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Metasurface holographic encryption, as an emerging and highly promising technique, presents the advantages of multi‐dimension and high security and consequently provides a flexible and convenient platform for information encryption. Recent years have witnessed rapid progress in meta‐holographic encryption through the utilization of different dimensions of electromagnetic (EM) waves, yet enhancing the capacity and security of encrypted information remains an endless goal. In this context, the concept of three physical dimensions encrypted metasurface holography is proposed in which orbital angular momentum (OAM) is fully synergized with frequency and polarization state. The mechanism relies on the simultaneous control of these dimensions and arbitrarily overlaying them in each operation channel, thereby acting as a multi‐dimensional key to realize information encryption and decryption. As a proof‐of‐concept experiment, a three‐physical dimension encrypted meta‐hologram is designed that conceals four real images as encrypted information in 32 information channels. Remarkably, a specific OAM order, frequency, and polarization state is required to decrypt the designed meta‐hologram. This work opens a new avenue for multi‐dimensional encrypted holography, which has broad application prospects for information security and data storage in the era of big data. [ABSTRACT FROM AUTHOR]

Metasurface‐based holography provides tremendous advances in multi‐dimensional detection, super‐resolution imaging, and cryptography applications. Current mainstream researches on holographic metasurface are primarily focused on exploring degrees of freedom to enhance information multiplexing capability. Nevertheless, from the information security point of view, it is necessary to integrate existing available freedom resources, such as multi‐polarization components, to enhance the security of holographic encryption. Herein, a full‐polarimetric synthetization scheme is proposed for holographic displaying to develop a novel approach for information and imaging encryption. By exploiting chirality‐assisted metasurface as the implementation platform, quadruplex circular polarization components are independently phase‐modulated with separate holographic sub‐imaging. For an intuitive demonstration, linear polarization is set as the encoded state to acquire the synthesized intensity image with "HIT" characters. Hence, the output holographic information in transmission field can be successfully distinguished with the valid polarization. Additionally, the sensitivity and robustness property of the synthesized holographic performance is experimentally evaluated against ergodic elliptical polarization states, where the optimal performance of working efficiency and signal‐to‐noise ratio only appear under the preset linear polarizations. These results effectively prove the feasibility of the polarization integration hologram, opening the door to novel solutions for future full‐polarimetric encryption strategies. [ABSTRACT FROM AUTHOR]

In the face of the challenges posed by the advent of 5G, Internet of Things (IoT) and requirements of future communication generations, the urgent need for cutting‐edge technologies capable of adapting their operation to a variety of dynamic tasks for efficient signal propagation management is clear. Connected intelligent metasurfaces, distinguished by their ability to dynamically and autonomously adapt their responses, and able to interact with an external uncontrollable and fast‐changing communication environment to benefit from collective intelligence so as to determine the required action, are emerging as an innovative solution in this field. Herein, an intelligent connected system based on a programmable metasurface reflector, is presented and assessed in two scenarios simulating dynamic uplink and downlink communications. For the uplink configuration, the metasurface is exploited for the estimation of the direction of arrival (DOA) of an incoming wave, while for the downlink scenario, it enables the automated tracking of mobile targets using global positioning system (GPS) data updated from a real‐time database. Experimental results highlight the responsiveness of the metasurface and demonstrate efficiency in wave focusing and mobile target tracking. [ABSTRACT FROM AUTHOR]

Metasurfaces have shown unparalleled controllability of electromagnetic (EM) waves. However, most of the metasurfaces need external spatial feeding sources, which renders practical implementation quite challenging. Here, a low‐profile programmable metasurface with 0.05λ0 thickness driven by guided waves is proposed to achieve dynamic control of both amplitude and phase simultaneously. The metasurface is fed by a guided wave traveling in a substrate‐integrated waveguide, avoiding external spatial sources and complex power divider networks. By manipulating the state of the p‐i‐n diodes embedded in each meta‐atom, the proposed metasurface enables 1‐bit amplitude switching between radiating and nonradiating states, as well as a 1‐bit phase switching between 0° and 180°. As a proof of concept, two advanced functionalities, namely, low sidelobe‐level beam scanning and Airy beam generation, are experimentally demonstrated with a single platform operating in the far‐ and near‐field respectively. Such complex‐amplitude, programmable, and low‐profile metasurfaces can overcome integration limitations of traditional metasurfaces, and open up new avenues for more accurate and advanced EM wave control within an unprecedented degree of freedom. [ABSTRACT FROM AUTHOR]

