arXiv subjects

Mário G. Silveirinha

Publications and source records attributed to Mário G. Silveirinha.

At least 73 records · Page 4Linked to original sources

arXiv2018

Quantized Angular Momentum in Topological Optical Systems

The Chern index characterizes the topological phases of nonreciprocal photonic systems. Unlike in electronic systems, the photonic Chern number has no clear physical meaning, except that it determines the net number of unidirectional edge states supported by an interface with a trivial mirror. Here, we fill in this gap by demonstrating that the photonic Chern number can be understood as a quantum of the light-angular momentum in a photonic insulator cavity. It is proven that for a large cavity, when the discrete spectrum can be approximated by a continuum, the spectral density of the thermal fluctuation-induced angular momentum is precisely quantized in the band-gaps of the bulk states. The nontrivial expectation of the light angular momentum is due to a circulation of thermal energy in closed orbits. Remarkably, this result can be extended to systems without a topological classification, and in such a case the "quantum" of the angular momentum density is determined by the net number of unidirectional edge states supported by the cavity walls.

physics.optics↗

arXiv2018

Drift-induced Unidirectional Graphene Plasmons

Nonreciprocal photonic devices enable "one-way" light flows and are essential building blocks of optical systems. Here, we investigate an alternative paradigm to break reciprocity and achieve unidirectional subwavelength light propagation fully compatible with modern all-photonic highly-integrated systems. In agreement with a few recent studies, our theoretical model predicts that a graphene sheet biased with a drift electric current has a strong nonreciprocal tunable response. Strikingly, we find that the propagation of the surface plasmon polaritons can be effectively "one-way" and may be largely immune to the backscattering from defects and obstacles. Furthermore, the drift-current biasing may boost the propagation length of the graphene plasmons by more than 100%. Our findings open new inroads in nonreciprocal photonics and offer a new opportunity to control the flow of light with one-atom thick nonreciprocal devices.

physics.optics↗

arXiv2018

Optical torque on a two-level system near a strongly nonreciprocal medium

We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagnetic environment described by the most general linear constitutive relations. The atom is modeled as a two-level system prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and under the Markov approximation, we show that the optical torque has a resonant and non-resonant part, associated respectively with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum, which can both be written explicitly in terms of the system Green function. Our formulation is valid for any inhomogeneous, dissipative, dispersive, nonreciprocal, and bianisotropic structure. We apply this general theory to a scenario in which the atom interacts with a material characterized by strong nonreciprocity and modal unidirectionality. In this case, the main decay channel of the atom energy is represented by the unidirectional surface waves launched at the nonreciprocal material-vacuum interface. To provide relevant physical insight into the role of these unidirectional surface waves in the emergence of non-trivial optical torque, we derive closed-form expressions for the induced torque under the quasi-static approximation. Finally, we investigate the equilibrium states of the atom polarization, along which the atom spontaneously tends to align due to the action of the torque. Our theoretical predictions may be experimentally tested with cold Rydberg atoms and superconducting qubits near a nonreciprocal material. We believe that our general theory may find broad application in the context of nano-mechanical and bio-mechanical systems.

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↗

arXiv2018

Asymmetric Cherenkov Emission in a Topological Plasmonic Waveguide

Here, we investigate the Cherenkov emission by an array of moving electric charges in the vicinity of a topologically nontrivial gyrotropic material. It is shown that the nonreciprocal material response may result in a robustly asymmetric Cherenkov emission, such that the spectrum of the emitted radiation and the stopping power depend strongly on the sign of the particle velocity. It is demonstrated that the main emission channels are determined by the unidirectional edge states supported by the topological material. We consider as examples both magnetized plasmas and Weyl semi-metals. The latter may exhibit a spontaneous nonreciprocal response without a biasing magnetic field.

