arXiv subjects

David E. Fernandes

Publications and source records attributed to David E. Fernandes.

At least 19 recordsLinked to original sources

arXiv2026

Continuous modal spectrum in nonreciprocal cavities

Nonreciprocal systems enable asymmetric energy transport and suppress backscattering, giving rise to unconventional wave phenomena. Here, we show that nonreciprocal cavities based on unidirectional waveguides exhibit a continuous modal spectrum, in contrast to conventional cavities with discrete eigenmodes. Using a ferrite-loaded microwave cavity as an example, we demonstrate that enforcing unidirectionality, by tailoring the waveguide geometry, drives a transition from discrete to continuous spectra, accompanied by strong spatial localization of electromagnetic fields. Our results reveal that dissipation alone fails to regularize these singular responses, highlighting the need for additional mechanisms to control localization in nonreciprocal systems.

physics.optics↗

arXiv2024

Plasmonic Time Crystals

We study plasmonic time crystals, an extension of dielectric-based photonic time crystals to plasmonic media. Remarkably, we demonstrate that such systems may amplify both longitudinal and transverse modes. In particular, we show that plasmonic time crystals support \emph{collective resonances} of longitudinal modes, which occur independently of the wave vector $k$, even in the presence of significant dissipation. These resonances originate from the coupling between the positive- and negative-frequency branches of the plasmonic dispersion relation of the unmodulated system and from the divergence of the density of states near the plasma ($\varepsilon$-near zero) frequency $\omega_p$. The strongest resonance arises at a modulation frequency $\Omega = 2 \omega_p$, corresponding to a direct interband transition. We demonstrate these resonances for various periodic modulation profiles and provide a generic perturbative formula for resonance widths in the weak modulation limit. Furthermore, we propose transparent conducting oxides as promising platforms for realizing plasmonic time crystals, as they enable significant modulation of the electron effective mass while maintaining moderate dissipation levels. Our findings provide new insights into leveraging time-modulated plasmonic media to enhance optical gain and control wave dynamics at the nanoscale.

physics.optics↗

arXiv2024

Chiral-Gain Photonics

Here, we present an exploratory study of the potential applications of electrically biased materials that possess a nonreciprocal and non-Hermitian electromagnetic response analogous to the electronic response of field-effect transistors. The most distinctive feature of such materials is their chiral-gain, meaning that their response can be active or dissipative depending on the handedness of the wave polarization. Here, we show how the chiral-gain can be harnessed to develop novel electromagnetic devices with unique properties such as chiral lasers, polarization-dependent mirrors, and coherent-perfect-absorber lasers. Furthermore, it is demonstrated that materials with chiral-gain can bypass a reciprocity constraint that typically limits the external coupling strength, thus facilitating the excitation of cavities with extremely large quality factors.

physics.optics↗

arXiv2024

High Q-factor Fano resonances in coupled wire arrays with bulk structural asymmetry

In this article we characterize the macroscopic electromagnetic response of a nested wire metamaterial with a high degree of bulk structural asymmetry. The unit cell of the considered metamaterial contains two sets of metallic wires, one set consisting of an array of straight wires and another formed by a racemic array of helical-shaped wires. We study the scattering of electromagnetic waves in a metamaterial slab and show that the electromagnetic coupling between both arrays of wires can originate Fano resonances with narrow lineshape. The origin of the resonances is rooted in the formation of a sub-radiant mode in the metamaterial wherein the net polarization vector vanishes. We envision that the proposed configuration, with sharp resonances whose quality factor can be greatly enhanced by tuning the structural parameters of the wires, may have promising applications in sensing and switching in a broad frequency range from the microwave regime up to THz frequencies.

physics.app-ph↗

arXiv2024

Exceptional points in transistor-metamaterial inspired transmission lines

Motivated by our recent findings in [Phys. Rev. Lett. 128, 013902, 2022], which introduces a new class of electromagnetic bulk materials whose response is similar to conventional semiconductor transistors, here we propose a one-dimensional (1D) version of such a material based on transmission lines coupled with FET isolators. We demonstrate that the response of this 1D system is nonreciprocal and non-Hermitian, analogous to the idealized transistor-metamaterial, and is also characterized by a broken time-reversal symmetry. We analyze the wave propagation in the system and find that the interaction between the eigenmodes can either lead to gain or loss depending on the propagation distance. Furthermore, it is also shown that the system may be operated at an exceptional point, wherein the response of the structure is singular, and the power gain is maximized. Finally, we demonstrate that the exceptional point coincides with the point of operation of typical microwave amplifiers, such as the distributed amplifier.

physics.app-ph↗

arXiv2023

Enhancing the Directional Violation of Kirchhoff's Law of Thermal Radiation with a Nonreciprocal Wire Medium