A multifunctional transmission line (TL) based on spoof surface plasmon polaritons (SSPPs) is proposed for dynamical switching features among bandpass, band‐stop, and dual‐band phase shifting functionalities. Different operating modes can be controlled in real‐time by changing the states of the Positive Intrinsic Negative (PIN) diodes synchronously. The dispersion properties of the spoof plasmonic unit cell and transmission characteristics of the tunable PIN‐diodes‐loaded spoof plasmonic TL are investigated to highlight the basic theory and method. A prototype of the proposed TL composed of five cascaded unit cells is fabricated and tested for proof‐of‐concept validation. In the ON state of the PIN diodes, the measured −3 dB transmission frequency bandwidth ranges from 3 to 8.1 GHz for the bandpass characteristic. In the OFF state, the measured −10 dB rejection band spans from 3.7 to 5.6 GHz for the bandstop characteristic. When the diodes are switched between ON and OFF states, the measured dual‐band phase shifts vary from 192° to 203° and 248° to 292° in the 3.1–3.4 GHz and 6.1–9.4 GHz frequency bands, respectively. The validated switchable features of the TL pave the way for applications of tunable SSPPs in large‐scale integrated circuits and smart wireless communication systems. [ABSTRACT FROM AUTHOR]

Bottle beams have drawn considerable research interest due to their attractive features and applications in beam trapping. Herein, the flexible generation and modulation of bottle beams utilizing a reconfigurable metamirror over a non‐negligible frequency band ranging from 8.5 to 9.5 GHz is investigated. A varactor diode is embedded into each meta‐atom on the metamirror enabling the achievement of a nearly 2π phase coverage by varying the external bias voltage. Based on the caustic theory, several configurations of bottle beam with distinct radius a are realized as proof‐of‐concept validation. Besides, the flexible modulation of the operating frequency is analyzed by judiciously controlling the bias voltages applied to the metasurface. The experimental results are in good agreement with those obtained from numerical simulations. Such a flexible bottle beam generator shows good potential in the microwave regime for near‐field applications, such as imaging and power transmission. [ABSTRACT FROM AUTHOR]

Beam diffraction management with on‐demand efficiency over compact devices is important in various applications, such as communications, spectroscopy, wireless power transfer, and others. Recently, the in‐depth study of metasurfaces, such as phase gradient metasurfaces (PGMs) or metagratings (MGs) made of discrete elements, has promoted an ultrathin platform to manipulate diffractions. However, most studies only focus on symmetrical diffraction orders or different propagating diffraction orders with equally distributed energy. It is difficult to efficiently excite beams with arbitrary energy distribution by phase‐only metasurfaces due to the complex optimization procedure. Here, to address these challenges, Fourier harmonic component engineering is proposed to allocate the energy between multiple diffraction beams. By introducing phase‐corrected gradient (PCG) on the metasurface platform, lossless transformation from the incidence to far‐field patterns can be obtained. A variety of diffraction situations are considered (symmetric and asymmetric, with equal or arbitrary energy ratio), where the simulated and measured far‐field patterns are in excellent agreement with the theoretical predictions and the achieved diffraction efficiency is up to 98.3%. The proposed method paves the way for multichannel wireless communication applications and can be readily extended to other frequency regions. [ABSTRACT FROM AUTHOR]

