arXiv subjects

E. N. Economou

Publications and source records attributed to E. N. Economou.

At least 19 recordsLinked to original sources

arXiv2025

Non-Hermitian off-diagonal disordered optical lattices

Within the framework of non-Hermitian photonics, we investigate the spectral and dynamical properties of one- and two-dimensional non-Hermitian off-diagonal disordered optical lattices, where randomness is applied to the couplings rather than to the on-site potential terms. We analyze eigenvalue distributions and the localization properties of the eigenmodes, comparing them with those of the corresponding Hermitian lattices. Furthermore, we study their transport behavior under single-channel excitation and identify unconventional phenomena such as jumps between distant lattice regions in systems with a purely real spectrum, as well as complex spectrum-induced Anderson jumps, reported here for the first time in two dimensions. Our results establish a reference framework for non-Hermitian off-diagonal disorder and open new directions for future studies of localization phenomena.

cond-mat.dis-nn↗

arXiv2025

Evolving disorder in non-Hermitian lattices

The impact of disorder on wave transport has been extensively studied in Hermitian systems, where static randomness gives rise to Anderson localization. In non-Hermitian lattices, static disorder can lead to peculiar transport features, including jumpy wave evolution. By contrast, much less is known about how transport is modified when the on-site disorder evolves during propagation. Here we address this problem by investigating two pertinent non-Hermitian lattice models with disorder altered at regular intervals, characterized by a finite disorder period. In lattices with symmetric couplings and complex on-site disorder, short disorder periods suppress localization and give rise to diffusion-like spreading, while longer periods allow the emergence of jumps. In Hatano-Nelson lattices with real on-site disorder, the non-Hermitian skin effect asymptotically dominates regardless of the disorder strength, while the disorder period reshapes the drift velocity and modulates its competition with Anderson localization. These results establish evolving disorder as a novel way of tuning non-Hermitian transport.

cond-mat.dis-nn↗

arXiv2025

Non-Hermitian impurity problem

The problem of a single Hermitian impurity has long served as a cornerstone in condensed matter physics, offering fundamental insights into the mechanisms of Anderson localization. Yet, despite the increased interest in the spectral and localization properties of non-Hermitian lattices with defects, the non-Hermitian extension of the single impurity problem remains largely unexplored. In this work, we investigate the role of a single complex impurity in one-, two-, and three-dimensional infinite tight-binding lattices. Our study reveals a series of counterintuitive phenomena, including regions where localization vanishes and re-emerges as the impurity strength varies. Next, we study the corresponding finite-sized lattices, which are highly relevant to experimental realizations in readily accessible photonic platforms, revealing a variety of exotic features, such as scale-free localized states, exceptional points, and peculiar cross-shaped localized eigenstates, whose profiles deviate from the conventional exponential localization. This work paves the way for future studies on transport phenomena in non-Hermitian disordered lattices.

cond-mat.dis-nn↗

arXiv2024

Anderson localization versus hopping asymmetry in a disordered lattice

In the framework of non-Hermitian photonics, we investigate the interplay between disorder and non-Hermiticity in a one-dimensional Hatano-Nelson lattice. While Anderson localization dictates the wave's evolution in conservative random systems, the introduction of non-Hermiticity tends to force the beam to unidirectionally propagate towards one edge of the potential due to the existence of skin modes. As we show, the antagonism between these effects results in qualitatively distinct phases of wave diffraction, including counter-intuitive characteristics regarding the relationship between the strength of disorder and the wavepacket's velocity.

cond-mat.dis-nn↗

arXiv2022

Topological Extraordinary Optical Transmission

The incumbent technology for bringing light to the nanoscale, the near-field scanning optical microscope, has notoriously small throughput efficiencies - of the order of 10^(-4) - 10^(-5), or less. We report on a broadband, topological, unidirectionally-guiding structure, not requiring adiabatic tapering and in principle enabling near-perfect (ideally, ~100%) optical transmission through an unstructured single (POTUS) arbitrarily-subdiffraction slit at its end. Specifically, for a slit width of just lambda_eff / 72 (lambda_0 / 138) the attained normalized transmission coefficient reaches a value of 1.52, while for a unidirectional-only (non-topological) device the normalized transmission through a lambda_eff / 21 (~lambda_0 / 107) slit reaches 1.14 - both, limited only by inherent material losses, and with zero reflection from the slit. The associated, under ideal conditions, near-perfect optical extraordinary transmission (POET) has implications, among diverse areas in wave physics and engineering, for high-efficiency, maximum-throughput nanoscopes and heat-assisted magnetic recording devices.