In this work, we develop a homogenization model to determine the effective response of a metallic nanowire array embedded in an electric gyrotropic material. We study the interaction of electromagnetic waves with the metamaterial and demonstrate that the nanowire array can greatly enhance the nonreciprocal response of the gyrotropic substrate. In particular, the metamaterial can either absorb the incoming energy almost entirely or reflect it with little loss, depending on the sign of the incidence angle. We explore the implications of our findings in the context of Kirchhoff's law of thermal radiation. Our results demonstrate that the wire array can boost the difference between the emissivity and absorptivity in a broad spectrum of frequencies and incidence angles as compared to an unstructured gyrotropic substrate. These findings suggest potential applications for the nonreciprocal wire medium in thermal management, radiative cooling, and others.

physics.app-ph↗

arXiv2023

Experimental verification of magnetic near-field channeling using a helical-shaped wire medium lens

We experimentally verify that a magnetic uniaxial wire medium lens consisting of a racemic array of helical-shaped metallic wires may enable channeling the normal component of the magnetic field of near-field sources with resolution well below the diffraction limit over a broad bandwidth. It is experimentally demonstrated that the helical-shaped wire medium lens can be regarded as the magnetic counterpart of the usual wire medium lenses formed by straight metallic wires. The experimental results are validated with full-wave numerical simulations. We envision that the proposed metamaterial lens may have potential applications in magnetic resonance imaging, near-field wireless power transfer, and sensing.

physics.app-ph↗

arXiv2023

Effective Medium Model for Graphene Superlattices with Electrostatic and Magnetic Vector Potentials

In this article we develop an effective medium model to characterize the electron wave propagation in graphene based nanostructures with an electrostatic and magnetic vector potentials imposed on their surface. We use a numerical algorithm to determine the effective medium parameters of the heterostructure and calculate the electronic band structure of the system. We apply our formalism to analyze superlattices with solely a magnetic potential and reveal that the response of the structure remains reciprocal and is characterized by a decrease in charge carrier's velocity. We also study the response of superlattices with both potentials superimposed on graphene and show that the response of the system becomes nonreciprocal with a dispersion characterized by a tilted Dirac cone. We demonstrate that it is possible to alternate between a type-I, type-II or type-III Dirac cones by properly tuning the amplitude of the potentials.

cond-mat.mes-hall↗

arXiv2022

Role of Time-Reversal Symmetry in the Dynamical Response of "One-Way" Nonlinear Devices

We study the role of time-reversal symmetry on the dynamical response of nonlinear optical systems that behave as unidirectional ("one-way") devices. It is shown that lossless nonlinear materials, despite being nonreciprocal, are typically time-reversal invariant. This property raises an apparent paradox because time-reversal invariant systems are forcibly bi-directional. Here, we present a solution for this conundrum, and theoretically explain why the "one-way" behavior can indeed be compatible with the time-reversal invariance. It is found that in the time-reversed problem the incident waves have a variation in time that is generally incompatible with the adiabatic approximation. Due to this reason the adiabatic approximation fails to predict the bi-directional nature of nonlinear system. We discuss the implications of this finding in the performance of practical nonlinear "one-way" devices.

physics.app-ph↗

arXiv2022

Nonreciprocal and non-Hermitian material response inspired by semiconductor transistors

Here, inspired by the operation of conventional semiconductor transistors, we introduce a novel class of bulk materials with nonreciprocal and non-Hermitian electromagnetic response. Our analysis shows that material nonlinearities combined with a static electric bias may lead to a linearized permittivity tensor that lacks the Hermitian and transpose symmetries. Remarkably, the material can either dissipate or generate energy, depending on the relative phase of the electric field components. We introduce a simple design for an electromagnetic isolator based on an idealized "MOSFET-metamaterial" and show that its performance can in principle surpass conventional Faraday isolators due to the material gain. Furthermore, it is suggested that analogous material responses may be engineered in natural media in nonequilibrium situations. Our solution determines an entirely novel paradigm to break the electromagnetic reciprocity in a bulk nonlinear material using a static electric bias.

physics.optics↗

arXiv2022

Optical Tractor Beam with Chiral Light

We suggest a novel mechanism to induce the motion of a chiral material body towards an optical source. Our solution is based on the interference between a chiral light beam and its reflection on an opaque mirror. Surprisingly, it is theoretically shown that the electromagnetic response of the material may be tailored in such a way that independent of the specific body location with the respect to the mirror, it is always pushed upstream against the photon flow associated with the incoming wave. Moreover, it is proven that by controlling the handedness of the incoming light it may be possible to harness the sign of the optical force, switching from a pulling force to a pushing force.

physics.optics↗

arXiv2021

Experimental Verification of Ill-defined Topologies and Energy Sinks in Electromagnetic Continua

