Showing papers by "Shah Nawaz Burokur published in 2018"
TL;DR: The proposed method provides an efficient approach to control the radius of vortex beam carrying OAM mode in microwave wireless applications for medium-short range distance.
Abstract: In this paper, ultra-thin metalenses are proposed to generate converging and non-diffractive vortex beam carrying orbital angular momentum (OAM) in microwave region. Phase changes are introduced to the transmission cross-polarized wave by tailoring spatial orientation of Pancharatnam-Berry phase unit cell. Based on the superposition of phase profile of spiral phase plate and that of a converging lens or an axicon, vortex beam carrying OAM mode generated by the metalens can also exhibit characteristics of a focusing beam or a Bessel beam. Measured field intensities and phase distributions at microwave frequencies verify the theoretical design procedure. The proposed method provides an efficient approach to control the radius of vortex beam carrying OAM mode in microwave wireless applications for medium-short range distance.
214 citations
115 citations
TL;DR: Frequency agility, beam deflection and beam focusing are the main functionalities demonstrated from the proposed reconfigurable meta-mirror for wavefronts control in microwave antenna applications.
Abstract: A planar metasurface composed of electronically tunable meta-atoms incorporating voltage-controlled varactor diodes is proposed as a reconfigurable meta-mirror for wavefronts control in microwave antenna applications. The dispersion responses of the cells are individually tailored in the reconfigurable metasurface so as to overcome the bandwidth limitations of passive metasurfaces and also to control the phase characteristics. By controlling the bias voltage of the varactor diodes on the planar metasurface, the phase characteristics of reflectors can be engineered. The reconfigurable meta-mirror is utilized to implement three different types of reflectors. As such, a reflectarray, a cylindrical parabolic reflector and a dihedral reflector are numerically verified in microwave regime through finite element method. Moreover, experimental measurements are performed on a fabricated prototype to validate the proposed device. Frequency agility, beam deflection and beam focusing are the main functionalities demonstrated from the proposed reconfigurable meta-mirror.
87 citations
TL;DR: In this article, a synergistic approach that combines metamaterials and gratings to achieve complete control of diffraction patterns was proposed. But the work in this paper is restricted to the case where the number of scatterers is significantly reduced.
Abstract: Although various wavefront-manipulation capabilities have been demonstrated with metasurfaces, both fundamental and practical difficulties remain. This study elaborates on a synergistic approach that combines metamaterials and gratings to achieve complete control of diffraction patterns. Unlike in a metasurface, in a metagrating the number of scatterers is significantly reduced, relaxing fabrication tolerance. Strong control of diffraction with simple excitation, ultrawide bandwidth, and significantly fewer scatterers is particularly interesting at optical and infrared frequencies, for $e.g.$ efficient, reconfigurable antennas in microwave communication systems.
67 citations
TL;DR: Simulation results verify the ability of the MS lens to achieve OAM beam focusing, which is advantageous for enhancing the propagation directivity and increasing the gain in the main lobes of vortex waves, of particular importance in microwave wireless communication applications.
Abstract: Vortex electromagnetic (EM) waves hold promise for their ability to significantly increase the transmission capacity of wireless communication systems via the torsion resistance defined by different topological charges associated with the orbital angular momentum (OAM). However, the application of vortex waves in remote distance transmission is limited by its characteristic of divergence. In this paper, a lens based on a phase-modulation metasurface (MS) is proposed that enables vortex EM waves to converge, thereby improving their propagation performance at microwave frequencies. A phase-shift distribution on the plane of the MS is obtained based on the concept of the optical converging axicon, which can convert a Laguerre-Gaussian (LG) beam to a Bessel beam based on changing the propagation direction. Simulation results verify the ability of the MS lens to achieve OAM beam focusing, which is advantageous for enhancing the propagation directivity and increasing the gain in the main lobes of vortex waves. This is of particular importance in microwave wireless communication applications.
48 citations
TL;DR: In this paper, a metasurface composed of active unit cells incorporating voltage-controlled varactor diodes, where the dispersion responses of the cells can be tailored, is proposed to overcome the limitations of passive surfaces.
Abstract: Metasurfaces relying solely on passive unit cells printed on a dielectric substrate present limited characteristics due to their intrinsic dispersions. A metasurface composed of active unit cells incorporating voltage-controlled varactor diodes, where the dispersion responses of the cells can be tailored, is proposed to overcome the limitations of passive metasurfaces. Two functionalities are numerically and experimentally demonstrated from the active metasurface used as an electronically reconfigurable reflector. First, we show that anomalous reflection properties can be produced within a broad frequency range and second, we show that the direction of the reflected beam can be scanned within an angular range.
