A programmable metasurface with dynamic polarization, scattering and focusing control

Graphene-Based Tunable Polarization Conversion Metasurface for Array Antenna Radar Cross-Section Reduction

A graphene-based tunable polarization conversion metasurface (PCM) was designed and analyzed for the purpose of reducing the radar cross-section (RCS) of array antennas. The metasurface comprises periodic shuttle-shaped metal patches, square-patterned graphene, and inclined grating-patterned graphene. By adjusting the Fermi energy levels of the upper (μ1) and lower (μ2) graphene layers, different states were achieved. In State 1, with μ1 = 0 eV and μ2 = 0.5 eV, the polarization conversion ratio (PCR) exceeded 0.9 in the bandwidths of 1.65-2.19 THz and 2.29-2.45 THz. In State 2, with μ1 = μ2 = 0.5 eV, the PCR was greater than 0.9 in the 1.23-1.85 THz and 2.24-2.60 THz bands. In State 3, with μ1 = μ2 = 1 eV, the PCR exceeded 0.9 in the 2.56-2.75 THz and 3.73-4.05 THz bands. By integrating the PCM with the array antenna, tunable RCS reduction was obtained without affecting the basic radiation functionality of the antenna. In State 1, RCS reduction was greater than 10 dB in the 1.60-2.43 THz and 3.63-3.72 THz frequency ranges. In State 2, the RCS reduction exceeded 10 dB in the 2.07-2.53 THz, 2.78-2.98 THz, and 3.70-3.81 THz bands. In State 3, RCS reduction was greater than 10 dB in the 1.32-1.43 THz, 2.51-2.76 THz, and 3.76-4.13 THz frequency ranges. This polarization conversion metasurface shows significant potential for applications in switchable and tunable antenna RCS reduction.

A filtering reconfigurable intelligent surface for interference-free wireless communications

The powerful capability of reconfigurable intelligent surfaces (RISs) in tailoring electromagnetic waves and fields has put them under the spotlight in wireless communications. However, the current designs are criticized due to their poor frequency selectivity, which hinders their applications in real-world scenarios where the spectrum is becoming increasingly congested. Here we propose a filtering RIS to feature sharp frequency-selecting and 2-bit phase-shifting properties. It permits the signals in a narrow bandwidth to transmit but rejects the out-of-band ones; meanwhile, the phase of the transmitted signals can be digitally controlled, enabling flexible manipulations of signal propagations. A prototype is designed, fabricated, and measured, and its high quality factor and phase-shifting characteristics are validated by scattering parameters and beam-steering phenomena. Further, we conduct a wireless communication experiment to illustrate the intriguing functions of the RIS. The filtering behavior enables the RIS to perform wireless signal manipulations with anti-interference ability, thus showing big potential to advance the development of next-generation wireless communications.

Microwave Speech Recognizer Empowered by a Programmable Metasurface

Speech recognition becomes increasingly important in the modern society, especially for human-machine interactions, but its deployment is still severely thwarted by the struggle of machines to recognize voiced commands in challenging real-life settings: oftentimes, ambient noise drowns the acoustic sound signals, and walls, face masks or other obstacles hide the mouth motion from optical sensors. To address these formidable challenges, an experimental prototype of a microwave speech recognizer empowered by programmable metasurface is presented here that can remotely recognize human voice commands and speaker identities even in noisy environments and if the speaker's mouth is hidden behind a wall or face mask. The programmable metasurface is the pivotal hardware ingredient of the system because its large aperture and huge number of degrees of freedom allows the system to perform a complex sequence of sensing tasks, orchestrated by artificial-intelligence tools. Relying solely on microwave data, the system avoids visual privacy infringements. The developed microwave speech recognizer can enable privacy-respecting voice-commanded human-machine interactions is experimentally demonstrated in many important but to-date inaccessible application scenarios. The presented strategy will unlock new possibilities and have expectations for future smart homes, ambient-assisted health monitoring, as well as intelligent surveillance and security.

Two-dimensionally high AR performance beam scanning utilizing randomly-rotated single-PIN-diode elements for circularly-polarized programmable metasurface

In this paper, we introduce a novel technique that utilizes randomly rotated elements (RREs) for the cross-polarization and axial ratio (AR) control of a circularly polarized programmable metasurface (CPPMS). We evaluate the CPPMS performance by comparing RREs layout with uniform elements (UEs) layout, and analyze far-field radiation parameters for 50 groups of CPPMS with different RREs layouts. Simulation results demonstrate consistent and improved performance across various RREs layouts, showcasing reduced cross-polarization and enhanced AR beamwidth. To validate these findings, we design a 1-bit CPPMS in Ku-band comprising 20 × 20 elements with the optimal RREs layout, and conduct measurements in an anechoic chamber. The CPPMS prototype achieves high gain (22.34 dBi), low cross-polarization (-20.5 dB), and a narrow 3 dB AR beamwidth (8.93°). Notably, it offers wide-angle beam scanning capabilities of up to ±60°. The gain bandwidth at -3 dB ranges from 14.54 to 16.65 GHz, with a relative bandwidth of 7.3%, while the 3 dB AR bandwidth extends from 14.24 to 16.07 GHz. Consequently, the proposed 1-bit CPPMS exhibits high-performance two-dimensional AR beam scanning, presenting promising applications in satellite communications.