In this article, it is experimentally verified that nonreciprocal photonic systems with a continuous translation symmetry may have an ill-defined topology. The topological classification of such systems is only feasible when the material response is regularized with a spatial-frequency cutoff. Here, we experimentally demonstrate that inserting a small air gap in between two materials may effectively imitate an idealized spatial cutoff that suppresses the nonreciprocal response for short wavelengths and regularizes the topology. Furthermore, it is experimentally verified that nonreciprocal systems with an ill-defined topology may be used to abruptly halt the energy flow in a unidirectional waveguide due to the violation of the bulk-edge correspondence. In particular, we report the formation of an energy sink that absorbs the incoming electromagnetic waves with a large field enhancement at the singularity.

physics.app-ph↗

arXiv2021

Time Evolution of Electron Waves in Graphene Superlattices

The time evolution of electron waves in graphene superlattices is studied using both microscopic and 'effective medium' formalisms. The numerical simulations reveal that in a wide range of physical scenarios it is possible to neglect the granularity of the superlattice and characterize the electron transport using a simple effective Hamiltonian. It is verified that as general rule the continuum approximation is rather accurate when the initial state is less localized than the characteristic spatial period of the superlattice. This property holds even when the microsocopic electric potential has a strong spatial modulation or in presence of interfaces between different superlattices. Detailed examples are given both of the time evolution of initial electronic states and of the propagation of stationary states in the context of wave scattering. The theory also confirms that electrons propagating in tailored graphene superlattices with extreme anisotropy experience virtually no diffraction.

cond-mat.mes-hall↗

arXiv2021

Topological origin of electromagnetic energy sinks

It was recently highlighted that nonreciprocal surface waves can be stopped in suitably designed waveguides, leading to the formation of hotspots wherein the electromagnetic fields are massively enhanced. Here, we prove that this phenomenon has a topological origin and is related to a breakdown of the bulk-edge correspondence in electromagnetic continua with no spatial cut-off. Our theoretical analysis shows that nonreciprocal electromagnetic continua can used to realize energy sinks with ultra-singular fields that absorb all the energy generated in their surroundings. Moreover, it is proven that similar energy sinks may be formed in fully reciprocal optical platforms with a parity-time-duality (P-T-D) symmetry.

physics.optics↗

arXiv2021

Single-Beam Optical Conveyor Belt for Chiral Particles

A different paradigm is proposed to selectively manipulate and transport small engineered chiral particles and discriminate different enantiomers using unstructured chiral light. It is theoretically shown that the response of a chiral metamaterial particle may be tailored to enable an optical conveyor belt operation with no optical traps, such that for a fixed incident light helicity the nanoparticle is either steadily pushed towards the direction of the photon flow or steadily pulled against the photon flow, independent of its position. Our findings create distinct opportunities for unconventional optical manipulations of tailored nanoparticles and may have applications in sorting racemic mixtures of artificial chiral molecules and in particle delivery.

physics.optics↗

arXiv2021

Topological pumping in photonic systems

The topology of typical Chern insulators is rooted in the periodicity of the system along two directions of real-space. In this article, we depart from this standard concept and demonstrate that a generic non-Hermitian photonic waveguide periodic along a single direction of real space can be regarded as a sub-component of an extended system with a synthetic dimension and with a nontrivial Chern topology. In particular, we show that the number of bands below a band-gap of a generic waveguide determines the gap Chern number of the extended system. It is theoretically and numerically demonstrated that in real-space the gap Chern number gives the number of gapless Tamm state branches localized at the system boundary, when its geometry is continuously displaced by one lattice period. In the non-Hermitian case, the Tamm states connect different bands in the complex plane.

physics.optics↗

arXiv2018

Asymmetric Transmission and Isolation in Nonlinear Devices: Why They Are Different

Here, we highlight the fundamental differences between nonlinear two-port devices with strongly asymmetric transmission responses and "isolators". We use a mushroom-structure loaded with nonlinear elements as a guiding example. The mushroom-metamaterial can be operated in a regime where it behaves as a nearly ideal electromagnetic diode, such that the individual excitations of the two ports lead to strongly asymmetric responses. We point out the limitations of using this type of nonlinear devices as microwave isolators. In particular, we underline the crucial importance of material loss to attain the isolator functionality and that in the lossless regime nonreciprocal devices may be time-reversal invariant.

physics.app-ph↗

arXiv2017

Bistability in mushroom-type metamaterials

Here, we study the electromagnetic response of asymmetric mushroom-type metamaterials loaded with nonlinear elements. It is shown that near a Fano resonance these structures may have a strong tunable, bi-stable, and switchable response and enable giant nonlinear effects. Using an effective medium theory and full wave simulations, it is proven that the nonlinear elements may allow the reflection and transmission coefficients to follow hysteresis loops, and to switch the metamaterial between "go" and "no-go" states similar to an ideal electromagnetic switch.

physics.app-ph↗