37 citations
TL;DR: This work proposes an innovative OAM generation method based on transformation optics (TO), which provides an effective way to realize a conversion from plane waves to vortex waves, which can greatly facilitate the potential implementation of OAM waves in microwave wireless communication systems.
Abstract: Orbital angular momentum (OAM) vortex waves generated by conventional spiral phase plates and metasurfaces have been widely discussed. In this work, we propose an innovative OAM generation method based on transformation optics (TO). By solving Laplace’s equation with specific boundary conditions, an oblate cylindrical shaped physical domain is designed to imitate a gradient shaped virtual domain which is able to generate a vortex beam upon reflection. As a proof-of-concept demonstration, a broadband all-dielectric microwave lens for vortex beam generation is presented with a topological charge of + 1. The corresponding far-field patterns as well as near-field helical phase and doughnut-shaped amplitude distributions of the lens, obtained from numerical simulations, are reported along with a wide operational bandwidth spanning from 8 to 16 GHz. As a transformation method, the proposed TO technique provides an effective way to realize a conversion from plane waves to vortex waves, which can greatly facilitate the potential implementation of OAM waves in microwave wireless communication systems.
31 citations
TL;DR: In this article, a general case of metagratings composed of N polarization electric line currents per supercell was considered and it was shown that a metagrating having N equal to the number of plane waves scattered in the far-field can be used for controlling the diffraction pattern.
Abstract: In this study we elaborate on the recent concept of metagratings proposed in Ra'di et al. [Phys. Rev. Lett. 119, 067404 (2017)] for efficient manipulation of reflected waves. Basically, a metagrating is a set of 1D arrays of polarization line currents which are engineered to cancel scattering in undesirable diffraction orders. We consider a general case of metagratings composed of N polarization electric line currents per supercell. This generalization is a necessary step to totally control diffraction patterns. We show that a metagrating having N equal to the number of plane waves scattered in the far-field can be used for controlling the diffraction pattern. To validate the developed theoretical approach, anomalous and multichannel reflections are demonstrated with 3D full-wave simulations in the microwave regime at 10 GHz. The results can be interesting for the metamaterials community as allow one to significantly decrease the number of used elements and simplify the design of wavefront manipulation devices, what is very convenient for optical and infra-red frequency ranges. Our findings also may serve as a way for development of efficient tunable antennas in the microwave domain.
28 citations
TL;DR: A transformation optics (TO) based all-dielectric converging lens design comprised by a graded permittivity profile can be fabricated by additive manufacturing technology, which greatly facilitates the potential development and application of vortex wave based wireless communications.
Abstract: Radio waves carrying orbital angular momentum (OAM) may potentially increase spectrum efficiency and channel capacity based on their extra rotational degree of freedom. However, due to their divergence characteristics, vortex waves are not suitable to transmit over a long distance in the radio frequency (RF) and microwave domains. In this paper, a transformation optics (TO) based all-dielectric converging lens is proposed. The beam divergence angle of the vortex wave passing through the lens can be decreased from 25° to 9°. The transformed material parameters of the converging lens are determined by solving Laplace's equation subject to specific boundary conditions. Far-field antenna radiation patterns as well as near-field helical phase and electric field amplitude distributions obtained from numerical simulations are reported, demonstrating the broadband characteristics of the proposed microwave lens. Moreover, the all-dielectric compact lens design comprised by a graded permittivity profile can be fabricated by additive manufacturing technology, which greatly facilitates the potential development and application of vortex wave based wireless communications.
16 citations
TL;DR: Modeling and experimental results bring evidence that metal-insulator-metal metasurfaces operating in the microwave or terahertz domains provide greater flexibility for Fano resonance engineering and provide strong enhancement of the spectral selectivity factor.
Abstract: The engineering of metal-insulator-metal metasurfaces (MSs) displaying sharp spectral features based on Fano-type interference between a symmetric bright mode and an antisymmetric dark mode is reported. The proposed mechanism for direct excitation of antisymmetric mode avoids the necessity of mode hybridization through near-field coupling. Modeling and experimental results bring evidence that such MSs operating in the microwave or terahertz domains provide greater flexibility for Fano resonance engineering and provide strong enhancement of the spectral selectivity factor. It is shown that the occurring Fano resonance interference is related to the broken eigenmode orthogonality in open systems and is independent of hybridization mechanism.