A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models

Terahertz (THz) band will play an important role in enabling sixth generation (6G) envisioned applications. Compared with lower frequency signals, THz waves are severely attenuated by the atmosphere temperature, pressure, and humidity. Thus, designing a THz communication system must take into account how to circumvent or diminish those issues to achieve a sufficient quality of service. Different solutions are being analyzed: intelligent communication environments, ubiquitous artificial intelligence, extensive network automation, and dynamic spectrum access, among others. This survey focuses on the benefits of integrating intelligent surfaces (ISs) and THz communication systems by providing an overview of IS in wireless communications with the scanning of the recent developments, a description of the architecture, and an explanation of the operation. The survey also covers THz channel models, differentiating them based on deterministic and statistical channel modeling. The IS-aided THz channels are elucidated at the end of the survey. Finally, discussions and research directions are given to help enrich the IS field of research and guide the reader through open issues.

Programmable multifunctional metasurface for polarization, phase, and amplitude manipulation

Metasurfaces have shown extraordinary capability in individually manipulating various electromagnetic (EM) properties, including polarization, phase, and amplitude. However, it is still a challenge to manipulate these EM properties in one metasurface simultaneously. In this paper, a programmable multifunctional metasurface (PMFMS) is demonstrated with polarization, phase, and amplitude manipulation abilities. By controlling tunable coding states and changing the direction of incident waves, the PMFMS can operate as a transmission cross-polarization converter, spatial wave manipulator, and low-RCS radome. Besides, the PMFMS possesses an ultra-wideband property, which can operate from 6.5 to 10.2 GHz with 44.3% relative bandwidth. More importantly, multiple functionalities can also be achieved in reflection operating mode by reassembling the PMFMS. As a proof of concept, the PMFMS is fabricated and experimentally verified. Measured results are in good agreement with simulated results. Benefiting from multifunctional EM manipulations in an ultra-wideband, such a design can be applied in wireless communication systems, radar detection, and EM stealth platform.

Toward Sub-Terahertz: Space-Time Coding Metasurface Transmitter for Wideband Wireless Communications

A space-time coding metasurface (STCM) operating in the sub-terahertz band to construct new-architecture wireless communication systems is proposed. Specifically, a programmable STCM is designed with varactor-diode-tuned metasurface elements, enabling precise regulation of harmonic amplitudes and phases by adjusting the time delay and duty cycle of square-wave modulation signal loaded on the varactor diodes. Independent electromagnetic (EM) regulations in the space and time domains are achieved by STCM to realize flexible beam manipulations and information modulations. Based on these features, a sub-terahertz wireless communication link is constructed by employing STCM as a transmitter. Experimental results demonstrate that the STCM supports multiple modulation schemes including frequency-shift keying, phase-shift keying, and quadrature amplitude modulations in a wide frequency band. It is also shown that the STCM is capable of realizing wide-angle beam scanning in the range of ±45o , which offers an opportunity for user tracking during the communication. Thus, the STCM transmitter with high device density and low power consumption can provide low-complexity, low-cost, low-power, and low-heat solutions for building the next-generation wireless communication systems in the sub-terahertz frequency and even terahertz band.

High-performance cost efficient simultaneous wireless information and power transfers deploying jointly modulated amplifying programmable metasurface

Programmable metasurfaces present significant capabilities in manipulating electromagnetic waves, making them a promising candidate for simultaneous wireless information and power transfer (SWIPT), which has the potential to enable sustainable wireless communication in complex electromagnetic environments. However, challenges remain in terms of maximum power transmission distance and stable phase manipulation with high-power scattered waves. Additionally, waveform limitations restrict average scattered power and rectifier conversion efficiency, affecting data transmission rates and energy transmission distance. Here we show an amplifying programmable metasurface (APM) and a joint modulation method to address these challenges. The APM mitigates the peak-to-average power ratio and improves maximum power, phase response stability, average output power, and rectifier conversion efficiency. Through experimental validation, we demonstrate the feasibility of the SWIPT system, showcasing simultaneous LED array powering and movie video transmission. This innovative SWIPT system holds promise for diverse applications, including 6 G wireless communications, IoT, implanted devices, and cognitive radio networks.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Simultaneous manipulation of spatial fundamental and harmonic electromagnetic waves by microwave nonlinear metasurfaces