physics.optics↗

arXiv2018

Fluctuation-induced forces on an atom near a photonic topological material

We theoretically study the Casimir-Polder force on an atom in a arbitrary initial state in a rather general electromagnetic environment wherein the materials may have a nonreciprocal bianisotropic dispersive response. It is shown that under the Markov approximation the force has resonant and nonresonant contributions. We obtain explicit expressions for the optical force both in terms of the system Green function and of the electromagnetic modes. We apply the theory to the particular case wherein a two-level system interacts with a topological gyrotropic material, showing that the nonreciprocity enables exotic light-matter interactions and the opportunity to sculpt and tune the Casimir-Polder forces on the nanoscale. With a quasi-static approximation, we obtain a simple analytical expression for the optical force and unveil the crucial role of surface plasmons in fluctuation induced forces. Finally, we derive the Green function for a gyrotropic material half-space in terms of a Sommerfeld integral.

quant-ph↗

arXiv2018

Topological classification of Chern-type insulators with the photonic Green function

The Chern topological numbers of a material system are traditionally written in terms of the Berry curvature which depends explicitly on the material band structure and on the Bloch eigenwaves. Here, we demonstrate that it is possible to calculate the gap Chern numbers of a photonic platform without having any detailed knowledge of its band structure, relying simply on the system photonic Green function. It is shown that the gap Chern number is given by an integral of the photonic Green function along a line of the complex frequency plane parallel to the imaginary axis. Our theory applies to arbitrary frequency dispersive fully three-dimensional photonic crystals, as well as to the case of electromagnetic continua with no intrinsic periodicity.

physics.optics↗

arXiv2017

Link between the photonic and electronic topological phases in artificial graphene

In recent years the study of topological phases of matter has emerged as a very exciting field of research, both in photonics and in electronics. However, up to now the electronic and photonic properties have been regarded as totally independent. Here, we establish a link between the electronic and the photonic topological phases of the same material system and theoretically demonstrate that they are intimately related. We propose a realization of the Haldane model as a patterned 2D electron gas and determine its optical response using the Kubo formula. It is shown that the electronic and photonic phase diagrams of the patterned electron gas are strictly related. In particular, the system has a trivial photonic topology when the inversion symmetry is the prevalent broken symmetry, whereas it has a nontrivial photonic topology for a dominant broken time-reversal symmetry, similar to the electronic case. To confirm these predictions, we numerically demonstrate the emergence of topologically protected unidirectional electromagnetic edge-states at the interface with a trivial photonic material.

cond-mat.mes-hall↗

arXiv2017

Spontaneous lateral atomic recoil force close to a photonic topological material

We investigate the quantum recoil force acting on an excited atom close to the surface of a nonreciprocal photonic topological insulator (PTI). The main atomic emission channel is the unidirectional surface-plasmon propagating at the PTI-vacuum interface, and we show that it enables a spontaneous lateral recoil force that scales at short distance as $1/d^4$, where $d$ is the atom-PTI separation. Remarkably, the sign of the recoil force is polarization and orientation-independent, and it occurs in a translation-invariant homogeneous system in thermal equilibrium. Surprisingly, the recoil force persists for very small values of the gyration pseudovector, which, for a biased plasma, corresponds to very low cyclotron frequencies. The ultra-strong recoil force is rooted on the quasi-hyperbolic dispersion of the surface-plasmons. We consider both an initially excited atom and a continuous pump scenario, the latter giving rise to a continuous lateral force whose direction can be changed at will by simply varying the orientation of the biasing magnetic field. Our predictions may be tested in experiments with cold Rydberg atoms and superconducting qubits.

quant-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↗

arXiv2017

Light tunneling anomaly in interlaced metallic wire meshes

For long wavelengths three-dimensional connected metallic wire meshes are impenetrable by light and have an electromagnetic response similar to that of an electron gas below the plasma frequency. Surprisingly, here it is shown that when two opaque metallic meshes are spatially-interlaced the combined structure enables an anomalous light tunneling in the long wavelength regime. The effect is due to the destructive interference of the waves scattered by the two wire meshes, which leads to a Fano-type resonance.

physics.optics↗

arXiv2017

Negative Landau damping in bilayer graphene

We theoretically demonstrate that a system formed by two coupled graphene sheets enables a negative damping regime wherein graphene plasmons are pumped by a DC current. This effect is triggered by electrons drifting through one of the graphene sheets and leads to the spontaneous light emission (spasing) and wave instabilities in the mid-infrared range. It is shown that there is a deep link between the drift-induced instabilities and wave instabilities in moving media, as both result from the hybridization of oscillators with oppositely signed frequencies. With a thickness of few nanometers and wide spectral tunability, the proposed structure may find interesting applications in nanophotonic circuitry as an on-chip light source.