13 citations
TL;DR: Such gradient phase reflective surface paves the way to low-cost easy-made microwave metal-dielectric surfaces incorporating functionalities such as beam control, forming and collimation.
Abstract: A metal-dielectric-metal gradient phase partially reflecting surface based on the combination of a gradient index dielectric substrate with an inductive and a capacitive grids, is designed at microwave frequencies for antenna applications. The gradient index is obtained by realizing air holes of different dimensions in a dielectric host material. A prototype of the gradient index dielectric substrate is fabricated through three-dimensional printing, an additive fabrication technology. It is then associated to two patterned metallic grids to realize a partially reflecting surface with a gradient phase behavior. For experimental validation, the partially reflective surface is used as reflector in a low-profile Fabry-Perot cavity antenna. An angular enhancement of the emitted beam in a desired direction is reported by further engineering the phase introduced by the inductive and the capacitive grids. Far-field measurements are performed on fabricated antenna prototypes to validate the concept. Such gradient phase reflective surface paves the way to low-cost easy-made microwave metal-dielectric surfaces incorporating functionalities such as beam control, forming and collimation.
TL;DR: This paper defines a coordinate transformation scheme that enables one to taper the electric field between two waveguides of different cross-sections and presents a 2D continuous design model and a potential 3D discretized realization model.
Abstract: Complex electromagnetic structures can be designed by exploiting the concept of spatial coordinate transformations. In this paper, we define a coordinate transformation scheme that enables one to taper the electric field between two waveguides of different cross-sections. The electromagnetic field launched from the wide input waveguide is compressed in the proposed field tapering device and guided into the narrow output waveguide. In closed rectangular waveguide configurations, the taper can further play the role of a mode selector due to the output waveguide’s cut-off frequency. Realizable permittivity and permeability values that can be achieved with common existing metamaterials are determined from the transformation equations and simplified by a proposed parameter reduction method. Both a 2D continuous design model and a potential 3D discretized realization model are presented at microwave frequencies and the performances of the tapering devices are verified by full-wave finite element numerical simulations. Finally, near-field distributions are shown to demonstrate the field tapering functionality.
01 Nov 2018
TL;DR: In this paper, an ultrathin metasurface is made reconfigurable through the use of unit cells incorporating voltage-controlled varactor diodes, and phase characteristics of each unit cell are individually controlled for reconfigurability mechanisms.
Abstract: An ultrathin metasurface is made reconfigurable through the use of unit cells incorporating voltage-controlled varactor diodes. The phase characteristics of each unit cell are individually controlled for reconfigurability mechanisms. Two main applications are proposed in the domain of microwave devices and antennas. The planar metasurface is illuminated by a primary source and is firstly used as reconfigurable reflector allowing beam steering performances and frequency agility. A second functionality is also demonstrated, where the metasurface is used as a reconfigurable polarizer. Measurements are performed on a fabricated prototype to validate the concept.
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TL;DR: In this paper, a 1D periodic array of super cells composed of N polarization electric line currents placed over a grounded dielectric substrate and excited by an incident plane wave was used to achieve total control over electromagnetic waves diffracted in the far-field.
Abstract: In this study, metagratings are used to achieve total control over electromagnetic waves diffracted in the far-field. To this end, we consider a 1D periodic array of super cells composed of N polarization electric line currents placed over a grounded dielectric substrate and excited by an incident plane wave. To validate the developed theoretical approach, broadband anomalous and multichannel reflections are demonstrated from 3D full-wave simulations in the microwave regime at 10 GHz.
01 Jan 2018
TL;DR: In this article, a planar active metasurface composed of meta-atoms incorporating voltage-controlled varactor diodes, is used as reflector for antenna applications.
Abstract: A planar active metasurface composed of meta-atoms incorporating voltage-controlled varactor diodes, is used as reflector for antenna applications. The dispersion responses of the cells are individually tailored for reconfigurability mechanisms. The phase characteristics of two reflectors are engineered by judiciously controlling the bias voltage of the varactor diodes on the planar metasurface. The metasurface is illuminated by a primary source to be used as parabolic and corner reflectors. Measurements are performed on a fabricated prototype to validate the concept. Beam steering performances are also demonstrated in the case of the parabolic-profile reflector.
09 Apr 2018
TL;DR: In this article, a 3D printed index-modulated dielectric medium intended to be utilized as substrate in a partially reflecting surface is designed, where the resulting metal-dielectric-metal structure is able to present a phase modulation along its surface.