Dispersion is one of the key performances of optical systems. As a man‐made device, metasurface is a notable alternative for dispersion manipulation and has been developed vigorously in recent years. However, the currently reported dispersion manipulation principle of meta‐atoms only relies on controlling the propagation phase in the operation frequency band or several working wavelengths. In this paper, the chirality‐assisted phase is introduced as an additional degree of freedom to engineer the dispersion characteristics of the meta‐atom, and the strategy is theoretically demonstrated. The dispersion characteristic of the chiral meta‐atom working in a reflective manner is discussed in detail within the working bandwidth. Then, two hybrid dispersion‐engineered metamirrors (HDEMs) are proposed and constructed to demonstrate versatile dispersion manipulation in the working frequency band, including achromatic focusing for the lower half band and hyper dispersive focusing for the upper half band, and hyper dispersive focusing and abnormal dispersive focusing in the lower and upper half band, respectively. Both full‐wave simulation and measured performances verify the validity and flexibility of the proposed strategy. This work exploits a new degree of freedom for dispersion manipulation, providing a new approach for dispersion‐engineered metasurfaces design. [ABSTRACT FROM AUTHOR]

Metasurfaces play an important role in holographic imaging to flexibly manipulate and control electromagnetic wave fronts. However, metasurface‐based holograms present weaknesses in space utilization and efficiency. In this work, a 2‐bit coding metasurface based on double‐face copper‐cladded patterns on a single‐layer substrate is proposed, where the electromagnetic waves can be independently manipulated in full space. When illuminated by an x‐polarized wave, the co‐polarized reflection and cross‐polarized transmission components of the coding metasurface can be simultaneously manipulated at two distinct frequencies. The co‐polarized reflection efficiency is greater than 0.7 at 15 GHz, and the cross‐polarized transmission efficiency attains to 0.4 at 14 GHz. The proposed coding metasurface hologram can simultaneously reconstruct two different holographic images without changing the polarization state of the incident wave, while avoiding crosstalk between different channels. A coding metasurface sample is fabricated and measured, and the experimental results agree well with the numerically simulated ones. The proposed hologram imaging concept highlights great potential applications in the fields of holographic display and information encryption. [ABSTRACT FROM AUTHOR]

Metasurface‐tuning is presented as a solution to camouflage high‐index and electrically large dielectric parasitic scatterers. By incorporating metasurfaces into the dielectric volume of the scatterer, it is possible to reduce its scattering signature with respect to a given dielectric reference. The metasurface's surface impedance is retrieved analytically from a transmission‐line model and the induced field ratio parameter is introduced to characterize the scattering behavior of the scatterer. The scattering reduction effect achieved by the metasurfaces is validated with microwave near‐field full‐wave simulations and experiments for both normal and off‐normal transverse‐electric and transverse‐magnetic polarized incident wave illuminations. The study highlights the capability of metasurface‐tuning as a polarization‐insensitive and frequency‐adjustable solution to reduce spurious scattering from high‐index dielectric mechanical pieces such as seams in large ground‐based radio‐frequency antenna radomes. [ABSTRACT FROM AUTHOR]

This article aims to analyze the effect of formal and informal financing on the growth of cottage industry firms in village Islampur in Swat one of the districts of Khyber Pakhtunkhwa Pakistan. Primary data were collected through Focused Group Discussions (FGD) and a total of 208 cottage industry firms were surveyed. Robust Regression techniques were employed to analyze the data. The results show that informal financing has a higher impact than formal financing on the growth of the small firms in the study area. There is a tendency for informal borrowing among the small size firms' owners due to the fact that access to formal sources is hindered by a multitude of factors including high interest rate, complex application procedure, and demand for a collateral among others. On the contrary, we find that in case of large size firms, formal financing has a significant influence on firms' growth for cottage industry. Moreover, in co‐financing where the ratio of informal is greater than formal financing has a significant positive effect on the firm's growth. The findings of the study have implications for devising loan policies of the financial institutions to take into account these factors. The financial institutions need to come up with packages that meets the need of the small and medium enterprises. [ABSTRACT FROM AUTHOR]