physics.optics↗

arXiv2021

Transport and spectral features in non-Hermitian open systems

We study the transport and spectral properties of a non-Hermitian one-dimensional disordered lattice, the diagonal matrix elements of which are random complex variables taking both positive (loss) and negative (gain) imaginary values: Their distribution is either the usual rectangular one or a binary pair-correlated one possessing, in its Hermitian version, delocalized states, and unusual transport properties. Contrary to the Hermitian case, all states in our non-Hermitian system are localized. In addition, the eigenvalue spectrum, for the binary pair-correlated case, exhibits an unexpected intricate fractallike structure on the complex plane and with increasing non-Hermitian disorder, the eigenvalues tend to coalesce in particular small areas of the complex plane, a feature termed "eigenvalue condensation". Despite the strong Anderson localization of all eigenstates, the system appears to exhibit transport not by diffusion but by a new mechanism through sudden jumps between states located even at distant sites. This seems to be a general feature of open non-Hermitian random systems. The relation of our findings to recent experimental results is also discussed.

cond-mat.dis-nn↗

arXiv2020

Shape-preserving beam transmission through non-Hermitian disordered lattices

We investigate the propagation of Gaussian beams through optical waveguide lattices characterized by correlated non-Hermitian disorder. In the framework of coupled mode theory, we demonstrate how the imaginary part of the refractive index needs to be adjusted to achieve perfect beam transmission, despite the presence of disorder. Remarkably, the effects of both diagonal and off-diagonal disorder in the waveguides and their couplings can be efficiently eliminated by our non-Hermitian design. Waveguide arrays thus provide an ideal platform for the experimental realization of non-Hermitian phenomena in the context of discrete photonics.

cond-mat.dis-nn↗

arXiv2019

Non-Hermitian disorder in two-dimensional optical lattices

In this paper, we study the properties of two-dimensional lattices in the presence of non-Hermitian disorder. In the context of coupled mode theory, we consider random gain-loss distributions on every waveguide channel (on site disorder). Our work provides a systematic study of the interplay between disorder and non-Hermiticity. In particular, we study the eigenspectrum in the complex frequency plane and we examine the localization properties of the eigenstates, either by the participation ratio or the level spacing, defined in the complex plane. A modified level distribution function vs disorder seems to fit our computational results.

cond-mat.dis-nn↗

arXiv2010

Planar designs for electromagnetically induced transparency in metamaterials

We present a planar design of a metamaterial exhibiting electromagnetically induced transparency that is amenable to experimental verification in the microwave frequency band. The design is based on the coupling of a split-ring resonator with a cut-wire in the same plane. We investigate the sensitivity of the parameters of the transmission window on the coupling strength and on the circuit elements of the individual resonators, and we interpret the results in terms of two linearly coupled Lorentzian resonators. Our metamaterial designs combine low losses with the extremely small group velocity associated with the resonant response in the transmission window, rendering them suitable for slow light applications at room temperature.

cond-mat.mtrl-sci↗

arXiv2010

Repulsive Casimir forces with finite-thickness slabs

We use the extended Lifshitz theory to study the behaviors of the Casimir forces between finite-thickness effective medium slabs. We first study the interaction between a semi-infinite Drude metal and a finite-thickness magnetic slab with or without substrate. For no substrate, the large distance $d$ dependence of the force is repulsive and goes as $1/d^5$; for the Drude metal substrate, a stable equilibrium point appears at an intermediate distance which can be tuned by the thickness of the slab. We then study the interaction between two identical chiral metamaterial slabs with and without substrate. For no substrate, the finite thickness of the slabs $D$ does not influence significantly the repulsive character of the force at short distances, while the attractive character at large distances becomes weaker and behaves as $1/d^6$; for the Drude metal substrate, the finite thickness of the slabs $D$ does not influence the repulsive force too much at short distances until $D=0.05λ_0$.

cond-mat.mes-hall↗

arXiv2010

Magnetic response of nanoscale left-handed metamaterials

Using detailed simulations we investigate the magnetic response of metamaterials consisting of pairs of parallel slabs or combinations of slabs with wires (including the fishnet design) as the length-scale of the structures is reduced from mm to nm. We observe the expected saturation of the magnetic resonance frequency when the structure length-scale goes to the sub-micron regime, as well as weakening of the effective permeability resonance and reduction of the spectral width of the negative permeability region. All these results are explained by using an equivalent resistor-inductor-capacitor (RLC) circuit model, taking into account the current-connected kinetic energy of the electrons inside the metallic parts through an equivalent inductance, added to the magnetic field inductance in the unit-cell. Using this model we derive simple optimization rules for achieving optical negative permeability metamaterials of improved performance. Finally, we analyze the magnetic response of the fishnet design and we explain its superior performance regarding the high attainable magnetic resonance frequency, as well as its inferior performance regarding the width of the negative permeability region.

physics.optics↗

arXiv2009

Comparison of Chiral Metamaterial Designs for Repulsive Casimir Force

In our previous work [Phys. Rev. Lett. 103, 103602 (2009)], we found that repulsive Casimir forces could be realized by using chiral metamaterials if the chirality is strong enough. In this work, we check four different chiral metamaterial designs (i.e., Twisted-Rosettes, Twisted-Crosswires, Four-U-SRRs, and Conjugate-Swastikas) and find that the designs of Four-U-SRRs and Conjugate-Swastikas are the most promising candidates to realize repulsive Casimir force because of their large chirality and the small ratio of structure length scale to resonance wavelength.