Abstract: A 3D printed index-modulated dielectric medium intended to be utilized as substrate in a partially reflecting surface is designed. When associated to uniform inductive and capacitive grids, the resulting metal-dielectric-metal structure is able to present a phase modulation along its surface. The latter phase-modulated metasurface is applied as a partially reflecting surface in a reflex-type Fabry-Perot cavity antenna. A prototype of the antenna is designed, fabricated and measured. Simulated and measured performances show that due to the phase modulation introduced in the partially reflecting surface, the radiated beam is steered from boresight to a desired off-normal direction.
01 Nov 2018
TL;DR: In this paper, an index modulated dielectric substrate was realized by 3D printing technology and combined with uniform copper inductive and capacitive grids to act as a phase modulated partially reflective surface.
Abstract: We report the realization of an index modulated dielectric substrate by three-dimensional (3D) printing technology. Such dielectric substrate is further combined with uniform copper inductive and capacitive grids to act as a phase-modulated partially reflective surface. The resulting surface is used in a Fabry-Perot cavity antenna in order to steer the radiated beam to an off-normal direction.
01 Jan 2018
TL;DR: In this paper, the authors revisited the engineering of metasurfaces displaying sharp spectral features and conventionally relying on electromagnetically induced transparency resulting from Fano-type interference between dark and bright resonant elements.
Abstract: We revisit the engineering of metasurfaces displaying sharp spectral features and conventionally relying on electromagnetically induced transparency resulting from Fano-type interference between dark and bright resonant elements. The aim of the developed approach based on symmetry considerations is to show that electromagnetically induced transparency and dark mode excitation are not necessarily associated. We bring theoretical and experimental evidence in the microwave domain that electromagnetically induced transparency and dark mode excitation can be achieved in an independent manner by using distinctly different mechanisms. The use of these distinctly different mechanisms provides higher flexibility for metasurfaces engineering and results in a great improvement of their spectral performances.
01 Jul 2018
TL;DR: Based on the superposition of phase profile of converging lens and spiral phase plate, the converging vortex beam with OAM mode can be generated by proposed metalens in microwave region.
Abstract: In this paper, a planar ultra-thin metalens is proposed to generate converging vortex beam carrying orbital angular momentum (OAM) in microwave region. Phase changes are introduced to manipulate the transmitted wave front by designing rotation angles of Pancharatnam- Berry phase unit cell. Based on the superposition of phase profile of converging lens and spiral phase plate, the converging vortex beam with OAM mode can be generated by proposed metalens in microwave region. The measurement results are in good accordance with simulation results, which verify the theoretical design procedures.
TL;DR: In this paper, a simulation-based design approach to construct metagratings in the "unit cell by unit cell" manner is presented, which represents an analog of the local periodic approximation that has been used to design space-modulated metasurfaces and allows one to overcome the limitations of straightforward numerical optimization and semianalytical procedures.
Abstract: Recently, metamaterials-inspired diffraction gratings (or metagratings) have demonstrated unprecedented efficiency in wavefront manipulation by means of relatively simple structures. Conventional one-dimensional (1D) gratings have a profile modulation in one direction and a translation symmetry in the other. In 1D metagratings, the translation invariant direction is engineered at a subwavelength scale, which allows one to accurately control polarization line currents and, consequently, the scattering pattern. In bright contrast to metasurfaces, metagratings cannot be described by means of surface impedances (or local reflection and transmission coefficients). In this paper, we present a simulation-based design approach to construct metagratings in the "unit cell by unit cell" manner. It represents an analog of the local periodic approximation that has been used to design space-modulated metasurfaces and allows one to overcome the limitations of straightforward numerical optimization and semianalytical procedures that have been used to date to design metagratings. Electric and magnetic metagrating structures responding to, respectively, transverse electric and transverse magnetic incident plane waves are presented to validate the proposed design approach.
TL;DR: In this paper, a symmetry matching approach for the excitation of dark modes was proposed to take advantage of the geometry symmetry of the structure with regard to the incident external electromagnetic field for a direct far-field coupling.
Abstract: We revisit the engineering of metasurfaces to obtain sharp features in their spectral response. We show that in contrast to conventional approach exploiting indirect mode hybridization mechanism based on strong near-field coupling, a more flexible and efficient engineering of the spectral response can be achieved using a symmetry matching approach for the excitation of dark modes. This distinctly different mechanism takes advantage of the geometry symmetry of the structure with regard to the incident external electromagnetic field for a direct far-field coupling.