In recent years, metasurfaces have shown extremely powerful abilities for manipulation of electromagnetic waves. However, the local electromagnetic response of conventional metasurfaces yields an intrinsic performance limitation in terms of efficiency, minimizing their implementation in real‐life applications. The efficiency of reconfigurable metasurfaces further decreases because of the high density of meta‐atoms, reaching 74 meta‐atoms per λ2 area, incorporating lossy tunable elements. To address these problems, strong electromagnetic non‐local features are implemented in a sparse metasurface composed of electronically reconfigurable meta‐atoms. As a proof‐of‐concept demonstration, a dynamic sparse metasurface having as few as 8 meta‐atoms per λ2 area is experimentally realized in the microwave domain to control 2D wavefronts in both near‐field and far‐field regions for focusing and beam‐forming, respectively. The proposed metasurface with its sparsity not only facilitates design and fabrication, but also opens the door to high‐efficiency real‐time reprogrammable functionalities in beam manipulations, wireless power transfer, and imaging holography. [ABSTRACT FROM AUTHOR]

Vortex beam carrying orbital angular momentum (OAM) has been widely explored in optical and microwave regions attributed to its potential characteristics in communication systems. For circular polarization incidence, Pancharatnam–Berry (PB) phase is a direct resource to generate phase gradient along the azimuthal direction required by specific OAM mode. The main drawback of PB phase is that it only affects cross‐polarized fields and keeps the copolarized part unmodulated. Here, a paradigm‐shift perspective of noninterleaved metasurfaces is proposed, which can simultaneously generate separate multiple integer and fractional OAM modes occupying both copolarized and cross‐polarized output channels. The scheme is validated by a series of experimental demonstrations in the microwave regime. By adjusting the polarization states of both input and receiving ends, different integer and fractional OAM modes are demonstrated in the full transmission channels. The results offer a unique recipe to enhance information capacity of metasurfaces and trigger versatile electromagnetic (EM) wave function integrations for advanced compact systems. A variety of applications can be readily expected in spin‐selective optics, spin‐Hall metadevices, and multitask metasurfaces operating in both reflection and transmission modes. [ABSTRACT FROM AUTHOR]

Bessel beams have attracted considerable interest due to their intriguing nondiffraction and self‐healing properties. Such beam can be generated by conventional devices that are more or less subjected to issues of low efficiency, complex manufacturing, and bulky size. Here, an electronically engineered reflective metasurface is designed to generate and flexibly manipulate Bessel‐like beams over a broad frequency range in the microwave regime. Due to the dynamic resonance feature of the metasurface, the phase profile can be easily adjusted for different applications. The nondiffraction feature is illustrated by analyzing several configurations with different nondiffraction area of the Bessel‐like beam, while the self‐healing property is verified by placing a metallic obstacle in the propagation path. The dynamic control of functional frequency and propagation direction of Bessel‐like beams is realized by controlling the external DC bias voltage applied to the metasurface. Experimental validation of the aforementioned features is perfectly consistent with numerical simulations. Owing to the flexible control, broad bandwidth, and high‐efficiency properties, the proposed Bessel‐like beam generator suggests potential applications in microwave domain such as focusing, wireless power transmission, and other complex demands such as beam steering. [ABSTRACT FROM AUTHOR]

Geometric metasurfaces primarily follow the physical mechanism of Pancharatnam–Berry (PB) phases, empowering wavefront control of cross‐polarized reflective/transmissive light components. However, inherently accompanying the cross‐polarized components, the copolarized output components have not been attempted in parallel in existing works. Here, a general method is proposed to construct phase‐modulated metasurfaces for implementing functionalities separately in co‐ and cross‐polarized output fields under circularly polarized (CP) incidence, which is impossible to achieve with solely a geometric phase. By introducing a propagation phase as an additional degree of freedom, the electromagnetic (EM) energy carried by co‐ and cross‐polarized transmitted fields can be fully phase‐modulated with independent wavefronts. Under one CP incidence, a metasurface for separate functionalities with controllable energy repartition is verified by simulations and proof‐of‐principle microwave experiments. A variety of applications can be readily expected in spin‐selective optics, spin‐Hall metasurfaces, and multitasked metasurfaces operating in both reflective and transmissive modes. [ABSTRACT FROM AUTHOR]