In the area of manipulating the spatial electromagnetic (EM) waves fields, the metasurfaces have become much more attractive and powerful in recent years. Here, we propose a design to realize the simultaneous control of spatial fundamental and harmonic EM waves applying nonlinear metasurfaces in microwave band. The proposed meta-atom composed of three topological layers which are transmitting antenna, nonlinear wave guiding and receiving antenna respectively. And the critical factor of generating the harmonic is the nonlinear chip which is integrated into the middle layer. The microstrip power divider and phase shifter in each meta-atom are preciously tailored to actualize the spatial control of the fundamental and harmonic transmission beams in the far field. One prototype of the nonlinear metasurfaces is fabricated and corresponding radiation patterns of fundamental and harmonic modes are observed very well in the experience that can verify the validity of our proposed method.

Electronically reconfigurable transmitarray for fully independent beam manipulation in two divided frequency bands

Electronically reconfigurable transmitarray (ERTA) combines the advantages of optic theory and coding metasurface mechanism with the characteristic of low-loss spatial feed and real-time beam manipulation. Designing a dual-band ERTA is challenging due to multiple factors, including large mutual coupling generated by dual-band operation and separate phase control in each band. In this paper, a dual-band ERTA is demonstrated with the capability of fully independent beam manipulation in two divided bands. This dual-band ERTA is constructed by two kinds of orthogonally polarized reconfigurable elements which share the aperture in an interleaved way. The low coupling is achieved by utilizing polarization isolation and a backed cavity connected to the ground. To separately control the 1-bit phase in each band, a hierarchical bias method is elaborately presented. As proof of concept, a dual-band ERTA prototype composed of 15 × 15 upper-band elements and 16 × 16 lower-band elements is designed, fabricated, and measured. Experimental results verify that fully independent beam manipulation with orthogonal polarization is implemented in 8.2-8.8 GHz and 11.1-11.4 GHz. The proposed dual-band ERTA may be a suitable candidate for space-based synthetic aperture radar imaging.

Recent Advances in Reconfigurable Metasurfaces: Principle and Applications

Metasurfaces have shown their great capability to manipulate electromagnetic waves. As a new concept, reconfigurable metasurfaces attract researchers' attention. There are many kinds of reconfigurable components, devices and materials that can be loaded on metasurfaces. When cooperating with reconfigurable structures, dynamic control of the responses of metasurfaces are realized under external excitations, offering new opportunities to manipulate electromagnetic waves dynamically. This review introduces some common methods to design reconfigurable metasurfaces classified by the techniques they use, such as special materials, semiconductor components and mechanical devices. Specifically, this review provides a comparison among all the methods mentioned and discusses their pros and cons. Finally, based on the unsolved problems in the designs and applications, the challenges and possible developments in the future are discussed.

Low-profile and wide-band RCS-reduction antenna array based on the metasurface polarization converter

Antenna elements with a low profile and high front-to-back (FB) ratio mean that no additional reflective cavity is required when forming the array, which greatly helps to simplify and lighten the entire array system. In this paper, we enhance the FB ratio of the antenna to 35 dB while maintaining an ultra-low profile of 0.05 λ₀ by attaching the broadband polarization conversion metasurfaces (PCMs) as the parasitic patches to the surface of the radiating patch. Meanwhile, the array formed by the proposed antenna is arranged in a checkerboard form, which makes it have a lower radar cross section (RCS) in the X- and Ku- bands. Even with PCMs loaded, the antenna element maintains a small size of 0.58 λ₀ × 0.58 λ₀, which ensures the proposed array can achieve the ± 45° beam scanning, making it suitable for the phased array. For verification, we propose a low-sidelobe array composed of the proposed antenna elements, which exhibits a low profile, high FB ratio, and broadband RCS reduction through simulation and measurement.

Space-frequency-polarization-division multiplexed wireless communication system using anisotropic space-time-coding digital metasurface

In the past few years, wireless communications based on digital coding metasurfaces have gained research interest owing to their simplified architectures and low cost. However, in most of the metasurface-based wireless systems, a single-polarization scenario is used, limiting the channel capacities. To solve the problem, multiplexing methods have been adopted, but the system complexity is inevitably increased. Here, a space-frequency-polarization-division multiplexed wireless communication system is proposed using an anisotropic space-time-coding digital metasurface. By separately designing time-varying control voltage sequences for differently oriented varactor diodes integrated on the metasurface, we achieve frequency-polarization-division multiplexed modulations. By further introducing different time-delay gradients to the control voltage sequences in two polarization directions, we successfully obtain space-frequency-polarization-division multiplexed modulations to realize a wireless communication system with a new architecture. The new communication system is designed with compact dual-polarized meta-elements, and can improve channel capacity and space utilization. Experimental results demonstrate the high-performance and real-time transmission capability of the proposed communication system, confirming its potential application in multiple-user collaborative wireless communications.

