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Terahertz spectrum splitting by a graphene-covered array of rectangular grooves
, Article Optics Letters ; Volume 42, Issue 23 , 2017 , Pages 4808-4811 ; 01469592 (ISSN) ; Saba, A ; Jafargholi, A ; Khavasi, A ; Sharif University of Technology
Abstract
We propose a bidirectional terahertz (THz) spectrum splitter using a practically simple metamaterial structure consisting of rectangular grooves covered by graphene. Thanks to the graphene optoelectronic tunability and by adjusting the grooves width, this structure provides nearly 2π phase shift. At the same time, the reflection efficiency is acceptable throughout the phase shifts. We design each of the meta-atoms using a circuit model, and then we synthesize the final supercell based on the generalized Snell’s law so that the structure reflects different frequency waves to totally different directions. The full-wave simulations demonstrate the beam splitting with a remarkable efficiency of...
Analytical and rigorous method for analysis of an array of magnetically-biased graphene ribbons
, Article Optics Express ; Volume 27, Issue 20 , 2019 ; 10944087 (ISSN) ; Rejaei, B ; Memarian, M ; Khavasi, A ; Sharif University of Technology
OSA - The Optical Society
2019
Abstract
A sheet of graphene under magnetic bias attains anisotropic surface conductivity, opening the door for realizing compact devices such as Faraday rotators, isolators and circulators. In this paper, an accurate and analytical method is proposed for a periodic array of graphene ribbons under magnetic bias. The method is based on integral equations governing the induced surface currents on the coplanar array of graphene ribbons. For subwavelength size ribbons subjected to an incident plane wave, the current distribution is derived leading to analytical expressions for the reflection/transmission coefficients. The results obtained are in excellent agreement with full-wave simulations and predict...
Perfect anomalous reflection using a compound metallic metagrating
, Article Optics Express ; Volume 28, Issue 11 , 2020 , Pages 16439-16452 ; Khavasi, A ; Sharif University of Technology
OSA - The Optical Society
2020
Abstract
Metagrating is a new concept for wavefront manipulation that, unlike phase gradient metasurfaces, does not suffer from low efficiency and also has a less complicated fabrication process. In this paper, a compound metallic grating (a periodic metallic structure with more than one slit in each period) is proposed for anomalous reflection. We propose an analytical method for analyzing the electromagnetic response of this grating. Closed-form and analytical expressions are presented for the reflection coefficients of zeroth diffracted order and also higher diffracted orders. The proposed method is verified against full-wave simulations and the results are in excellent agreement. Thanks to the...
Analytical method for the diffraction of an electromagnetic wave by subwavelength graphene ribbons
, Article Journal of the Optical Society of America B: Optical Physics ; Volume 38, Issue 3 , 2021 , Pages 953-960 ; 07403224 (ISSN) ; Khavasi, A ; Rejaei, B ; Sharif University of Technology
The Optical Society
2021
Abstract
Theoretical study of arrays of graphene ribbons is currently of high interest due to its potential application in beam splitters, absorbers, and polarizers. In this paper, an analytical method is presented for diffraction analysis of graphene ribbon arrays. Previous analytical studies were carried out in the regime where the lateral separation between the ribbons is much smaller than the wavelength of the incident wave. As such, they cannot be used to calculate the reflection coefficients of higher diffracted orders. In contrast, the method proposed here can predict the electromagnetic response of graphene ribbon arrays even when the array constant is larger than the wavelength. To reach our...
Circuit model in design of THz transparent electrodes based on two-dimensional arrays of metallic square holes
, Article IEEE Transactions on Terahertz Science and Technology ; Vol. 4, issue. 3 , April , 2014 , p. 383-390 ; Yarmoghaddam, E ; Khavasi, A ; Mehrany, K ; Sharif University of Technology
Abstract
In this paper, we propose a circuit model for two-dimensional arrays of metallic square holes located on a homogeneous substrate, in order to propose a new scheme containing this type of metamaterials to obtain transparent electrodes with simultaneous terahertz transparency and low electrical resistance. The results of the introduced circuit model, which is a fully analytical model with explicit expressions, are in almost complete agreement with the full-wave simulations. Thanks to this analytical model, we can employ standard binomial matching transformer in order to minimize the reflected power from the structure at a desired frequency. Furthermore, taking advantage of this model, we...