cond-mat.mes-hall↗

arXiv2017

Berry phase, Berry Connection, and Chern Number for a Continuum Bianisotropic Material from a Classical Electromagnetics Perspective

The properties that quantify photonic topological insulators (PTIs), Berry phase, Berry connection, and Chern number, are typically obtained by making analogies between classical Maxwell's equations and the quantum mechanical Schrödinger equation, writing both in Hamiltonian form. However, the aforementioned quantities are not necessarily quantum in nature, and for photonic systems they can be explained using only classical concepts. Here we provide a derivation and description of PTI quantities using classical Maxwell's equations, we demonstrate how an electromagnetic mode can acquire Berry phase, and we discuss the ramifications of this effect. We consider several examples, including wave propagation in a biased plasma, and radiation by a rotating isotropic emitter. These concepts are discussed without invoking quantum mechanics, and can be easily understood from an engineering electromagnetics perspective.

quant-ph↗

arXiv2016

Negative spontaneous emission by a moving two-level atom

In this paper we investigate how the dynamics of a two-level atom is affected by its interaction with the quantized near field of a plasmonic slab in relative motion. We demonstrate that for small separation distances and a relative velocity greater than a certain threshold, this interaction can lead to a population inversion, such that the probability of the excited state exceeds the probability of the ground state, corresponding to a negative spontaneous emission rate. It is shown that the developed theory is intimately related to a classical problem. The problem of quantum friction is analyzed and the differences with respect to the corresponding classical effect are highlighted.

quant-ph↗

arXiv2016

Wave Instabilities and Unidirectional Light Flow in a Cavity with Rotating Walls

We investigate the conditions for the emergence of wave instabilities in a vacuum cavity delimited by cylindrical metallic walls under rotation. It is shown that for a small vacuum gap and for an angular velocity exceeding a certain threshold, the interactions between the surface plasmon polaritons supported by each wall give rise to an unstable behavior of the electromagnetic field manifested in an exponential growth with time. The instabilities occur only for certain modes of oscillation and are due to the transformation of kinetic energy into electromagnetic energy. We also study the possibility of having asymmetric light flows and optical isolation relying on the relative motion of the cavity walls.

physics.optics↗

arXiv2016

Single-Interface Casimir Torque

It is shown that the quantum fluctuations of the electromagnetic field generally induce a torque at the interface of an anisotropic material with another anisotropic or isotropic material. It is proven that this torque depends on an interface zero-point energy determined by the dispersion of the interface (localized and extended) modes. Our theory demonstrates that the single-interface torque is essential to understand the Casimir physics of material systems with anisotropic elements and determines the equilibrium positions of the system.

cond-mat.mes-hall↗

arXiv2015

Analytical solution for the stopping power of the Cherenkov radiation in a uniaxial nanowire material

We derive closed analytical formulae for the power emitted by moving charged particles in a uniaxial wire medium by means of an eigenfunction expansion. Our analytical expressions demonstrate that in the absence of material dispersion the stopping power of the uniaxial wire medium is proportional to the charges velocity, and that there is no velocity threshold for the Cherenkov emission. It is shown that the eigenfunction expansion formalism can be extended to the case of dispersive lossless media. Furthermore, in presence of material dispersion the optimal charge velocity that maximizes the emitted Cherenkov power may be less than the speed of light in vacuum.

cond-mat.mes-hall↗

arXiv2015

Super collimation of the radiation by a point source in a uniaxial wire medium

We investigate the radiation properties of a short horizontal dipole embedded in a uniaxial wire medium. It is shown that the uniaxial wire medium enables a super-collimation of the dipole radiation such that the radiation pattern has a singularity and the radiated fields are non-diffractive in the broadside direction. We derive a closed analytical formula for the power radiated by the dipole. Our theory demonstrates that as a consequence of the ultrahigh density of photonic states of the nanowire array, the power radiated by the dipole is strongly enhanced as compared to that emitted in the dielectric host material.

cond-mat.mes-hall↗