First-principles analysis of energy exchange in time-varying capacitors for energy trapping applications

Time-varying networks, consisting of lumped elements, such as resistors, capacitors, and inductors, actively modulated in time, have introduced a host of novel wave phenomena and witnessed a remarkable development during recent years. This paper investigates the scattering from a time varying capacitor and how such a load can be fully reflectionless when the capacitance is suitably modulated in time. We analytically derive the required temporal dependence of the capacitance and show how in contrast to other techniques it avoids extreme and negative values and, as a result, can be implemented in a feasible way, when the capacitor is charged with a DC voltage source. We also derive from first principles the energy balance of such a time-varying capacitor, proving that the energy of an incoming pulse is transferred to the modulation source. Our findings clarify scattering of waves from time-varying capacitors and open up a new way to matching of broadband pulses

Transition Functions for Closed Form Representation of Metasurface Reactance

Metasurfaces are thin metamaterials used for manipulating propagation of plane waves and surface-waves (SWs). They can be characterized by homogenized-boundary conditions, which, in absence of losses, can be represented through an equivalent reactance. In this paper, we introduce a general representation of isotropic frequency-dependent reactance which is valid along the dispersion curve of the relevant TM SW. This representation is written in terms of a transition function derived from a manipulation of the Cardano's formula for third-degree algebraic equations. Throughout a large portion of the dispersion curve, this transition function depends on one parameter only, which is an equivalent quasi-static capacitance. Approaching the Floquet-Bloch region, where many higher order Floquet modes are excited, two additional parameters should be extracted from the full-wave data to complete the transitional representation of the reactance until the upper boundary of the Brillouin region. The final formula is valid for a generic isotropic reactance and for an anisotropic reactance when the direction of propagation is along a symmetry axis of the unit cell element

Addressing surface waves on modulated metasurfaces

This work presents an extension of Transformation Optics (TO) to control the wavefront of surface waves (SW) through the use of modulated metasurfaces. As the outputs of the conventional TO approach are the metamaterial constitutive parameters able to perform a certain modification of the ray-field path, here the outcomes are the components of the metasurface reactance tensor. This methodology can be applied to design a large number of planar devices including lenses, beam splitters and invisibility cloaks. © 2013 IEEE

Reconfigurable nonlinear optical element using tunable couplers and inverse-designed structure

In recent years, wave-based analog computing has been at the center of attention for providing ultra-fast and power-efficient signal processing enabled by wave propagation through artificially engineered structures. Building on these structures, various proposals have been put forward for performing computations with waves. Most of these proposals have been aimed at linear operations, such as vector-matrix multiplications. The weak and hardly controllable nonlinear response of electromagnetic materials imposes challenges in the design of wave-based structures for performing nonlinear operations. In the present work, first, by using the method of inverse design we propose a three-port device, which consists of a combination of linear and Kerr nonlinear materials, exhibiting the desired power-dependent transmission properties. Then, combining a proper arrangement of such devices with a collection of Mach–Zehnder interferometers (MZIs), we propose a reconfigurable nonlinear optical architecture capable of implementing a variety of nonlinear functions of the input signal. The proposed device may pave the way for wave-based reconfigurable nonlinear signal processing that can be combined with linear networks for full-fledged wave-based analog computing

Compact Binary Coalescences: Astrophysical Processes and Lessons Learned

On 11 February 2016, the LIGO and Virgo scientific collaborations announced the first direct detection of gravitational waves, a signal caught by the LIGO interferometers on 14 September 2015, and produced by the coalescence of two stellar-mass black holes. The discovery represented the beginning of an entirely new way to investigate the Universe. The latest gravitational-wave catalog by LIGO, Virgo and KAGRA brings the total number of gravitational-wave events to 90, and the count is expected to significantly increase in the next years, when additional ground-based and space-born interferometers will be operational. From the theoretical point of view, we have only fuzzy ideas about where the detected events came from, and the answers to most of the five Ws and How for the astrophysics of compact binary coalescences are still unknown. In this work, we review our current knowledge and uncertainties on the astrophysical processes behind merging compact-object binaries. Furthermore, we discuss the astrophysical lessons learned through the latest gravitational-wave detections, paying specific attention to the theoretical challenges coming from exceptional events (e.g., GW190521 and GW190814)