Analytical circuit model for periodic arrays of graphene disks
, Article IEEE Journal of Quantum Electronics ; Volume 51, Issue 9 , July , 2015 ; 00189197 (ISSN) ; Rejaei, B ; Khavasi, A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2015
Abstract
In this paper, an analytical circuit model is proposed for 2-D arrays of graphene disks. First, we derive an analytical expression for the surface current density on a single graphene disk in the subwavelength regime, induced by a normally incident plane wave. The solution is then extended to 2-D arrays of graphene disks using perturbation theory. Finally, by applying appropriate boundary conditions, an R - L - C equivalent circuit of the structure is obtained. It is shown that both a single graphene disk and periodic array of graphene disks have dual capacitive-inductive nature. The results of the proposed model are in excellent agreement with those obtained by full-wave simulations
Phase resonance tuning and multi-band absorption Via graphene-covered compound metallic gratings
, Article IEEE Journal of Quantum Electronics ; Volume 53, Issue 5 , 2017 ; 00189197 (ISSN) ; Rahmani, B ; Bagheri, A ; Khavasi, A ; Sharif University of Technology
Abstract
1-D compound metallic grating (CMG) is a periodic structure with more than one slit in each period. When CMG is combined with a graphene sheet as its cover, the incident light is effectively coupled to the plasmons in graphene which in turn can result in strong manipulation of light for both major polarizations. We show that tunable phase resonance and perfect absorption of the incident light are interesting outcomes of this manipulation. In this paper, we demonstrate that fano-like phase resonances which can be observed in CMGs under transverse magnetic polarized incident wave are tuned by changing the Fermi level of graphene. It is shown that while the spectral position of the phase...
Deriving surface impedance for 2-d arrays of graphene patches using a variational method
, Article IEEE Journal of Quantum Electronics ; Volume 53, Issue 1 , 2017 ; 00189197 (ISSN) ; Tavakol, M. R ; Khavasi, A ; Sharif University of Technology
Abstract
In this paper, we extract the fundamental resonant mode of a graphene patch using a variational method. We use 2-D eigenvalue problem obtained from the integral equation governing the surface current on graphene patterns under quasi-static approximation. To compute the eigenvalues, we propose three trial eigenfunctions, which meet the boundary conditions. We investigate the accuracy of these eigenfunctions with comparing to the results obtained by full wave simulations. Finally, we analyze square-lattice arrangements of graphene patches using the most accurate proposed eigenfunction and derive a very accurate surface impedance for it. The proposed surface impedance is much more precise than...
Modeling of periodic array of cut-through slits with sinusoidal surface conductivity at the interfaces of an anisotropic medium
, Article IEEE Transactions on Antennas and Propagation ; Volume 66, Issue 10 , 2018 , Pages 5630-5633 ; 0018926X (ISSN) ; Mehrany, K ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2018
Abstract
A periodic arrangement of 1-D slits carved in a perfect electric conductor is investigated, and an equivalent model which features sinusoidal surface conductivity on its upper and lower interfaces is derived. Therefore, and in sheer contrast to all the previous attempts that were successful in mimicking only the zeroth-order diffracted waves, it is capable of emulating both specular and nonspecular diffraction orders. The accuracy of the model is verified by the full-wave simulations. The proposed model is used to study the sharp Fano-type extraordinary transmission resonances arising in the structure. The transmission spectrum near these resonances is derived in terms of Fano resonance...
Generating uniform irradiance in the Fresnel region by quasi-optical beam shaping of a millimeter-wave source
, Article Optics Express ; Volume 27, Issue 22 , 2019 , Pages 32135-32146 ; 10944087 (ISSN) ; Ahmadi-Boroujeni, M ; Tehranian, A ; Sharif University of Technology
OSA - The Optical Society
2019
Abstract
In this paper, we report on the design, simulation and measurement of a flat-top beam shaper (FBS) lens for a millimeter-wave (mm-wave) antenna coupled source. The beam shaper is designed to transform the pseudo-Gaussian beam of the mm-wave source into a flat-top beam of sharp roll-off at a specific distance in the Fresnel region. Firstly, relying on the geometrical optics principles, we propose an analytical formulation for the design of lens profiles. Next, a simple optimization method based on the full-wave simulation of the lens and the mm-wave antenna is suggested to tune the analytically extracted lens profile. The optimized lens profile is then studied to assess its performance...
