arXiv subjects

Carlos Molero

Publications and source records attributed to Carlos Molero.

16 recordsLinked to original sources

arXiv2026

A Dual-Mode FM/AM Modulator Based on a Time-Varying Inverting Integrator

This paper presents the analysis, design, fabrication, and experimental validation of a dual-mode frequency/amplitude modulator based on a time-modulated varactor diode. By exploiting the varactor as a time-varying capacitor in combination with an operational amplifier configured as an inverting integrator and a passband filter, the proposed circuit generates frequency-modulated (FM) signals in an efficient manner. Amplitude-modulated (AM) signals can also be obtained with a simple modification. The implementation, realized in microstrip technology, leverages the unique properties of time-modulated electronic components, particularly their inherent frequency-mixing capability. Analytical expressions are derived to predict the characteristics of the generated waveforms, and their accuracy is verified through numerical simulations performed in Keysight ADS. A microstrip PCB prototype is then fabricated and experimentally characterized. The measured results show excellent agreement with both the theoretical predictions and the numerical simulations. The proposed approach demonstrates the potential of time-varying capacitors as an attractive alternative to conventional FM techniques for telecommunications and radar applications.

eess.SP↗

arXiv2026

Temporal Paraxial Optics under Adiabatic Modulations

This paper presents a temporal paraxial formulation for the propagation of ultrashort optical pulses in time-modulated media with slowly varying refractive index. By deriving the paraxial wave equation directly in the time domain from the Helmholtz equation under an adiabatic approximation, the model remains analytically tractable while extending paraxial optics beyond time-invariant backgrounds commonly treated by frequency-domain expansions. The resulting equation preserves a Schrödinger-like structure in the presence of explicit temporal modulation and admits closed-form solutions for ultrashort Gaussian pulses. The framework supports a Green's-function description and an operator-based Hamiltonian formalism, from which an ABCD matrix representation for temporal propagation in time-varying media is obtained. The results demonstrate that temporal modulation provides a dynamic means to control ultrashort pulse dynamics, enabling tailored evolution of pulse characteristics such as temporal width and chirp, with potential applications in ultrafast pulse shaping and a direct connection to temporal wave-packet dynamics.

physics.optics↗

arXiv2026

Gain and One-Way Propagation in Synthetically Moving non-Foster Gratings

In this paper, we analyze the electromagnetic properties of space-time grooved metal surfaces governed by uniform-velocity modulations. We begin by deriving the electromagnetic fields of a uniform-velocity-modulated parallel-plate waveguide (PPW) using Lorentz transformations, establishing it as the fundamental building block of a more complex space-time structures. We then analyze the dispersion and scattering characteristics of a space-time grooved surface and later extend the study to the interaction between two facing surfaces. Our findings show that these systems can indeed amplify electromagnetic waves and exhibit non-reciprocal as well as non-Foster behavior. Moreover, under specific conditions, they enable the formation of unidirectional propagation channels, effectively constraining light to be guided along a single direction. These results unveil new opportunities for the design of advanced electromagnetic and photonic devices.

physics.optics↗

arXiv2026

Time-Controlled Resonances in 2-D Metasurfaces via Equivalent Circuits

This work introduces a semi-analytical frequency-domain framework for the analysis of two-dimensional, time-modulated (2+1)-D metasurfaces controlled by PIN diodes. The formulation focuses on the unit-cell level, modeled as a waveguide discontinuity problem, where the space-time periodicity of the structure enables the representation of scattered fields via Floquet expansions. After appropriate mathematical treatment, these expansions lead to an equivalent circuit description of the metasurface, providing physical insight into its spatiotemporal scattering behavior and facilitating the design of reconfigurable electromagnetic devices. The model is employed to explore key phenomena present in space-time systems, such as frequency mixing and spatiotemporal scattering. In addition, dynamic tuning is explored in resonant metasurfaces, where time becomes an additional degree of freedom for the design. The dynamic control of resonances opens a new way to explore multi-band and wideband behaviors from very thin metasurfaces under temporal coupling.

