arXiv subjects

Mario G. Silveirinha

Publications and source records attributed to Mario G. Silveirinha.

At least 19 recordsLinked to original sources

arXiv2026

Driven Time Crystal in Low-Symmetry ENZ Conductors

In recent years, epsilon-near-zero (ENZ) materials have attracted a great deal of attention in nonlinear optics, as they combine field enhancement with strong, ultrafast nonlinearities. In particular, transparent conducting oxides, such as ITO, have emerged as a promising class of materials and have been extensively exploited to achieve temporal optical responses varying on the femtosecond scale using an optical pump. Most of the solutions discussed so far in the literature rely on effective $χ^{(3)}$ modulations, wherein the dominant material response is controlled by the envelope of the optical pump. Here, it is shown that low-symmetry conductors can provide an interesting alternative to transparent conducting oxides and a more natural implementation of time-crystalline behavior in optical systems with optical-cycle modulation. I demonstrate that ENZ confinement, combined with the strong anomalous-velocity nonlinearity of low-symmetry conductors, enables a subwavelength nanoparticle to develop a time-crystalline response under optical pumping. For sufficiently strong pumping, this response can overcome dissipative losses and lead to parametric amplification. Furthermore, the pump can strongly tailor the scattering and extinction of a weak probe and, in extreme cases, render the extinction negative. In this regime, the driven nanoparticle effectively amplifies the probe beam.

physics.optics↗

arXiv2026

Can crystal symmetry reshape ENZ photonics?: Opinion

Over the last decade, epsilon-near-zero (ENZ) photonics has been driven by the search for lower losses and stronger nonlinear responses. Here, I ask a different question: can crystal symmetry also be used to shape the ENZ response? I focus on low-symmetry conductors, where the geometry of the electronic states can produce electric currents that are not present in ordinary Drude materials. These currents may enable polarization-dependent gain, nonreciprocal effects, and ultrafast nonlinearities controlled by symmetry and an external bias. I suggest that such materials may be useful for active and time-varying nanophotonics.

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↗

arXiv2025

Thermodynamic Paradox and Non-Hermitian Topological Singularities

Unidirectional modes in magnetically biased electromagnetic systems have long been associated with a thermodynamic paradox: the absence of counter-propagating channels may produce field "hotspots" that can act as unphysical sinks of thermal radiation. Here we revisit this problem and show that, surprisingly, material dissipation alone cannot fully regularize the singular behavior of the normal modes of a nonreciprocal cavity. We demonstrate that the paradox is resolved by nonlocal effects, which suppress the material response at short wavelengths and eliminate field singularities altogether. Our analysis reveals a fundamental link between nonlocality, topology, and thermodynamic consistency, showing that real-space singularities and ill-defined topologies go hand in hand, even in strongly dissipative platforms. These findings clarify the physical origin of the paradox and establish nonlocality as a natural and robust mechanism for its resolution, opening new avenues to explore the intertwined roles of topology and nonlocality in passive nonreciprocal photonics.

physics.optics↗

arXiv2025

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 $ω_p$. The strongest resonance arises at a modulation frequency $Ω= 2 ω_p$, 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↗

arXiv2025

Impact of Chiral-Transitions in Quantum Friction

We theoretically investigate the role of chiral-transitions in the quantum friction force that acts on a two-level atom that moves with relative velocity v parallel to a planar metallic surface. We find that the friction force has a component that is sensitive to the handedness of the atomic transition dipole moment. In the particular, we show that the friction force can be enhanced by an atomic transition with a dipole moment with a certain handedness, and almost suppressed by the dipole moment with the opposite handedness. Curiously, the handedness of the transition dipole moment that boosts the ground-state friction force is the opposite of what is classically expected from the spin-momentum locking. We explain this discrepancy in terms of the interaction between positive and negative frequency oscillators.

quant-ph↗

arXiv2023

Chiral terahertz lasing with Berry curvature dipoles

Materials with Berry curvature dipoles (BDs) support a non-Hermitian electro-optic (EO) effect that is investigated here for lasing at terahertz (THz) frequencies. Such a system is here conceived as a stack of low-symmetry 2D materials. We show that a cavity made of such a material supports a nonreciprocal growing mode with elliptical polarization that generates an unstable resonance leading to self-sustained oscillations. Notably, we demonstrate that the chiral nature of the gain derived from the Berry dipole allows for the manipulation of the laser light's handedness by a simple reversal of the electric field bias.