Rigorous derivation of temporal coupled mode theory expressions for travelling and standing wave resonators coupled to optical waveguides
, Article 7th International Conference on Photonics, Optics and Laser Technology, PHOTOPTICS 2019, 25 February 2019 through 27 February 2019 ; 2019 , Pages 201-208 ; 9789897583643 (ISBN) ; Memarian, M ; Mehrany, K ; Ribeiro P ; Raposo M ; Andrews D ; Institute for Systems and Technologies of Information, Control and Communication (INSTICC) ; Sharif University of Technology
SciTePress
2019
Abstract
Temporal coupled mode theory (CMT) has so far been applied phenomenologically in the analysis of optical cavity-waveguide structures, and relies on a priori knowledge of the to-be-excited resonator mode. Thus a rigorous derivation from Maxwell's equations, and without any prior knowledge of the resonator type is needed. In this paper we derive temporal CMT of optical cavities coupled to waveguides. Starting from Maxwell's equations and considering a proper expansion of the modes of the waveguide and resonator, and using mode orthogonality, the temporal CMT for this structure is obtained. We show that this formulation is general and can be applied to both traveling wave and standing wave type...
Generating uniform irradiance in the fresnel region by quasi-optical beam shaping of a millimeter-wave source
, Article Optics Express ; Volume 27, Issue 22 , 2019 , Pages 32135-32146 ; 10944087 (ISSN) ; Ahmadi Boroujeni, M ; Tehranian, A ; Sharif University of Technology
OSA - The Optical Society
2019
Abstract
In this paper, we report on the design, simulation and measurement of a flat-top beam shaper (FBS) lens for a millimeter-wave (mm-wave) antenna coupled source. The beam shaper is designed to transform the pseudo-Gaussian beam of the mm-wave source into a flat-top beam of sharp roll-off at a specific distance in the Fresnel region. Firstly, relying on the geometrical optics principles, we propose an analytical formulation for the design of lens profiles. Next, a simple optimization method based on the full-wave simulation of the lens and the mm-wave antenna is suggested to tune the analytically extracted lens profile. The optimized lens profile is then studied to assess its performance...
Rigorous derivation of temporal coupled mode theory expressions for travelling and standing wave resonators coupled to optical waveguides
, Article 7th International Conference on Photonics, Optics and Laser Technology, PHOTOPTICS 2019, 25 February 2019 through 27 February 2019 ; 2019 , Pages 201-208 ; 9789897583643 (ISBN) ; Memarian, M ; Mehrany, K ; Sharif University of Technology
SciTePress
2019
Abstract
Temporal coupled mode theory (CMT) has so far been applied phenomenologically in the analysis of optical cavity-waveguide structures, and relies on a priori knowledge of the to-be-excited resonator mode. Thus a rigorous derivation from Maxwell's equations, and without any prior knowledge of the resonator type is needed. In this paper we derive temporal CMT of optical cavities coupled to waveguides. Starting from Maxwell's equations and considering a proper expansion of the modes of the waveguide and resonator, and using mode orthogonality, the temporal CMT for this structure is obtained. We show that this formulation is general and can be applied to both traveling wave and standing wave type...
Wide-band/angle blazed dual mode metallic groove gratings
, Article IEEE Transactions on Antennas and Propagation ; 16 June , 2020 ; Abbassi, M. A ; Rahmani, B ; Memarian, M ; Mehrany, K ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2020
Abstract
A simple approach to acquire wide-band/angle blazing operation in one-dimensional metallic gratings is investigated by means of mode-matching and equivalent circuit analysis. The gratings are single-groove perfectly conducting gratings that support two propagating guided modes (opposed to typical single mode cases). It is shown under what conditions, one can achieve blazing over a wide range of frequencies and angles. Most importantly, it is identified what governs blazing, in particular the true matching condition that needs to be satisfied, and how the structures’ multiple resonances play role in achieving such matching and tailoring the bandwidth. Parameters of the proposed equivalent...
Frequency conversion and parametric amplification using a virtually rotating metasurface
, Article Optics Express ; Volume 28, Issue 5 , 2020 , Pages 6378-6394 ; Rejaei, B ; Memarian, M ; Sharif University of Technology
OSA - The Optical Society
2020
Abstract
We analyze the scattering of circularly polarized electromagnetic waves from a time-varying metasurface having a time-dependent surface susceptibility that locally mimics a rotating, anisotropic surface. Such virtually rotating metasurfaces (VRM) can be realized by means of electronically tunable surface elements and reach microwave-range rotation frequencies. It is shown that the scattered field contains the incident tone, as well as a single up-or down converted tone which differs by twice the rotation frequency of the surface. A simple full frequency converter is then proposed by augmenting the VRM with a metal screen separated by a proper distance. It is shown that after reflection from...
