arXiv subjects

Nader Engheta

Publications and source records attributed to Nader Engheta.

At least 73 records · Page 4Linked to original sources

arXiv2014

Tunneling of Obliquely-Incident Waves through PT-Symmetric Epsilon-Near-Zero Bi-Layers

We show that obliquely-incident, transversely-magnetic-polarized plane waves can be totally transmitted (with zero reflection) through epsilon-near-zero (ENZ) bi-layers characterized by balanced loss and gain with parity-time (PT) symmetry. This tunneling phenomenon is mediated by the excitation of a surface-wave localized at the interface separating the loss and gain regions. We determine the parameter configurations for which the phenomenon may occur and, in particular, the relationship between the incidence direction and the electrical thickness. We show that, below a critical threshold of gain and loss, there always exists a tunneling angle which, for moderately thick (wavelength-sized) structures, approaches a critical value dictated by the surface-wave phase-matching condition. We also investigate the unidirectional character of the tunneling phenomenon, as well as the possible onset of spontaneous symmetry breaking, typical of PT-symmetric systems. Our results constitute an interesting example of a PT-symmetry-induced tunneling phenomenon, and may open up intriguing venues in the applications of ENZ materials featuring loss and gain.

physics.optics↗

arXiv2013

Optical isolation via unidirectional resonant photon tunneling

We show that tri-layer structures combining epsilon-negative and magneto-optical material layers can exhibit unidirectional resonant photon tunneling phenomena that can discriminate between circularly-polarized (CP) waves of given handedness impinging from opposite directions, or between CP waves with different handedness impinging from the same direction. This physical principle, which can also be interpreted in terms of a Fabry-Perot-type resonance, may be utilized to design compact optical isolators for CP waves. Within this framework, we derive simple analytical conditions and design formulae, and quantitatively assess the isolation performance, also taking into account the unavoidable imperfections and nonidealities.

physics.optics↗

arXiv2013

One-way Phonon Isolation In Acoustic Waveguides

We present a method to achieve one-way mode isolation in acoustic waveguides by considering spatio-temporal modulation of material properties. Mode conversion between symmetric and antisymmetric shear horizontal (SH) modes of an unbounded plate acoustic waveguide is demonstrated using this method. Analytical and numerical evidence for the possibility of mode conversion is presented. Possible experimental implementation and applications for on chip signal isolation in MEMS devices are also suggested.

physics.optics↗

arXiv2013

Enhanced Faraday Rotation via Resonant Tunnelling in Tri-Layers Containing Magneto-Optical Metals

We study resonant tunnelling effects that can occur in tri-layer structures featuring a dielectric layer sandwiched between two magneto-optical-metal layers. We show that the resonance splitting associated with these phenomena can be exploited to enhance Faraday rotation at optical frequencies. Our results indicate that, in the presence of realistic loss levels, a tri-layer structure of sub-wavelength thickness is capable of yielding sensible (~10°) Faraday rotation with transmittance levels that are an order of magnitude larger than those attainable with a standalone slab of magneto-optical metal of same thickness.

physics.optics↗

arXiv2013

Giant nonlinearity in Zero-Gap Semiconductor Superlattices

Using an effective medium approach, we describe how by combining mercury-cadmium-telluride semiconductor alloys with band gap energies of opposite signs it may be possible to design a superlattice where the electrons have isotropic zero-effective mass and a single valley linear energy-momentum dispersion. We demonstrate that because of the zero-mass property, the superlattice may have a strong nonlinear response under the action of an external electromagnetic field.

cond-mat.mtrl-sci↗

arXiv2013

Magnetized Epsilon-Near-Zero (ENZ) Structures: Hall Opacity, Hall Transparency, and One-Way Photonic Surface States

We study propagation of transverse-magnetic (TM) electromagnetic waves in the bulk and at the surface of magnetized epsilon-near-zero (ENZ) medium in a Voigt configuration. We reveal that in a certain range of material parameters novel regimes of wave propagation emerge: we show that the transparency of the medium can be altered with the magnetization leading either to magnetically induced Hall opacity or Hall transparency of the ENZ. In our theoretical study, we demonstrate that surface waves at the interface between either a transparent or an opaque Hall medium and a homogeneous medium may, under certain conditions, be predominantly one-way. Moreover, we predict that one-way photonic surface states may exist at the interface of an opaque Hall ENZ and a regular metal, giving rise to a possibility for backscattering immune wave propagation and isolation.