TL;DR: In this article, the authors show that each propagating diffraction order requires 2 degrees of freedom represented by passive and lossless loaded thin "wires" and provide a solution to the old problem of power management between diffraction orders created by a grating.
Abstract: We report that metamaterial-inspired one-dimensional gratings (or metagratings) can be used to control nonpropagating diffraction orders as well as propagating ones. By accurate engineering of the near field, it becomes possible to satisfy power-conservation conditions and achieve perfect control over all propagating diffraction orders with passive and lossless metagratings. We show that each propagating diffraction order requires 2 degrees of freedom represented by passive and lossless loaded thin "wires." This provides a solution to the old problem of power management between diffraction orders created by a grating. The theory developed is verified by both three-dimensional full-wave numerical simulations and experimental measurements, and can be readily applied to the design of wavefront-manipulation devices over the entire electromagnetic spectrum as well as in different fields of physics.
01 Dec 2018
TL;DR: A reconfigurable Fabry-Perot cavity antenna is proposed and it is shown that the main beam can be steered to an off-normal direction by modulating the phase profile along the metasurface.
Abstract: A reconfigurable Fabry-Perot (FP) cavity antenna is proposed in this letter. The antenna is composed of a ground plane and a reconfigurable metasurface used as partially reflective surface (PRS). By considering the possibility of incorporating electronic components in the metasurface, reconfigurability mechanism can be achieved. We start by designing a FP cavity that presents a directive beam along boresight direction. Then, by modulating the phase profile along the metasurface we show that the main beam can be steered to an off-normal direction.
01 Jul 2018
TL;DR: In this article, a planar reconfigurable reflective metasurface is used as a polarizer, which allows the conversion of a linear incident wave to a circular one, by varying the dispersion responses of each meta-atom.
Abstract: A planar reconfigurable reflective metasurface is used as a polarizer. The metasurface allows the conversion of a linear incident wave to a circular one. The proposed structure is composed of meta-atoms incorporating voltage-controlled varactor diodes. By varying the dispersion responses of each meta-atom., the operating frequency can be tailored. Simulations and measurements on a fabricated prototype are presented to validate the concept at microwave frequencies.
01 Jul 2018
TL;DR: In this paper, a two-step procedure is introduced to simplify the tensor properties of the material by eliminating non-diagonal components, and the resulting permittivity and permeability distributions for the taper device are calculated and presented.
Abstract: Spatial coordinate transformations can be used in the design of electromagnetic devices with unconventional and exotic behavior. However, their realizations are generally limited by the complex material properties that are required. In this paper, the coordinate transformation technique is employed to successfully design two field tapering devices. 2D continuous models are presented that confirm the desired field tapering functionality for the targeted microwave frequency band. Next, a scheme that utilizes a method of parameter reduction to simplify the complicated tensors produced by the conventional coordinate transformation technique is applied to one of the taper designs. A two-step procedure is introduced to simplify the tensor properties of the material by eliminating non-diagonal components. Furthermore, the resulting permittivity and permeability distributions for the taper device are calculated and presented.
01 Jul 2018
TL;DR: In this article, phase-engineered partially reflecting surfaces with either phase gradient or phase modulation along the dielectric-metal structures are designed, fabricated and measured, and the radiated beam is steered from boresight to a desired off-normal direction.
Abstract: 3D printing is used to realize index-gradient and index-modulated dielectric media. The engineered media are associated to uniform inductive and capacitive grids to realize partially reflecting surfaces with either phase gradient or phase modulation along the dielectric-metal structures. Fabry-Perot cavity antennas utilizing the phase-engineered partially reflecting surfaces are designed, fabricated and measured. Simulated and measured performances show that the radiated beam is steered from boresight to a desired off-normal direction.
08 Jul 2018
TL;DR: In this article, an all-dielectric oblate cylindrical shaped reflecting lens is proposed, where the plane wave emitted from a wave port can be transformed into a vortex wave after being transmitted through the proposed lens and reflected by a planar metallic surface.
Abstract: An all-dielectric oblate cylindrical shaped reflecting lens is proposed. The plane wave emitted from a wave port can be transformed into a vortex wave after being transmitted through the proposed lens and reflected by a planar metallic surface. The space transformation concept is applied for theoretical design of the lens, which results in a graded permittivity distribution profile. The far-field patterns are numerically simulated and reported in order to validate the proposed microwave lens over a wide frequency range spanning from 8 GHz to 12 GHz. Such an all-dielectric microwave lens can be potentially fabricated by low-cost materials through three-dimensional (3D) printing technology.