Recent Progress in Reconfigurable and Intelligent Metasurfaces: A Comprehensive Review of Tuning Mechanisms, Hardware Designs, and Applications

Intelligent metasurfaces have gained significant importance in recent years due to their ability to dynamically manipulate electromagnetic (EM) waves. Their multifunctional characteristics, realized by incorporating active elements into the metasurface designs, have huge potential in numerous novel devices and exciting applications. In this article, recent progress in the field of intelligent metasurfaces are reviewed, focusing particularly on tuning mechanisms, hardware designs, and applications. Reconfigurable and programmable metasurfaces, classified as space gradient, time modulated, and space-time modulated metasurfaces, are discussed. Then, reconfigurable intelligent surfaces (RISs) that can alter their wireless environments, and are considered as a promising technology for sixth-generation communication networks, are explored. Next, the recent progress made in simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) that can achieve full-space EM wave control are summarized. Finally, the perspective on the challenges and future directions of intelligent metasurfaces are presented.

Design of Low-RCS Broadband High-Gain Antennas Based on Transmission Array Metasurface

In this paper, a low-RCS broadband high-gain antenna based on metasurface transmission array is proposed, consisting of two parts: a metasurface transmission array and a feed antenna. When designing the metasurface transmission array, the phase compensation method is used to achieve the beam convergence effect of metasurface in the broadband. By designing the elements and using the checkerboard arrangement, the RCS of the incident wave with fixed polarization can be reduced more than 10 dB at X band or Ku band. The feed antenna is designed as a microstrip magnetic and electric dipole antenna, which has the characteristics of small structure and wide impedance bandwidth. An antenna that can reduce RCS by more than 10 dB in Ku band is simulated and measured. The measurement and simulation results show that the -10 dB operating bandwidth of the high-gain antenna designed in this paper is 6.7~13.5 GHz, and the relative bandwidth is 67%. The designed metasurface can effectively improve the gain of the antenna in the operating frequency band. In this way, the design of high-gain antenna is realized, and the antenna has an obvious RCS reduction effect on the vertically incident y-polarized wave in the whole Ku band. The method to design an antenna in this paper realizes the regulation of radiation and scattering at the same time, which has important reference significance for expanding the function of transmission array antennae and has great application value.

Intelligent metasurfaces: digitalized, programmable, and intelligent platforms

Distinguished from conventional ones, intelligent metasurfaces are endowed with three important characteristics: digitalization, programmability, and intelligence, which can be further integrated with detection, artificial neural networks, and feedback systems into a smart platform. Metasurface-based smart systems are expected to play an important role in wireless communications, advanced sensing technologies and artificial intelligence.

Airy Beam-enabled Binary Acoustic Metasurfaces for Underwater Ultrasound Beam Manipulation

Airy beams are peculiar beams that are non-diffracting, self-accelerating, and self-healing, and they have offered great opportunities for ultrasound beam manipulation. However, one critical barrier that limits the broad applications of Airy beams in ultrasound is the lack of simply built device to generate Airy beams in water. This work presents a family of Airy beam-enabled binary acoustic metasurfaces (AB-BAMs) to generate Airy beams for underwater ultrasound beam manipulation. AB-BAMs are designed and fabricated by 3D printing with two coding bits: a polylactic acid (which is the commonly used 3D printing material) unit acting as a bit "1" and a water unit acting as a bit "0". The distribution of the binary units on the metasurface is determined by the pattern of Airy beam. To showcase the wavefront engineering capability of the AB-BAMs, several examples of AB-BAMs are designed, 3D printed, and coupled with a planar single-element ultrasound transducer for experimental validation. We demonstrate the capability of AB-BAMs in flexibly tuning the focal region size and beam focusing in 3D space by changing the design of the AB-BAMs. The focal depth of AB-BAMs can be continuous and electronical tuned by adjusting the operating frequency of the planar transducer without replacing the AB-BAMs. The superimposing method is leveraged to enable the generation of complex acoustic fields, e.g., multi-foci and letter patterns (e.g., "W" and "U"). The more complex focal patterns are shown to be also continuously steerable by simply adjusting the operating frequency. Furthermore, the proposed 3D-printed AB-BAMs are simple to design, easy to fabricate, and low-cost to produce with the capabilities to achieve tunable focal size, flexible 3D beam focusing, arbitrary multipoint focusing, and continuous steerability, which creates unprecedented potential for ultrasound beam manipulation.