physics.app-ph↗

arXiv2025

Transfer ABCD Matrix for Time-Varying Media and Time Crystals

This paper introduces a formal definition of the transfer ABCD parameters in time-varying electromagnetic systems. The formal definition comes after the rearrangement of the fields $D$ and $B$ at the inputs and outputs of the temporal system based on the time-varying boundary conditions. Then, we derive the ABCD parameters of a temporal transmission line, i.e., a temporal slab, and compute the associated scattering parameters (reflection and transmission coefficients). The results presented here open up an alternative way, based on network theory, to analyze multilayer temporal configurations. Moreover, we show that the ABCD parameters can be used to compute the dispersion diagram ($ω$ vs $k$) of time crystals.

physics.app-ph↗

arXiv2024

Transient States to Control the Electromagnetic Response of Space-time Dispersive Media

In this paper, we study the dynamic formation of transients when plane waves impinge on a dispersive slab that abruptly changes its electrical properties in time. The time-varying slab alternates between air and metal-like states, whose frequency dispersion is described by the Drude model. It is shown how the physics of this complex system can be well described with the joint combination of two terms: one associated with temporal refractions and the other associated with spatial refractions. To test the validity of the approach, some analytical results are compared with a self-implemented finite-difference time-domain (FDTD) method. Results show how the transients that occurred after the abrupt temporal changes can shape the overall steady-state response of the space-time system. In fact, far from always being detrimental, these transient states can be conveniently used to perform frequency conversion or to amplify/attenuate the electromagnetic fields.

physics.app-ph↗

arXiv2024

Space-time Metallic Metasurfaces for Frequency Conversion and Beamforming

This paper details a class of metal-based space-time metasurfaces for application in wireless communications scenarios. Concretely, we describe space-time metasurfaces that periodically alternate their properties in time between three spatial states: "air", "conductor" and "grating". We analyze the physics of these metastructures via a computationally-efficient analytical technique based on the use of Floquet-Bloch series, integral equations and circuit models. By doing so, we reveal important features of these spatiotemporal metasurfaces: scattering parameters, field profiles, diffraction angles and nature of the space-time harmonics. The results, corroborated with a self-implemented numerical FDTD approach, show the potential application of these space-time metasurfaces as beamformers acting in reflection, in transmission or both. The amplitude and direction of the diffracted orders can be electronically controlled with the paramaters of the metasurface. Moreover, the intrinsic ability of time-modulated diffractive metasurfaces to mix and multiply frequencies is tested. We show how two different modulations can lead to the same diffraction angle but with different mixed output frequencies.

physics.app-ph↗

arXiv2023

Analytical Equivalent Circuits for Three-dimensional Metamaterials and Metagratings

In recent times, three-dimensional (3D) metamaterials have undergone a revolution driven mainly by the popularization of 3D-printing techniques, which has enabled the implementation of modern microwave and photonic devices with advanced functionalities. However, the analysis of 3D metamaterials is complex and computationally costly in comparison to their 1D and 2D counterparts due to the intricate geometries involved. In this paper, we present a fully-analytical framework based on Floquet-Bloch modal expansions of the electromagnetic fields and integral-equation methods for the analysis of 3D metamaterials and metagratings. Concretely, we focus on 3D configurations formed by periodic arrangements of rectangular waveguides with longitudinal slot insertions. The analytical framework is computationally efficient compared to full-wave solutions and also works under oblique incidence conditions. Furthermore, it comes associated with an equivalent circuit that allows to gain physical insight into the scattering and diffraction phenomena. The analytical equivalent circuit is tested against full-wave simulations in commercial software CST. Simulation results show that the proposed 3D structures provide independent polarization control of the two orthogonal polarizations states. This key property is of potential interest for the production of full-metal polarizers, such as the one illustrated.