Dispersion Characteristics of Additive-Manufactured Metasurfaces

International audienceThis paper presents an efficient and accurate approach for the analysis of additive-manufactured metasurfaces (MTSs) consisting of dielectric posts grown on a grounded slab. The formulation is based conventional effective medium theory that allows modeling such MTSs as uniaxially anisotropic grounded slabs. This procedure can be used to effectively characterize the surface waves supported by additive-manufactured MTSs, as needed in the design of planar lenses and leaky-wave antennas

Compact Binary Coalescences: Astrophysical Processes and Lessons Learned

On 11 February 2016, the LIGO and Virgo scientific collaborations announced the first direct detection of gravitational waves, a signal caught by the LIGO interferometers on 14 September 2015, and produced by the coalescence of two stellar-mass black holes. The discovery represented the beginning of an entirely new way to investigate the Universe. The latest gravitational-wave catalog by LIGO, Virgo and KAGRA brings the total number of gravitational-wave events to 90, and the count is expected to significantly increase in the next years, when additional ground-based and space-born interferometers will be operational. From the theoretical point of view, we have only fuzzy ideas about where the detected events came from, and the answers to most of the five Ws and How for the astrophysics of compact binary coalescences are still unknown. In this work, we review our current knowledge and uncertainties on the astrophysical processes behind merging compact-object binaries. Furthermore, we discuss the astrophysical lessons learned through the latest gravitational-wave detections, paying specific attention to the theoretical challenges coming from exceptional events (e.g., GW190521 and GW190814)

A Closed-Form Representation of Isofrequency Dispersion Curve and Group Velocity for Surface Waves Supported by Anisotropic and Spatially Dispersive Metasurfaces

A general closed-form representation is introduced for representing the isofrequency dispersion curve (IDC) of an anisotropic, spatially, and frequency dispersive metasurface (MTS) constituted by a dense periodic texture of metallic elements printed on a grounded substrate. The formulation is restricted to printed elements isolated from each other (namely, patches and not slots) whose geometry exhibits at least two axes of symmetry. The expression is valid for the dominant TM surface wave (SW) until the limit of the Floquet-Bloch (FB) region and generalizes our previous formulation to arbitrary direction of propagation. This generalization permits a closed-form representation of the IDCs and of the group velocity as a function of two parameters only; these are the equivalent quasi-static capacitances along the symmetry directions of the geometry. The limit of validity of the closed-form representation has been defined and the formulation has been tested by full-wave analysis. The present formulation simplifies the design of MTS antennas and flat transformation optics devices

Surface wave dispersion for anisotropic metasurfaces constituted by elliptical patches

This paper presents an effective approach for the derivation of the two-dimensional (2-D) frequency-wavenumber dispersion surface of anisotropic metasurfaces (MTSs) consisting of elliptical patches printed over a grounded slab. These MTSs are important in the design of leaky-wave antennas and transformation optics (TO) surface-wave based devices. The formulation resorts to an analytical expression of the currents excited on the element of the periodic texture to define a reduced spectral method of moments (MoM) procedure with only three basis functions. An exact compact formula, which links the MoM matrix to the homogenized equivalent anisotropic impedance of the MTS, is derived. The formulation presented here has been found accurate and useful for designing MTS antennas and TO devices

Static-to-dynamic field conversion with time-varying media

International audienceWe theoretically demonstrate that a uniform static electric field distribution can be partially converted to radiation fields when a portion of the medium undergoes a temporal change of its permittivity. An in-depth theoretical investigation of this phenomenon is developed for a dielectric block with a steplike temporal change located inside a waveguide charged with a DC voltage source. Closed analytical expressions are derived for the radiated electric and magnetic fields. The exchange of energy between the electrostatic and electromagnetic fields is discussed. The reconciliation between the seemingly contradictory temporal and spatial boundary conditions for the electric and magnetic fields at the interface of the time-varying dielectric block is analyzed and elucidated. Our findings may provide an alternative solution for generating electromagnetic radiation based on time-varying media