Abstract: The design of a beam steerable Fabry‐Perot cavity antenna based on the use of a phase modulated partially reflective surface is reported. The latter partially reflective surface makes use of a 3D‐printed index‐modulated dielectric substrate so as to incorporate the phase modulation. The engineered substrate is combined with uniform inductive and capacitive grids in order to design the phase‐modulated metal‐dielectric‐metal metasurface acting as a partially reflecting surface in the cavity antenna. Thanks to the phase modulation, the radiated beam of the antenna is steered to an off‐normal direction. [ABSTRACT FROM AUTHOR]

Abstract: Huygens metasurface, an implementation of Huygens principle with metasurface, shows great potential in the manipulation of electromagnetic wave by elaborately designed subwavelength scale metaatoms with full phase coverage and high transmission amplitude. Here, a multiphase hologram with Huygens metasurface is demonstrated in microwave regime, and a novel algorithm method is proposed to modulate energy distribution among focal points. The proof‐of‐concept experiments show superior microwave holographic images with 89% transmittance efficiency, 59% imaging efficiency, and 1.65 root‐mean‐square error of focal intensity ratio. The proposed Huygens metasurface hologram expands the route to microwave applications for holographic technologies, including computer‐generated holograms, antennas, security, and data storage. [ABSTRACT FROM AUTHOR]

ABSTRACT Electrically small antennas are characterized by large reactance and small radiation resistance. Conventional passive impedance matching is limited by the gain-bandwidth theory which predicts narrow bandwidth characteristics. Furthermore, Wheeler-Chu limitation forecast a very narrow band relative to the size of the small antenna and a high quality factor impedance. Non-Foster circuits can overcome these restrictions and achieve broadband performances by using negative circuit elements. These circuits based on negative impedance converters allow cancelling the antenna's reactance and transforming its frequency-dependent radiation resistance to a constant value over a broad frequency band. In this article a miniaturized antenna using a non-Foster impedance matching circuit is presented. This active external matching circuit allows shifting the resonance frequency of an antenna initially designed at 1100-125 MHz while keeping its original dimension of 7 cm and also extending its relative bandwidth from 10% to 104%. An analytical study is used to design the active impedance matching circuit and to implement it. A prototype is fabricated and experimentally tested. Measurements show that non-Foster circuits can be successfully used to overcome fundamental limitations of electrically small passive antennas in terms of bandwidth even if this gain remains weak. © 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:986-991, 2017 [ABSTRACT FROM AUTHOR]

ABSTRACT We present the design and characterization of a triband low-profile antenna based on the use of artificial magnetic conductor (AMC) surfaces. The engineered double AMC substrate allows reducing the separation between the radiating element and the ground plane. The radiating element is a wide band microstrip elliptical dipole and is associated to the double AMC substrate to cover the (790-860 MHz) GSM, the (1710-1880 MHz) DCS and the (1920-2170 MHz) UMTS telecommunication frequency bands. The whole antenna system presents a reduced level of backward radiation and is intended to be inserted in the back of a jacket. Presented experimental performances agree qualitatively with the numerical ones. The fabricated prototype demonstrates the feasibility of a triband antenna with a substantial reduction of the thickness through the use of AMC surfaces. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:737-741, 2015 [ABSTRACT FROM AUTHOR]

This article deals with the modeling, practical implementation, and characterization of an azimuthal directive antenna around 10 GHz. The design of the antenna is based on transformation optics concept by transforming the radiation of a plane source into an azimuthal radiation. This coordinate transformation procedure is achieved by modifying the electromagnetic properties of the space around the plane source. Metamaterials presenting electric and magnetic resonances are used to produce the effective material parameters necessary for the transformation. S11 parameter and direct far-field measurements are performed on a fabricated prototype to experimentally demonstrate the narrow beam profile and the beam deflection. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2536-2540, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27122 [ABSTRACT FROM AUTHOR]