physics.optics↗

arXiv2023

"Shaking" Photons out of a Topological Material

Over the past decade, there has been a great interest in topological effects, with concepts originally developed in the context of electron transport in condensed matter platforms now being extended to optical systems. While topological properties in electronic systems are often linked to the quantization of electric conductivity observed in the integer quantum Hall effect, a direct analogue in optics remains elusive. In this study, we bridge this gap by demonstrating that the response of the Poynting vector (which may be regarded as a "photon current") to the mechanical acceleration of a medium provides a precise photonic analogue of the electric conductivity. In particular, it is shown that the photonic conductivity determines the energy irreversibly transferred from a periodic mechanical driving of the medium to the electromagnetic field. Furthermore, it is demonstrated that for nonreciprocal systems enclosed in a cavity, the constant acceleration of the system induces a flow of photons along a direction perpendicular to the acceleration, analogous to the Hall effect but for light. The spectral density of the photonic conductivity is quantized in the band gaps of the bulk region with the conductivity quantum determined by the gap Chern number.

physics.optics↗

arXiv2022

Geometry and Topological photonics

Topological photonics provides a powerful framework to describe and understand many nontrivial wave phenomena in complex electromagnetic platforms. The topological index of a physical system is an abstract global property that depends on the family of operators that describes the propagation of Bloch waves. Here, we highlight that there is a profound geometrical connection between topological physics and the topological theory of mathematical surfaces. We show that topological band theory can be understood as a generalization of the topological theory of surfaces and that the genus of a surface can be regarded as a Chern number of a suitable operator defined over the surface. We point out some nontrivial implications of topology in the context of radiation problems and discuss why for physical problems the topological index is often associated with a bulk-edge correspondence.

cond-mat.mes-hall↗

arXiv2022

Time-Crystal Particles and Classical Spin 4-vector

Time crystals are exotic phases of matter characterized by a broken time-translational symmetry, such that the ground state of the system evolves in time in a periodic fashion. Even though the time-crystal concept was introduced relatively recently, a related proposal can be traced back to Louis de Broglie. In his thesis, de Broglie conjectured that elementary particles may have some sort of internal clock that rules their behavior in the microscopic world. Here, I revisit de Broglie's idea and demonstrate that a special extreme case of classical mechanics yields in a natural way time-crystal type dynamics. Remarkably, it is found that time-crystal particles are characterized by a spin 4-vector that has a purely kinematic origin and is determined by the binormal of the velocity trajectory in the Bloch sphere. The dynamics of time-crystal particles is ruled by a generalized least action principle, such that the particle dynamically probes the nearby space and moves on average towards the direction that minimizes the action. I apply the theory to the case of charged particles and find that it predicts spin precession and that it provides a simple and intuitive picture for the origin of the anomalous magnetic moment of the electron. The classical formalism is recovered as an "effective theory" valid on a coarse time scale.

physics.class-ph↗

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↗

arXiv2020

Homogenisation Theory of Space-Time Metamaterials

We present a general framework for the homogenisation theory of space-time metamaterials. By mapping to a frame co-moving with the space-time modulation, we derive analytical formulae for the effective material parameters for travelling wave modulations in the low frequency limit: electric permittivity, magnetic permeability and magnetoelectric coupling. Remarkably, we show that the theory is exact at all frequencies in the absence of back-reflections, and exact at low frequencies when that condition is relaxed. This allows us to derive exact formulae for the Fresnel drag experienced by light travelling through travelling-wave modulations of electromagnetic media.

cond-mat.mes-hall↗

arXiv2019

Multiple Embedded Eigenstates in Nonlocal Plasmonic Nanostructures

Trapping light in open cavities is a long sought "holy grail" of nanophotonics. Plasmonic materials may offer a unique opportunity in this context, as they may fully suppress the radiation loss and enable the observation of spatially localized light states with infinite lifetime in an open system. Here, we investigate how the spatial dispersion effects, e.g., caused by the electron-electron interactions in a metal, affect the trapped eigenstates. Heuristically, one may expect that the repulsive-type electron-electron interactions should act against light localization, and thereby that they should have a negative impact on the formation of the embedded eigenstates. Surprisingly, here we find that the nonlocality of the material response creates new degrees of freedom and relaxes the requirements for the observation of trapped light. In particular, a zero-permittivity condition is no longer mandatory and the same resonator shell can potentially suppress the radiation loss at multiple frequencies.

