arXiv subjects

Ali Abdolali

Publications and source records attributed to Ali Abdolali.

30 records · Page 2Linked to original sources

arXiv2019

Real-time Terahertz Wave Channeling via Multifunctional Metagratings: A Sparse Array of All-Graphene Scatterers

Acquiring full control over a large number of diffraction orders can be strongly attractive in the case of realizing multifunctional devices such as multichannel reflectors. Recently, the concept of metagrating has been introduced which enables obtaining the desired diffraction pattern through a sparse periodic array of engineered scatterers. In this letter, for the first time, a tunable all-graphene multichannel meta-reflector is proposed for operating at terahertz (THz) frequencies. In the supercell level, the designed metagrating is composed of three graphene ribbons of different controllable chemical potentials which can be regarded as a five-channel THz meta-reflector. Several illustrative examples have been presented in which by choosing proper distribution of DC voltages feeding the ribbons, our design can realize different intriguing functionalities such as anomalous reflection, retro-reflection, and three-channel power splitting within a single shared aperture and with high efficiency. This work paves the way toward designing highly-efficient and tunable THz multichannel meta-reflectors with many potential applications in photonics and optoelectronics.

physics.app-ph↗

arXiv2019

Omnidirectional perfect acoustic cloak realized by homogeneous materials

Acoustic cloaks derived by coordinate transformation have opened up a new field of considerable interest in the last two decades. However, since perfect omnidirectional acoustic cloak relies on inhomogeneous and anisotropic materials that posses extreme values in certain regions, this cloak was deemed impossible to be attained even with metamaterials. Recently, our group was competent to introduce a new kind of extreme acoustic material, named as acoustic null medium (ANM), for attaining various devices. In this letter, an extreme mapping function is exploited to achieve an omnidirectional acoustic cloak. The attained extreme homogeneous material through this transformation becomes ANM that can satisfy the extreme parameters requirement and as a result, can hide the object from any incident acoustic waves. Several numerical simulations were carried out to demonstrate the capability of the propounded cloak in cloaking objects from sound fields. The propounded work will pave the way towards cloaking arbitrary objects from any incident angle and obviate the conventional challenges regarding this device.

physics.app-ph↗

arXiv2019

Self-biased Tri-state Power-Multiplexed Digital Metasurface Operating at Microwave Frequencies

Exploiting of nonlinearity has opened doors into undiscovered areas for achieving multiplexed performances in recent years. Although efforts have been made to obtain diverse nonlinear architectures at visible frequencies, the room is still free for incorporating non-linearity into the design of microwave metasurfaces. In this paper, a passive dual-band power intensity-dependent metasurface is presented. Here, power-multiplexing performance is achieved by embedding PIN-diodes in the coding particles. The proposed digital metasurface has three operational states: 1) it acts as a normal reflector at low power intensities whereas provides a dual-band nonlinear response upon illuminating by high-power incidences where 2) it perfectly absorbs the radiations at f1=6.7 GHz and 3) re-distributes the scattered beams by arranging the meta-atoms with a certain coding pattern at f2=9.37 GHz. The nonlinear performance of the designed tri-state coding elements has been characterized by using the scattering parameters captured in the full-wave simulations where the accurate models of the nonlinear diodes are involved. The emergence of microwave self-biased metasurfaces with smart re-actions upon illuminating by incidences of different power levels, revealing great opportunities for designing smart windows, smart camouflage coating surfaces and so on.

physics.app-ph↗

arXiv2019

Tunable Multifocal THz Metalens Based on Metal-Insulator Transition of VO2 Film

Recently, metalenses which consist of metasurface arrays, have attracted attention due to their more condensed size in comparison with conventional lenses. In this paper, we propose a reconfigurable coding metasurface hybridized with vanadium dioxide (VO2) for wavefront manipulation at terahertz (THz) frequencies. At room temperature, the unit-cell can reflect as a "1" bit under linearly y polarized illuminated waves. Besides, when the temperature is increased, VO2 would be in a fully metallic state; therefore, unit-cell can act as a "0" reflection phase. Furthermore, by changing the unit-cells arrangements on a metalens surface, the proposed device can focus the incident beam at any position according to a particular design. Numerical simulations demonstrate that the designed VO2-assisted metasurface can generate one and multi-focal spot in reflection mode as expected. Also, theoretical results depict an excellent agreement with obtained simulation results. The presented metalens has notable potential in THz high-resolution imaging and optical coding.