A Novel Circularly Polarized Folded Transmitarray Antenna with Integrated Radiation and Scattering Performance

In this paper, a novel design method of circularly polarized folded transmitarray antenna (CPFTA) is presented by applying sequential rotation technology. Compared with the general design method, the novel design method can reduce the design difficulty and improve the axis ratio (AR) bandwidth significantly without adding any additional structure. To verify the proposed method, both a general CPFTA (GCPFTA) and a novel CPFTA (NCPFTA) are designed, fabricated, measured and compared. Good agreements between simulated and measured results are obtained. Thanks to the creative design, the integrated radiation and scattering control of the CPFTA is realized for the first time. The proposed NCPFTA has the advantages of broadband, high gain, planar structure, low profile, convenience in installation and low radar cross section (RCS), which has potential application in mobile satellite communication.

Dynamic radiation steering with transmission-type coding metasurface

In this paper, we integrate continuous transmission amplitude modulation and phase coding into a metasurface, and dynamic transmitted electromagnetic field manipulation is realized in microwave regime. The active metasurface is obtained by placing a PIN diodes loaded amplitude modulation metasurface on a binary phase coding metasurface whose coding matrix is optimized by genetic algorithm. Changing the bias voltage applied on the diodes, the transmission amplitude of the phase coding units covered by amplitude modulation units can be tuned continuously while other coding units are not affected, leading to the fact that the transmitted field pattern of the metasurface varies from strongly directional transmission to diffusion-like radiation. By this means, two degrees of freedom of dynamic amplitude modulation and predesigned phase coding are achieved to control the electromagnetic waves. Additionally, a bias network is designed to ensure the polarization-stability of the metasurface. The proposed concept is predicted by analytical model, and verified by numerical simulations and experiment. This design with low profile and diverse functionalities can yield potential applications ranging from radio frequency energy harvesting to wireless communication systems.

Steer by Image Technology for Intelligent Reflecting Surface Based on Reconfigurable Metasurface with Photodiodes as Tunable Elements

Lately, metasurface has become an essential and promising component in implementing Intelligent Reflecting Surface (IRS) for 5G and 6G. A novel method that simplifies the ability to reconfigure the metasurface is presented in this paper. The suggested technology uses a PIN photodiode as a tuning element. The desired image is projected on the metasurface’s backside, where the PIN photodiodes are placed and reconfigures the metasurface. The projected image’s color and intensity pattern influence the PIN photodiode’s junction capacitance, which leads to local reflection phase control. This enables the required pattern reflection phase distribution to manipulate the reflection beam, for example, 2D beam steering or focusing, and any other beam forming combination, instead of wiring many digital-to-analog converters (DACs) or FPGA outputs, which bias the standard tuning element such as PIN diode or varactor using a complex RF circuit. Using a PIN photodiode as a tunable element instead of a varactor diode, PIN diode, Liquid Crystal and MEMS allows the changing of the internal junction capacitance without direct contact and thus continuously controlling the reflection phase. In addition, an open circuit work mode with negligible energy consumption can be obtained. This technology can be used to implement metasurface based on discrete or continuous phases and is called Steer by Image (SBI). A full description of the SBI technology using PIN photodiode is presented in this paper.

Intelligent Reflecting Surface Assisted Localization: Opportunities and Challenges

Future generation communication systems are aiming to provide a tremendous high data rate with low-latency high reliable and three-dimensional coverage. To achieve such a challenging goal, it is required to have very precise location information related to the mobile terminal. The advancement of signal processing techniques and communication technologies enables the path for improving localization performance. Recently, intelligent reflecting surface (IRS) has been widely considered as the key element for the future generation of wireless communication. Over the past few years, the performance of IRS-assisted networks is extensively investigated from the point of view of communication purposes and its improvement. However, by virtue of its potential, IRS finds its application for wireless localization. In this paper, we discuss and summarize the works that have already been carried out targeting localization performance improvement. In addition, we figured out the associated challenges and the opportunities to scale up the localization accuracy. Particularly in this paper, the authors have discussed the challenging issues such as channel modeling, channel estimation, system architecture, hardware impairment, IRS deployment strategies, phase optimization, mobility management, and near-field environments. Although these challenges are associated with opportunities to make the IRS-assisted system more effective and efficient.