physics.optics↗

arXiv2019

Spontaneous Rotational Symmetry Breaking in a Kramers Two-Level System

Here, I develop a model for a two-level system that respects the time-reversal symmetry of the atom Hamiltonian and the Kramers theorem. The two-level system is formed by two Kramers pairs of excited and ground states. It is shown that due to the spin-orbit interaction it is in general impossible to find a basis of atomic states for which the crossed transition dipole moment vanishes. The parametric electric polarizability of the Kramers two-level system for a definite ground-state is generically nonreciprocal. I apply the developed formalism to study Casimir-Polder forces and torques when the two-level system is placed nearby either a reciprocal or a nonreciprocal substrate. In particular, I investigate the stable equilibrium orientation of the two-level system when both the atom and the reciprocal substrate have symmetry of revolution about some axis. Surprisingly, it is found that when chiral-type dipole transitions are dominant the stable ground state is not the one in which the symmetry axes of the atom and substrate are aligned. The reason is that the rotational symmetry may be spontaneously broken by the quantum vacuum fluctuations, so that the ground state has less symmetry than the system itself.

physics.optics↗

arXiv2019

Unidirectional and diffractionless surface plasmon-polaritons on three-dimensional nonreciprocal plasmonic platforms

Light-matter interactions in conventional nanophotonic structures typically lack directionality. Furthermore, surface waves supported by conventional material substrates do not usually have a preferential direction of propagation, and their wavefront tends to spread as it propagates along the surface, unless the surface or the excitation are properly engineered and structured. In this article, we theoretically demonstrate the possibility of realizing \emph{unidirectional and diffractionless surface-plasmon-polariton modes} on a nonreciprocal platform, namely, a gyrotropic magnetized plasma. Based on a rigorous Green function approach, we provide a comprehensive and systematic analysis of all the available physical mechanisms that may bestow the system with directionality, both in the sense of one-way excitation of surface waves, and in the sense of directive diffractionless propagation along the surface. The considered mechanisms include (i) the effect of strong and weak forms of nonreciprocity, (ii) the elliptic-like or hyperbolic-like topology of the modal dispersion surfaces, and (iii) the source polarization state, with the associated possibility of chiral surface-wave excitation governed by angular-momentum matching. We find that three-dimensional gyrotropic plasmonic platforms support a previously-unnoticed wave-propagation regime that exhibit several of these physical mechanisms simultaneously, allowing us to theoretically demonstrate, for the first time, unidirectional surface-plasmon-polariton modes that propagate as a single ultra-narrow diffractionless beam. We also assess the impact of dissipation and nonlocal effects. Our theoretical findings may enable a new generation of plasmonic structures and devices with highly directional response.

physics.optics↗

arXiv2019

Non-Markovian Transient Casimir-Polder force and population dynamics on excited and ground state atoms: weak and strong coupling regimes in generally non-reciprocal environments

The transient Casimir-Polder force on a two-level atom introduced into a three-dimensional, inhomogeneous, generally non-reciprocal environment is evaluated using non-Markovian Weisskopf-Wigner theory in the strong and weak coupling regimes. Ground-state and excited atoms are considered as two separate initial-value problems, and both the short-time and long-time atomic population and force are evaluated. The results are compared with various Markov approximation of the Weisskopf-Wigner theory, and with previous Markov results from the Heisenberg picture.

quant-ph↗

arXiv2019

Hidden Time-Reversal Symmetry in Dissipative Reciprocal Systems

It is proven, without using the microscopic reversibility argument of Onsager, that lossy reciprocal systems have a hidden time-reversal symmetry. The key idea is that the dissipation channels of lossy dielectrics can be mimicked by a distributed network of lossless transmission lines. It is highlighted that the reciprocity of lossy dielectrics is fundamentally rooted on the hidden time-reversal invariance and on linearity of the materials. Furthermore, it is demonstrated that the upper-half plane response of dissipative materials can be approximated as much as desired by the response of some lossless material.

physics.class-ph↗

arXiv2019

Time-reversal Symmetry in Antenna Theory

Here, I discuss some implications of the time-reversal invariance of lossless radiating systems. I highlight that time-reversal symmetry provides a rather intuitive explanation for the conditions of polarization and impedance matching of a receiving antenna. Furthermore, I describe a solution to generate the time-reversed electromagnetic field through the illumination of a matched receiving antenna with a Herglotz wave.

physics.class-ph↗