physics.optics↗

arXiv2019

Illusion Mechanisms with Cylindrical Metasurfaces: A General Synthesis Approach

We explore the use of cylindrical metasurfaces in providing several illusion mechanisms including scattering cancellation and creating fictitious line sources. We present the general synthesis approach that leads to such phenomena by modeling the metasurface with effective polarizability tensors and by applying boundary conditions to connect the tangential components of the desired fields to the required surface polarization current densities that generate such fields. We then use these required surface polarizations to obtain the effective polarizabilities for the synthesis of the metasurface. We demonstrate the use of this general method for the synthesis of metasurfaces that lead to scattering cancellation and illusion effects, and discuss practical scenarios by using loaded dipole antennas to realize the discretized polarization current densities. This study is the first fundamental step that may lead to interesting electromagnetic applications, like stealth technology, antenna synthesis, wireless power transfer, sensors, cylindrical absorbers, etc.

physics.app-ph↗

arXiv2019

Non-Closed Acoustic Cloak Enabled by Sequential-Step Linear Coordinate Transformations

Hitherto acoustic cloaking devices, which conceal objects externally, have depended on the objects' characteristics. Despite previous works, we design cloaking device placed neighbor an arbitrary object and makes it invisible without the need to make it enclosed. Applying sequential linear coordinate transformations, leads to a non-closed acoustic cloak (NCAC) with homogeneous materials that creates an open invisible region. We propose a non-closed carpet cloak to conceal objects on a reflecting plane. Numerical simulations verify the cloaking effect, which is completely independent of the geometry and material properties of the hidden object. Due to the simple acoustic constitutive parameters of presented structures, this work paves the way toward realization of non-closed acoustic devices, which could find applications in air born sound manipulation and underwater demands.

physics.app-ph↗

arXiv2019

Geometry free materials enabled by transformation optics for enhancing the intensity of electromagnetic waves in an arbitrary domain

The concentration of electromagnetic waves (EM) is of utmost importance in many engineering applications such as solar-cells. According to the transformation optics (TO) methodology, a feasible approach for obtaining arbitrary shape concentrators is proposed. In contrary to the previous works, the obtained materials are homogeneous and independent of the concentrator shape. That is regardless of the input geometry always one constant material, which is optic-null medium (ONM), is used and the performance of device will not alter. This competency will ease the design process and circumvent the sophisticated calculations of the necessitating materials. To authenticate the concept, several numerical full-wave simulations were performed for different shape concentrators. It was observed that numerical results exhibit strong agreement with the theoretical investigations and corroborate the generality and effectiveness of the proposed designing method.

physics.optics↗

arXiv2019

Addition Theorem Revisiting for Phase/Amplitude-Encoded Metasurfaces: Asymmetric Spatial Power Dividers

Recent years have witnessed an extraordinary spurt in attention toward the wave manipulating strategies revealed by coding metasurfaces as they build up a bridge between the physical and digital worlds. Newly, it has been shown that when two different coding patterns responsible for doing separate missions are added together based on the superposition theorem, the mixed coding pattern will perform both missions at the same time. In this paper, via a semi-analytical procedure, we demonstrate that such a theorem is not necessarily valid for all possible functionalities with considering phase-only coding distributions and ignoring the element pattern function. By revisiting the addition theorem, we introduce the concept of asymmetric spatial power divider (ASPD) with arbitrary power ratio levels in which modulating both amplitude and phase of the meta-atoms is inevitable to fully control the power intensity pattern of the metasurface. Numerical simulations illustrate that the proposed ASPD driven by proper coding sequences can directly generate a desired number of beams with pre-determined orientations and power budgets. The phase/amplitude-encoded Pancharatnam-Berry meta-atoms realize the required coding pattern in each case and good conformity between simulations and theoretical predictions verifies the presented formalism. This work exposes a new opportunity to implement spatial power dividers for various applications such as multiple-target radar systems, beamforming networks, and multiple-input multiple-output (MIMO) communication.