Various easy made planar electric meta-atoms are presented as alternative designs to the conventional electric-LC resonator for achieving negative permittivity. Transforming the LC topology of the resonator helps to facilitate transposition of geometrical parameters for the optical regime and also to improve the metamaterial homogeneity. Our approach aims to simplify the resonator design and achieve a lower resonance frequency without being limited by fabrication techniques. A parametric sweep is performed to evaluate the influence of the different geometrical parameters on the resonance frequencies of the meta-atoms. The electromagnetic behavior is investigated through both simulations and experiments in the microwave regime. Our results show that the developed meta-atoms exhibit a purely strong electric response to normally incident radiation and can be used very effectively in producing materials with negative permittivity. For instance, a Z-shaped structure is shown to present a higher effective medium ratio. The proposed meta-atoms can find various applications in planar high-frequency passive circuits due the simplicity of fabrication. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2287-2295, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27050 [ABSTRACT FROM AUTHOR]

We report the theoretical and experimental investigations of a frequency selective surface (FSS) operating in the microwave regime. Numerical simulations using finite element method and reflection-transmission microwave measurements have been performed and a good quantitative agreement has been observed. Angular robustness is also demonstrated which makes it promising in telecommunication applications. The potential application of the FSS as a partially reflecting surface (PRS) reflector Fabry-Pérot (FP) cavity is proposed and evaluated. Measured return losses and radiations patterns done in an anechoic chamber agree very well with the simulated ones. A high directivity of 20 dBi is observed for the FP cavity. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1327-1332, 2012 [ABSTRACT FROM AUTHOR]

The analysis and design of subwavelength metamaterial-based resonant cavity antennas are presented. The antennas under investigation are formed by embedding a feeding source in a Fabry-Perot (FP) cavity composed of a Perfect Electrical Conductor (PEC) surface and a metasurface reflector. The embedded feeding source used in this study is a 2 × 2 microstrip patch array. The influence of the interelement spacing of the patch array feed on the radiation patterns is investigated and reported. The effect of the cavity thickness is also considered. Finally, the design of highly directive subwavelength cavity antennas is presented. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1883–1888, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24485 [ABSTRACT FROM AUTHOR]

The design of a highly directive metamaterial-based cavity antenna for a 2.46 GHz application is reported. The proposed structure is evolved from a Fabry-Perot (FP) cavity with a patch antenna acting as the feed and four metallic walls on the lateral sides. One of the two reflectors is the antenna's ground plane and the other is made from a bilayered metamaterial-based partially reflecting surface (PRS) constituted by a capacitive and an inductive grid. Calculations together with experimental results obtained from the fabricated prototype are presented. The results show a high directivity and low levels for the secondary lobes compared to a 2 × 2 patch antenna array. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1393–1396, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24391 [ABSTRACT FROM AUTHOR]

A new type of backward coupling is observed between two parallel microstrip transmission lines on a slot Split-Ring Resonators (SSRRs) defected ground plane, even though the lines are separated with a large gap spacing. This enhanced coupling is achieved at a fixed desired frequency on only one channel and the coupling at other frequencies is done on another channel. The design of novel types couplers with relaxed gap spacing and possible applications for diplexers and transponders can be considered for the future. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 138–141, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21287 [ABSTRACT FROM AUTHOR]

The effect of dielectric split-ring resonators (SRRs) on the transmitted signal of a microstrip line is evaluated. The ground plane of the microstrip line is modified with the inclusion of the SRRs. This technique leads to the design of a band-stop phenomenon. The frequency response measured in the X-band constitutes the behavior of a band-stop filter. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 445–448, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20662 [ABSTRACT FROM AUTHOR]