cond-mat.mes-hall↗

arXiv2009

Repulsive Casimir Force in Chiral Metamaterials

We demonstrate theoretically that one can obtain repulsive Casimir forces and stable nanolevitations by using chiral metamaterials. By extending the Lifshitz theory to treat chiral metamaterials, we find that a repulsive force and a minimum of the interaction energy exist for strong chirality, under realistic frequency dependencies and correct limiting values (for zero and infinite frequencies) of the permittivity, permeability, and chiral coefficients.

cond-mat.mtrl-sci↗

arXiv2008

Strong diamagnetic response of metamaterials

We demonstrate that there is a strong diamagnetic response of metamaterials, consisting of open or closed split ring resonators (SRRs). Detailed numerical work shows that for densely packed SRRs the magnetic permeability, $μ(ω)$, does not approach unity, as expected for frequencies lower and higher than the resonance frequency, $ω_0$. Below $ω_0$, $μ(ω)$ gives values ranging from 0.9 to 0.6 depending of the width of the metallic ring, while above $ω_0$, $μ(ω)$ is close to 0.5. Closed rings have $μ\approx 0.5$ over a wide frequency range independently of the width of the ring. A simple model that uses the inner and outer current loop of the SRRs can easily explain theoretically this strong diamagnetic response, which can be used in magnetic levitation.

cond-mat.mtrl-sci↗

arXiv2008

Low Loss Metamaterials Based on Classical Electromagnetically Induced Transparency

We demonstrate theoretically that electromagnetically induced transparency can be achieved in metamaterials, in which electromagnetic radiation is interacting resonantly with mesoscopic oscillators rather than with atoms. We describe novel metamaterial designs that can support full dark resonant state upon interaction with an electromagnetic beam and we present results of its frequency-dependent effective permeability and permittivity. These results, showing a transparency window with extremely low absorption and strong dispersion, are confirmed by accurate simulations of the electromagnetic field propagation in the metamaterial.

cond-mat.mtrl-sci↗

arXiv2005

Study of electromagnetic wave propagation in active medium and the equivalent Schrodinger equation with energy-dependent complex potential

We study the massless limit of the Klein-Gordon (K-G) equation in 1+1 dimensions with static complex potentials as an attempt to give an alternative, but equivalent, representation of plane electromagnetic (em) wave propagation in active medium. In the case of dispersionless em medium, the analogy dictates that the potential in the K-G equation is complex and energy-dependent. In the non-relativistic domain we study an analogous inertial system by considering wave packet propagation through a complex potential barrier and solve the time-independent Schrodinger equation with a potential that has the same energy dependence as that of the K-G equation. The behavior of these solutions is compared with those found elsewhere in the literature for the propagation of electromagnetic plane waves in a uniform active medium with complex dielectric constant. Our study concluded that the discrepancy between the time dependent and stationary results is due to the energy poles crossing the real axis in the complex energy plane. It was demonstrated unambiguously that there is a frequency (energy) and size dependent gain threshold above which the stationary results become unstable. This threshold corresponds to the value of the gain at which the pole crosses the real axis.

cond-mat.dis-nn↗

arXiv2004

Impact of the inherent periodic structure on the effective medium description of left-handed and related meta-materials

We study the frequency dependence of the effective electromagnetic parameters of left-handed and related meta-materials of the split ring resonator and wire type. We show that the reduced translational symmetry (periodic structure) inherent to these meta-materials influences their effective electromagnetic response. To anticipate this periodicity, we formulate a periodic effective medium model which enables us to distinguish the resonant behavior of electromagnetic parameters from effects of the periodicity of the structure. We use this model for the analysis of numerical data for the transmission and reflection of periodic arrays of split ring resonators, thin metallic wires, cut wires as well as the left-handed structures. The present method enables us to identify the origin of the previously observed resonance/anti-resonance coupling as well as the occurrence of negative imaginary parts in the effective permittivities and permeabilities of those materials. Our analysis shows that the periodicity of the structure can be neglected only for the wavelength of the electromagnetic wave larger than 30 space periods of the investigated structure.

cond-mat.mtrl-sci↗

arXiv2004

Electric coupling to the magnetic resonance of split ring resonators

We study both theoretically and experimentally the transmission properties of a lattice of split ring resonators (SRRs) for different electromagnetic (EM) field polarizations and propagation directions. We find unexpectedly that the incident electric field E couples to the magnetic resonance of the SRR when the EM waves propagate perpendicular to the SRR plane and the incident E is parallel to the gap-bearing sides of the SRR. This is manifested by a dip in the transmission spectrum. A simple analytic model is introduced to explain this interesting behavior.

cond-mat.mtrl-sci↗