physics.app-ph↗

arXiv2018

Arbitrary Shaped Acoustic Concentrators Enabled by Nihility Media

Based on the transformation acoustics (TA) methodology, an innovative approach for designing arbitrary shape concentrators is proposed. unlike previous works, which utilized inhomogenous and anisotropic materials to localize the incident waves in an arbitrary domain, the same functionality will be attained by introducing only one homogenous anisotropic medium, which is called acoustic nihility media (ANM). A great advantage of this method is that the attained materials are not dependent on the shape of the concentrator. That is regardless of the device geometry, a constant ANM will be used for each new shape and the output results do not alter. This will circumvent the conventional transformation acoustics' sophisticated and tedious calculations and could be easily implemented in real life scenarios.

physics.app-ph↗

arXiv2018

Generalized Optical Signal Processing Based on Multi-Operator Metasurfaces Synthesized by Susceptibility Tensors

This paper theoretically proposes a multichannel functional metasurface computer characterized by Generalized Sheet Transition Conditions (GSTCs) and surface susceptibility tensors. The study explores a polarization- and angle-multiplexed metasurfaces enabling multiple and independent parallel analog spatial computations when illuminated by differently polarized incident beams from different directions. The proposed synthesis overcomes substantial restrictions imposed by previous designs such as large architectures arising from the need of additional subblocks, slow responses, and most importantly, supporting only the even reflection symmetry operations for normal incidences, working for a certain incident angle or polarization, and executing only single mathematical operation. The versatility of the design is demonstrated in a way that an ultra-compact, integrable and planar metasurface-assisted platform can execute a variety of optical signal processing operations such as spatial differentiation and integration. It is demonstrated that a metasurface featuring non-reciprocal property can be thought of as a new paradigm to break the even symmetry of reflection and perform both even- and odd-symmetry mathematical operations at normal incidences. Numerical simulations also illustrate different aspects of multichannel edge detection scheme through projecting multiple images on the metasurface from different directions. Such appealing findings not only circumvent the major potential drawbacks of previous designs but also may offer an efficient, easy-to-fabricate, and flexible approach in wave-based signal processing, edge detection, image contrast enhancement, hidden object detection, and equation solving without any Fourier lens.

physics.optics↗

arXiv2018

Noninvasive Blockade of Action Potential by Electromagnetic Induction

Conventional anesthesia methods such as injective anesthetic agents may cause various side effects such as injuries, allergies, and infections. We aim to investigate a noninvasive scheme of an electromagnetic radiator system to block action potential (AP) in neuron fibers. We achieved a high-gradient and unipolar tangential electric field by designing circular geometric coils on an electric rectifier filter layer. An asymmetric sawtooth pulse shape supplied the coils in order to create an effective blockage. The entire setup was placed 5 cm above 50 motor and sensory neurons of the spinal cord. A validated time-domain full-wave analysis code Based on cable model of the neurons and the electric and magnetic potentials is used to simulate and investigate the proposed scheme. We observed action potential blockage on both motor and sensory neurons. In addition, the introduced approach shows promising potential for AP manipulation in the spinal cord.

q-bio.NC↗

arXiv2017

A novel analytical method for analysis of electromagnetic scattering from inhomogeneous spherical structures using duality principles

In this article, a novel analytical approach is presented for the analysis of electromagnetic (EM) scattering from radially inhomogeneous spherical structures (RISSs) based on the duality principle. According to the spherical symmetry, similar angular dependencies in all the regions are considered using spherical harmonics. To extract the radial dependency, the system of differential equations of wave propagation toward the inhomogeneity direction is equated with the dual planar ones. A general duality between electromagnetic fields and parameters and scattering parameters of the two structures is introduced. The validity of the proposed approach is verified through a comprehensive example. The presented approach substitutes a complicated problem in spherical coordinate to an easy, well posed, and a previously solved problem in planar geometry. This approach is valid for all continuously varying inhomogeneity profiles. One of the major advantages of the proposed method is the capability of studying two general and applicable types of RISSs. As an interesting application, a new class of lens antenna based on the physical concept of the gradient refractive index material is introduced. The approach is used to analyze the EM scattering from the structure and validate strong performance of the lens.

physics.class-ph↗