physics.optics↗

arXiv2013

PT Metamaterials via Complex-Coordinate Transformation Optics

We extend the transformation-optics paradigm to a complex spatial coordinate domain, in order to deal with electromagnetic metamaterials characterized by balanced loss and gain, giving special emphasis to parity-time (PT) symmetric metamaterials. We apply this general theory to complex-source-point radiation and anisotropic transmission resonances, illustrating the capability and potentials of our approach in terms of systematic design, analytical modeling and physical insights into complex-coordinate wave-objects and resonant states.

physics.optics↗

arXiv2013

Electric Levitation Using Epsilon-Near-Zero Metamaterials

Levitation of objects with action at a distance has always been intriguing to humans. Several ways to achieve this, such as aerodynamic, acoustic, or electromagnetic methods, including radiation pressure, stable potential wells, and quantum Casimir-Lifshitz forces, exist. A fascinating approach for levitation is that of magnets over superconductors based on the Meissner effect -the expulsion of the magnetic field by a superconductor. With the advent of metamaterials -designed structures with electromagnetic properties that may not be found in nature- we ask whether a material may be conceived exhibiting similar field expulsion, but involving the electric field. We show how a special subcategory of metamaterials, called epsilon-near-zero materials, exhibits such electric classic analog to the Meissner effect, exerting a repulsion on nearby sources. Repulsive forces using anisotropic and chiral metamaterials have been investigated, but our proposal uses a different mechanism based on field expulsion, and is very robust to both losses and material dispersion.

physics.class-ph↗

arXiv2013

Nanoscale plasmonic circulator

Here, we propose a conceptual approach for design of an ultracompact nanoscale passive optical circulator based on the excitation of plasmonic resonances. We study a three-port Y-junction with a deep subwavelength plasmonic nanorod structure integrated into its core. We show theoretically that such a structure immersed in the magneto-optical media may function as magnetically tunable scatterer tilting and rotating its near-field distribution and corresponding radiation. We demonstrate, using numerical simulations, that such a rotation of the near-field radiation yields a break in the symmetry of the coupling between the junction arms and the structure in such a way that the signal launched from any of the three ports is mostly transmitted into the next port in the circular order, while the other port is essentially isolated, thus providing the functionality of an optical circulator with subwavelength dimensions.

physics.optics↗

arXiv2012

Nonreciprocal rotating power flow within plasmonic nanostructures

We theoretically explore the notion of nonreciprocal near-zone manipulation of electromagnetic fields within subwavelength plasmonic nanostructures embedded in magneto-optical materials. We derive an analytical model predicting a strong, magneto-optically induced time-reversal symmetry breaking of localized plasmonic resonances in topologically symmetric structures. Our numerical simulations of plasmon excitations reveal a considerable near-zone power flow rotation within such hybrid nanostructures, demonstrating nanoscale nonreciprocity. This can be considered as another mechanism for tuning plasmonic phenomena at the nanoscale.

physics.optics↗

arXiv2012

Optical isolation with epsilon-near-zero metamaterials

We suggest a principle for isolation of circularly polarized waves in magnetically active extreme-parameter metamaterials. Using theoretical analysis and numerical simulations, we show that metamaterials with extreme parameters, such as epsilon-near-zero materials (ENZ), when merged with magneto-optical materials, become transparent for forward circularly polarized waves of a given handedness and opaque for backward propagating waves of the same handedness. We theoretically study two possible implementations of such hybrid materials: (1) the case of metal-dielectric stacks; and (2) rectangular waveguide near its cut-off frequency. We prove that these structures can be utilized as compact isolators for circularly polarized waves.

physics.optics↗

arXiv2012

Analytical Study of Sub-Wavelength Imaging by Uniaxial Epsilon-Near-Zero Metamaterial Slabs

We discuss the imaging properties of uniaxial epsilon-near-zero metamaterial slabs with possibly tilted optical axis, analyzing their sub-wavelength focusing properties as a function of the design parameters. We derive in closed analytical form the associated two-dimensional Green's function in terms of special cylindrical functions. For the near-field parameter ranges of interest, we are also able to derive a small-argument approximation in terms of simpler analytical functions. Our results, validated and calibrated against a full-wave reference solution, expand the analytical tools available for computationally-efficient and physically-incisive modeling and design of metamaterial-based sub-wavelength imaging systems.