The first family of application-specific integrated circuits for programmable and reconfigurable metasurfaces

Reconfigurable metasurfaces are man-made surfaces, which consist of sub-wavelength periodic elements-meta-atoms-that can be reconfigured to manipulate incoming electromagnetic waves. However, reconfigurable metasurfaces developed to-date, have limitations in terms of loading impedance range, reconfiguration delay and power consumption. Also, these systems are costly and they require bulky electronics and complex control circuits, which makes them unattractive for commercial use. Here, we report the first family of CMOS application-specific integrated circuits that enable microsecond and microwatt reconfiguration of complex impedances at microwave frequencies. Our approach utilizes asynchronous digital control circuitry with chip-to-chip communication capabilities, allowing simple and fast reconfiguration via digital devices and user-friendly software. Our solution is low-cost and can cover arbitrary board-to-board metasurfaces, with different sizes and shapes.

Research on full-polarization electromagnetic holographic imaging based on quasi-symmetrical structure reconfigurable metasurfaces

In the paper, a quasi-symmetrical structure reconfigurable metasurfaces (QSRMS) is proposed to generate the full-polarization electromagnetic (EM) holographic imaging. A combination of metasurfaces and varactor that involves the position and the gap of loading varactor is explored to achieve low-loss characteristics. The loading of the capacitor allows the element of reconfigurable metasurfaces (RMS) to present quasi-central symmetry, thus reducing the coupling between co- and cross-polarization. Therefore, the phase shift of 310° and the amplitude loss of 1.3 dB in the two orthogonal directions are acquired at 5.2 GHz. And the 3dB-loss bandwidth reaches 15.67%. Based on the dual-polarization QSRMS, the amplitude and phase modulation (APM) of the EM field are implemented simultaneously using L-BFGS-B algorithm. The implementation process of holographic imaging shows that all polarization state of the Poincaré sphere can be realized by designing the phase distribution of the QSRMS. Furthermore, the multi-polarization multiplexing holographic imaging is also investigated in this research, indicating that the polarization carrying capacity (PCC) can be enhanced by increasing the aperture of the metasurfaces. The results of simulation and experiment reveal that there will be a broad application prospect in next-generation large-scale, multi-channel EM intellisense systems.

Design and Application of Intelligent Reflecting Surface (IRS) for Beyond 5G Wireless Networks: A Review

The existing sub-6 GHz band is insufficient to support the bandwidth requirement of emerging data-rate-hungry applications and Internet of Things devices, requiring ultrareliable low latency communication (URLLC), thus making the migration to millimeter-wave (mmWave) bands inevitable. A notable disadvantage of a mmWave band is the significant losses suffered at higher frequencies that may not be overcome by novel optimization algorithms at the transmitter and receiver and thus result in a performance degradation. To address this, Intelligent Reflecting Surface (IRS) is a new technology capable of transforming the wireless channel from a highly probabilistic to a highly deterministic channel and as a result, overcome the significant losses experienced in the mmWave band. This paper aims to survey the design and applications of an IRS, a 2-dimensional (2D) passive metasurface with the ability to control the wireless propagation channel and thus achieve better spectral efficiency (SE) and energy efficiency (EE) to aid the fifth and beyond generation to deliver the required data rate to support current and emerging technologies. It is imperative that the future wireless technology evolves toward an intelligent software paradigm, and the IRS is expected to be a key enabler in achieving this task. This work provides a detailed survey of the IRS technology, limitations in the current research, and the related research opportunities and possible solutions.

Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links

The forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.

Reconfigurable terahertz metasurfaces coherently controlled by wavelength-scale-structured light

Structuring light-matter interaction at a deeply subwavelength scale is fundamental to optical metamaterials and metasurfaces. Conventionally, the operation of a metasurface is determined by the collective electric polarization response of its lithographically defined structures. The inseparability of electric polarization and current density provides the opportunity to construct metasurfaces from current elements instead of nanostructures. Here, we realize metasurfaces using structured light rather than structured materials. Using coherent control, we transfer structure from light to transient currents in a semiconductor, which act as a source for terahertz radiation. A spatial light modulator is used to control the spatial structure of the currents and the resulting terahertz radiation with a resolution of 5.6 ± 0.8  μm , or approximately λ / 54 at a frequency of 1 THz. The independence of the currents from any predefined structures and the maturity of spatial light modulator technology enable this metasurface to be reconfigured with unprecedented flexibility.

