicity density of the illu-minating field, scattered field, and an interference helicity term. Moreover, we illustrate when the nanoan-tenna is illuminated by a proper combination of azimuthal and radially polarized beams, the scattered nearfields satisfy the helicity maximization conditions beyond the diffraction limit. The application of the concept of helicity maximization to nanoantennas and generating optimally chiral nearfield result in helici-ty enhancement which is of great advantage in areas like detection of nanoscale chiral samples, microsco-py, and optical manipulation of chiral nanoparticles.

physics.app-ph↗