physics.app-ph↗

arXiv2023

Analysis of Metallic Space-Time Gratings using Lorentz Transformations

This paper presents an analytical framework for the study of scattering and diffraction phenomena in spacetime-modulated metallic gratings. Using a Lorentz transformation, it is shown that a particular class of spacetime-modulated gratings behave effectively as moving media. We take advantage of this property to derive a closed analytical solution for the wave scattering problem. In particular, using our formalism it is possible to avoid spacetime Floquet-Bloch expansions, as the solution of the problem in the original laboratory frame (grating parameters are periodic in space and time) is directly linked to a co-moving frame where the metallic grating is time-invariant (grating parameters are periodic only in space). In this way, we identify a fundamental connection between moving metallic gratings and spacetime-modulated metamaterials, and exploit this link to study the nonreciprocal response of the structure. Some limitations and difficulties of the alternative nonrelativistic Galilean approach are discussed and the benefits of the Lorentz approach are highlighted. Finally, some analytical results are presented in order to validate the formalism. The results include scenarios involving TM(p) and TE(s) normal and oblique incidence, even beyond the onset of the diffraction regime. Furthermore, we show how the synthetic Fresnel drag can tailor the Goos-Hänchen effect and create a specular point shifted towards the direction of the synthetic motion, independent of the sign of the incidence angle.

physics.app-ph↗

arXiv2023

Analytical Framework to Model Reconfigurable Metasurfaces including Lumped Elements

This paper presents an analytical framework, based on Floquet modal expansions of the electromagnetic fields and equivalent circuits, to model reconfigurable metasurfaces loaded with generic lumped elements (resistors, capacitors, inductors, varactors, etc.). The analytical approach is computationally efficient compared to full-wave solvers. Additionally, it works under oblique-incidence conditions in a wideband range of frequencies, even far beyond the onset of the first grating lobe (diffraction regime). The analytical framework is validated with some numerical examples in the commercial software CST Studio Suite, demonstrating its potential for analyzing and designing RF and microwave devices, including lumped elements, such as absorbers, polarizers, and reflectarray/transmitarray cells.

physics.app-ph↗

arXiv2023

Three-Dimensional Fully Metallic Dual Polarization Frequency Selective Surface Design Using Coupled-Resonator Circuit Information

This work employs a new approach to analyze coupled-resonator circuits to design and manufacture a fully metallic dual polarization frequency selective surface (FSS). The proposed filtering structure is composed of a series of unit cells with resonators fundamentally coupled along the z-direction and then repeated periodically in the xy-plane. The fully metallic cascaded unit cell is rigorously analyzed within an infinite periodic environment as a coupled-resonator electromagnetic (EM) circuit. The convenient design of the EM resonators makes it possible to push the evanescent EM field through the metallic structure in the desired frequency band for both polarizations. An FSS prototype is manufactured and measured, and good agreement is found between the simulation results and the final prototype.

physics.app-ph↗

arXiv2023

Diffraction Phenomena in Time-varying Metal-based Metasurfaces

This paper presents an analytical framework for the analysis of time-varying metal-based metamaterials. Concretely, we particularize the study to time-modulated metal-air interfaces embedded between two different semi-infinite media that are illuminated by monochromatic plane waves of frequency $ω_0$. The formulation is based on a Floquet-Bloch modal expansion, which takes into account the time periodicity of the structure ($T_s = 2π/ ω_s)$, and integral-equation techniques. It allows to extract the reflection/transmission coefficients as well as to derive nontrivial features about the dynamic response and dispersion curves of time-modulated metal-based screens. In addition, the proposed formulation has an associated analytical equivalent circuit that gives physical insight to the diffraction phenomenon. Similarities and differences between space- and time-modulated metamaterials are discussed via the proposed circuit model. Finally, some analytical results are presented to validate the present framework. A good agreement is observed with numerical computations provided by a self-implemented finite-difference time-domain (FDTD) method. Interestingly, the present results suggest that time-modulated metal-based screens can be used as pulsed sources (when $ω_s \ll ω_0$), beamformers ($ω_s \sim ω_0$) to redirect energy in specific regions of space, and analog samplers ($ω_s \gg ω_0$).