physics.optics↗

arXiv2012

Transformation Electronics: Tailoring Electron's Effective Mass

The speed of integrated circuits is ultimately limited by the mobility of electrons or holes, which depend on the effective mass in a semiconductor. Here, building on an analogy with electromagnetic metamaterials and transformation optics, we describe a new transport regime in a semiconductor superlattice characterized by extreme anisotropy of the effective mass and a low intrinsic resistance to movement - with zero effective mass - along some preferred direction of electron motion. We theoretically demonstrate that such regime may permit an ultra fast, extremely strong electron response, and significantly high conductivity, which, notably may be weakly dependent on the temperature at low temperatures. These ideas may pave the way for faster electronic devices and detectors and new functional materials with a strong electrical response in the infrared regime.

cond-mat.mes-hall↗

arXiv2012

A Metamaterial-Inspired Model for Electron Waves in Bulk Semiconductors

Based on an analogy with electromagnetic metamaterials, we develop an effective medium description for the propagation of electron matter waves in bulk semiconductors with a zincblende structure. It is formally demonstrated that even though departing from a different starting point, our theory gives results for the energy stationary states consistent with Bastard's envelope function approximation in the long-wavelength limit. Using the proposed approach, we discuss the time evolution of a wave packet in a bulk semiconductor with a zero-gap and linear energy-momentum dispersion.

cond-mat.mes-hall↗

arXiv2012

An Effective Medium Approach to Electron Waves: Graphene Superlattices

We develop an effective medium approach to characterize the propagation of matter waves in periodic structures, such as graphene or semiconductor superlattices. It is proven that the time evolution of the states that are not more localized in space than the characteristic period of the structure can be described exactly through an effective Hamiltonian, and that the electronic band structure of the system can be exactly determined from the effective Hamiltonian. As an illustration of the application of the method, we characterize the mesoscopic response of graphene superlattices. It is shown that these structures may be described using simply two effective parameters: a dispersive potential, and an anisotropy tensor that characterizes the pseudospin. Our model predicts that a graphene superlattice characterized by an indefinite anisotropy tensor - such that the eigenvalues of the tensor have opposite signs - may permit the perfect tunneling of all the stationary states with a specific value of the energy when it is paired with a dual graphene superlattice with positive definite anisotropy tensor.

cond-mat.mtrl-sci↗

arXiv2011

Nonlocal transformation optics

We show that the powerful framework of transformation optics may be exploited for engineering the nonlocal response of artificial electromagnetic materials. Relying on the form-invariant properties of coordinate-transformed Maxwell's equations in the spectral domain, we derive the general constitutive "blueprints" of transformation media yielding prescribed nonlocal field-manipulation effects, and provide a physically-incisive and powerful geometrical interpretation in terms of deformation of the equi-frequency contours. In order to illustrate the potentials of our approach, we present an example of application to a wave-splitting refraction scenario, which may be implemented via a simple class of artificial materials. Our results provide a systematic and versatile framework which may open intriguing venues in dispersion engineering of artificial materials.

physics.optics↗

arXiv2011

Sampling and Squeezing Electromagnetic Waves through Subwavelength Ultranarrow Regions or Openings

Here, we investigate the physical mechanisms that may enable squeezing a complex electromagnetic field distribution through a narrow and/or partially obstructed region with little amplitude and phase distortions. Following our recent works, such field manipulations may be made possible by a procedure in which the incoming wave is first "sampled" "pixel by pixel" using an array of metallic waveguides, and in a second step the energy corresponding to each individual pixel is "squeezed" through a very narrow channel filled with a permittivity-near zero material. In this work, we study in detail these processes in scenarios where the electromagnetic wave is compressed along a single direction of space, and present theoretical models that enable the analytical modeling of such phenomena. Full-wave results obtained with an electromagnetic simulator, demonstrate the possibility of compressing an incoming wave several folds through ultranarrow channels filled with silicon carbide. The "sampling and squeezing" concept may enable unparalleled control of electromagnetic waves in the nanoscale.

cond-mat.mtrl-sci↗

arXiv2011

Avoiding metallic walls: Use of modal superposition in plasmonic waveguides to reduce propagation loss

We theoretically explore the possibility of reducing the propagation loss in a metal-insulator-metal (MIM) waveguide, using mode combinations to achieve wall-avoiding field distributions along a certain propagation length. We present analytical results for several waveguides showing notable loss reduction, and we discuss the tradeoffs between low loss and high confinement present in this technique.

physics.optics↗