Reconfigurable Intelligent Surfaces for 5G and beyond Wireless Communications: A Comprehensive Survey

With possible new use cases and demanding requirements of future 5th generation (5G) and beyond cellular networks, the future of mobile communications sounds promising. However, the propagation medium has been considered a randomly acting agent between the transmitter and the receiver. With the advent of the digital age of wireless communications, the received signal quality is degrading due to the uncontrollable interactions of the transmitted radio waves with the surrounding artifacts. This paper presents a comprehensive literature review on reconfigurable intelligent surfaces (RISs) and assisted application areas. With the RIS, the network operators can control radio waves’ scattering, reflection, and refraction characteristics by resolving the harmful properties of environmental wireless propagation. Further, the RIS can effectively control the wavefront, such as amplitude, phase, frequency, and even polarization, without requiring complex encoding, decoding, or radio wave processing techniques. Motivated by technological advances, the metasurfaces, reflectarrays, phase shift, and liquid crystals are potential candidates for implementing RIS. Thus, they can be considered the front runner for realizing the 5G and beyond network. Furthermore, the current research activities in the evolving field of wireless networks operated by RIS are reviewed and discussed thoroughly. Finally, to fully explore the potential of RISs in wireless networks, the fundamental research issues to be addressed have been discussed.

Genetic Algorithm-Driven Surface-Enhanced Raman Spectroscopy Substrate Optimization

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive and molecule-specific detection technique that uses surface plasmon resonances to enhance Raman scattering from analytes. In SERS system design, the substrates must have minimal or no background at the incident laser wavelength and large Raman signal enhancement via plasmonic confinement and grating modes over large areas (i.e., squared millimeters). These requirements impose many competing design constraints that make exhaustive parametric computational optimization of SERS substrates prohibitively time consuming. Here, we demonstrate a genetic-algorithm (GA)-based optimization method for SERS substrates to achieve strong electric field localization over wide areas for reconfigurable and programmable photonic SERS sensors. We analyzed the GA parameters and tuned them for SERS substrate optimization in detail. We experimentally validated the model results by fabricating the predicted nanostructures using electron beam lithography. The experimental Raman spectrum signal enhancements of the optimized SERS substrates validated the model predictions and enabled the generation of a detailed Raman profile of methylene blue fluorescence dye. The GA and its optimization shown here could pave the way for photonic chips and components with arbitrary design constraints, wavelength bands, and performance targets.

Intelligent Reflective Surfaces for Wireless Networks: An Overview of Applications, Approached Issues, and Open Problems

An intelligent reflective surface (IRS) is a novel and revolutionizing communication technology destined to enable the control of the radio environment. An IRS is a real-time controllable reflectarray with a massive number of low-cost passive elements which introduce a phase shift to the incoming signals from the sources before the propagation towards the destination. This technology introduces the notion of a smart propagation environment with the aim of improving the system performance. In this paper, we provide a comprehensive literature overview on IRS technology, including its basic concepts and reconfiguration, as well as its design aspects and applications for wireless communication systems. We also study the performance metrics and the setups considered in recent publications related to IRS and provide suggestions of future research lines based on still unexplored use cases in the state-of-the-art.

Transparent dynamic metasurface for a visually unaffected reconfigurable intelligent surface: controlling transmission/reflection and making a window into an RF lens

Reconfigurable intelligent surfaces (RISs) that dynamically manipulate scattered waves have attracted much attention regarding accommodating coverage holes in wireless communication systems using radio wave frequencies higher than millimeter waves. RISs generally actualized through metasurface technologies must be visually unaffected so that they can be installed in various locations such as existing walls and glass windows in environments where propagation should be controlled. We propose a novel method that dynamically controls scattering characteristics of metasurfaces while achieving a large area and high optical transparency. For transparency in the visible light range, we use transparent glass as a substrate and meshed metal patterns. Furthermore, by stacking a metasurface substrate onto another transparent substrate and controlling the interlayer distance, we achieve dynamic control of the scattered waves over a large area in the 28-GHz band. Fabricated prototypes successfully operate when switching transmission and reflection modes, exhibiting extremely low loss of less than -1 dB. In metasurface lenses that can be attached to glass windows in outdoor-to-indoor scenarios, the lens gain of 25.4 dB is achieved for a static lens, and dynamic switching operation between single focus and dual focus is also successfully verified for a dynamic lens.

Reconfigurable magnetic near-field distributions based on the coding metasurfaces in MHz band

Magnetic near-field control has attracted extensive attention and have a wide range of applications in filters, sensors, and energy-transfer devices. Coding metamaterial has the convenience of miniaturization and integration, which not only provides a bridge between physics and information science, but also exploits a whole new perspective for magnetic near-field control. In this work, we theoretically propose and experimentally verify that the coding metasurface can realize the reconfigurable magnetic near-field distributions. By adjusting the digital voltage signals which drive the resonant units of metasurface, capacitance of the resonator can be dynamically controlled, thus the dynamic modulation of magnetic near-field distribution can be achieved. Specially, this magnetic near-field control can be explained with the help of coupled mode theory and coherent superposition method. Moreover, simulated annealing algorithm is employed to determine working frequency of the whole modulation system, which can avoid time-consuming frequency scanning process. The experimental results are in good agreement with the calculated results. This work reveals the significant potential of coding metasurface for the experimental study of the magnetic near-field control and promotes the use of metasurface for numerous integrated functional devices.