physics.app-ph↗

arXiv2022

Time-periodic Metallic Metamaterials defined by Floquet Circuits

In this paper, we study the scattering and diffraction phenomena in time-modulated metamaterials of metallic nature by means of Floquet equivalent circuits. Concretely, we focus on a time-periodic screen that alternates between "metal" and "air" states. We generalize our previous approaches by introducing the concepts of "macroperiod" and "duty cycle" to the time modulation. This allows to analyze time-periodic metallic metamaterials whose modulation ratios are, in general, rational numbers. Furthermore, with the introduction of the duty cycle, perfect temporal symmetry is broken within the time modulation as the time screen could remain a different amount of time in metal and air states. Previous statements lead to an enrichment of the diffraction phenomenon and to new degrees of freedom that can be exploited in engineering to control the reflection and transmission of electromagnetic waves. Finally, we present some analytical results that are validated with a self-implemented finite-difference time-domain (FDTD) approach. Results show that the scattering level and diffraction angles can be controlled independently by means of the duty cycle and the modulation ratio, respectively. Thus, novel time-based pulsed sources and beamformers can be efficiently designed

physics.app-ph↗

arXiv2022

3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques

Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow the creation of negative refractive indexes, light trapping with epsilon-near-zero compounds, bandgap selection, superconductivity phenomena, non-Hermitian responses and, more generally, to manipulate the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited the attention to 1-D and 2-D metamaterials. However, the true potential of metamaterials will be ultimately reached in 3-D configurations, when the degrees of freedom associated to the propagating direction are finally exploited in design. This is expected to lead to a new era in metamaterial field, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs and functional manufacturing techniques.

physics.optics↗

arXiv2021

Cross-polarization Control in FSSs by means of an Equivalent Circuit Approach

This paper presents an efficient equivalent circuit approach (ECA), based on a Floquet modal expansion, for the study of the co- and cross-polarization in frequency selective surfaces (FSS) formed by periodic arrays of patches/apertures in either single or stacked configurations. The ECA makes it possible the derivation of analytical expressions for the generalized scattering parameters associated with the proposed circuit networks. Furthermore, the proposed circuit approach is an efficient surrogate model that can be combined with optimization techniques and artificial intelligence algorithms for the efficient design of FSS structures, saving efforts in the computation compared to time-consuming full-wave simulators and tedious synthesis (simulation-assisted) techniques. Due to the simplicity of the topology of the involved networks, the ECA can also be advantageously used to gain physical insight. The proposed approach is applied and validated in different FSS configurations where the cross-pol component plays a fundamental role in the design, as in circular polarizers, polarization rotators, and reflectarray cells.

physics.app-ph↗

arXiv2021

Exploring the Potentials of the Multi-modal Equivalent Circuit Approach for Stacks of 2-D Aperture Arrays

Many frequency selective surface (FSS) structures are based on the use of a single periodic array of slot/apertures in a conducting sheet embedded in a layered medium. However, it is well known that stacking several conducting sheets and breaking the alignment of the stack can bring multiple benefits to the structure. In this paper, the analysis and design of stacks of 2-D aperture arrays are carried out by exploiting as much as possible all the potentialities of a rigorous and systematic formulation based on the multi-modal equivalent circuit approach (ECA). A key feature of the formulation is that linear transformations between the apertures of adjacent plates (rotation, translation, and scaling) can be dealt with from a purely analytical perspective. This fact is of potential interest for many practical applications, such as the design of polarization converters, absorbers, filters, and thin matching layers. When the apertures have an arbitrary geometry, it can be applied a hybrid approach that combines the ability of commercial simulators to handle arbitrary geometries with the fast computation times and physical insight of the ECA. In general, either the purely analytical or the hybrid approach can be applied in those many practical scenarios where the spatial profile of the electric field on the considered apertures hardly changes with frequency. As an additional feature of the approach, the dispersion properties (phase/attenuation constants and Bloch impedance) of infinite periodic stacks can be derived and, in particular, analytical expressions for mirror- and glide-symmetric configurations are provided.

physics.app-ph↗