Analytical method for diffraction analysis and design of perfect-electric-conductor backed graphene ribbon metagratings
, Article Optics Express ; Volume 29, Issue 18 , 2021 , Pages 28935-28952 ; 10944087 (ISSN) ; Khavasi, A ; Rejaei, B ; Sharif University of Technology
The Optical Society
2021
Abstract
Graphene-based gratings and metagratings have attracted great interest in the last few years because they could realize various multi-functional beam manipulation, such as beam splitting, focusing, and anomalous reflection in the terahertz (THz) regime. However, most of graphene-based metagratings are designed through numerical simulations, which are very time-consuming. In this paper, an accurate analytical method is proposed for diffraction analysis of a perfect electric conductor (PEC)-backed array of graphene ribbons. In contrast to previous analytical treatments, the proposed method can predict the electromagnetic performance of graphene ribbons not only in the subwavelength regime, but...
Wide-band/angle blazed dual-mode metallic groove gratings
, Article IEEE Transactions on Antennas and Propagation ; Volume 69, Issue 1 , 2021 , Pages 379-386 ; 0018926X (ISSN) ; Abbassi, M. A ; Rahmani, B ; Memarian, M ; Mehrany, K ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2021
Abstract
A simple approach to acquire wide-band/angle blazing operation in 1-D metallic gratings is investigated by means of mode-matching and equivalent circuit analysis. The gratings are single-groove perfectly conducting gratings that support two propagating guided modes (opposed to typical single-mode cases). It is shown under what conditions one can achieve blazing over a wide range of frequencies and angles. Most importantly, it is identified what governs blazing, in particular the true matching condition that needs to be satisfied, and how the structures' multiple resonances play role in achieving such matching and tailoring the bandwidth. Parameters of the proposed equivalent circuit model...
Direct coupled resonator filters realized by gap waveguide technology
, Article IEEE Transactions on Microwave Theory and Techniques ; Volume 63, Issue 10 , August , 2015 , Pages 3445-3452 ; 00189480 (ISSN) ; Banai, A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2015
Abstract
Novel types of narrow band filters based on the newly introduced gap waveguide technology are presented in this paper. The proposed filters have a central frequency of 35 GHz with approximately 1% fractional bandwidth. The filter resonators are composed of two separate plates and are manufactured by milling metallic blocks. The gap between the two metallic plates eliminates the need for electrical contact between them. This feature allows the resonators to be stacked in different layers. The filtering function is realized by producing a coupling between the stacked resonators. The measurement results of the manufactured filters are in good agreement with full-wave simulations, even without...
An exact method for the extraction of effective bulk and surface parameters of periodic artificial media
, Article IEEE Transactions on Antennas and Propagation ; Volume 63, Issue 6 , 2015 , Pages 2521-2531 ; 0018926X (ISSN) ; Rejaei, B ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2015
Abstract
A new method is presented for the extraction of the bulk and surface parameters of a periodic artificial medium which uses the eigenvectors of the generalized transfer matrix of a unit layer. These eigenvectors correspond to the Bloch modes of the periodic structure. The eigenvector related to the propagating Bloch mode directly yields an expression for the effective, intrinsic wave impedance of the medium. Moreover, the interface between the artificial material and a surrounding, conventional (dielectric) region is described by an interface impedance matrix which accounts for the excitation of higher order, nonpropagating Bloch modes at the interface. Although these modes do not propagate...
Design of terahertz photoconductive antenna arrays based on defective photonic crystal substrates
, Article Optics and Laser Technology ; Volume 114 , 2019 , Pages 89-94 ; 00303992 (ISSN) ; Ahmadi Boroujeni, M ; Sharif University of Technology
Elsevier Ltd
2019
Abstract
In this paper, arrays of two different terahertz (THz) antennas based on defective photonic crystal (DPC) substrates are proposed and their radiation characteristics are investigated by using full-wave simulation. We show that employing DPC substrates in THz photoconductive antenna (PCA) arrays reduces the amount of power coupled to the substrate surface waves and, as a result, decreases the mutual coupling between the elements of the antenna array and increases the directivity in the boresight direction. By studying 1 × 2 arrays of bowtie and four-leaf-clover-shaped antennas, we show that either the improvement of directivity in a broad bandwidth or the enhancement of photomixer antenna...