Radiation Pattern Prediction for Metasurfaces: A Neural Network-Based Approach

As the current standardization for the 5G networks nears completion, work towards understanding the potential technologies for the 6G wireless networks is already underway. One of these potential technologies for the 6G networks is reconfigurable intelligent surfaces. They offer unprecedented degrees of freedom towards engineering the wireless channel, i.e., the ability to modify the characteristics of the channel whenever and however required. Nevertheless, such properties demand that the response of the associated metasurface is well understood under all possible operational conditions. While an understanding of the radiation pattern characteristics can be obtained through either analytical models or full-wave simulations, they suffer from inaccuracy and extremely high computational complexity, respectively. Hence, in this paper, we propose a neural network-based approach that enables a fast and accurate characterization of the metasurface response. We analyze multiple scenarios and demonstrate the capabilities and utility of the proposed methodology. Concretely, we show that this method can learn and predict the parameters governing the reflected wave radiation pattern with an accuracy of a full-wave simulation (98.8–99.8%) and the time and computational complexity of an analytical model. The aforementioned result and methodology will be of specific importance for the design, fault tolerance, and maintenance of the thousands of reconfigurable intelligent surfaces that will be deployed in the 6G network environment.

6G Enabled Smart Infrastructure for Sustainable Society: Opportunities, Challenges, and Research Roadmap

The 5G wireless communication network is currently faced with the challenge of limited data speed exacerbated by the proliferation of billions of data-intensive applications. To address this problem, researchers are developing cutting-edge technologies for the envisioned 6G wireless communication standards to satisfy the escalating wireless services demands. Though some of the candidate technologies in the 5G standards will apply to 6G wireless networks, key disruptive technologies that will guarantee the desired quality of physical experience to achieve ubiquitous wireless connectivity are expected in 6G. This article first provides a foundational background on the evolution of different wireless communication standards to have a proper insight into the vision and requirements of 6G. Second, we provide a panoramic view of the enabling technologies proposed to facilitate 6G and introduce emerging 6G applications such as multi-sensory–extended reality, digital replica, and more. Next, the technology-driven challenges, social, psychological, health and commercialization issues posed to actualizing 6G, and the probable solutions to tackle these challenges are discussed extensively. Additionally, we present new use cases of the 6G technology in agriculture, education, media and entertainment, logistics and transportation, and tourism. Furthermore, we discuss the multi-faceted communication capabilities of 6G that will contribute significantly to global sustainability and how 6G will bring about a dramatic change in the business arena. Finally, we highlight the research trends, open research issues, and key take-away lessons for future research exploration in 6G wireless communication.

Dual-band multi-bit programmable reflective metasurface unit cell: design and experiment

Programmable reflective metasurfaces that combine the features of reconfigurable phased array antennas and reflectors are an effective solution for radar and modern communication systems. However, most of the demonstrated active metasurfaces support tunable responses for a specific frequency band. Thus, we propose a programmable metasurface that combines the advantages of multi-bit phase quantization and dual-band operations. To actively control the diverse functions, two PIN diodes are integrated on the radiating element, and these diodes are controlled by the biasing voltage. The unit cell is fabricated, and experimental characterization is performed in the waveguide measurement setup. The proposed design can be applied for imaging and high-capacity wireless communications.

Experimental demonstration of a time-domain digital-coding metasurface for a Doppler cloak

By generating an artificial Doppler shift, a Doppler cloak can compensate for the Doppler shift from a moving object. An object covered by a Doppler cloak will be detected as a static object, even if it is actually moving. Herein, we experimentally demonstrate the Doppler cloak in a radar system using a time-domain digital-coding metasurface. We theoretically illustrate an active metasurface with a modulated reflection phase that can imitate the motion of moving, thereby generating an artificial Doppler shift for a Doppler cloak. Moreover, a reflective metasurface composed of voltage-controlled varactor diodes with a 3-bit reflection phase was designed and fabricated. Finally, we experimentally demonstrate that an artificial Doppler shift for a Doppler cloak is obtained from the proposed metasurface using a discrete time-varying bias voltage. Simulation and measurement results show that the proposed time-domain digital-coding metasurface can cancel the Doppler shift and serve as a Doppler cloak. The proposed metasurface may have potential applications in a Doppler radar illusion, Doppler cancellation in vehicle-to-vehicle communications, and wireless communications.

Electronically reconfigurable unit cell for transmit-reflect-arrays in the X-band

This paper proposes an electronically reconfigurable unit cell for transmit-reflect-arrays in the X-band, which makes it possible to control the reflection or transmission phase independently by combining the mechanisms of reconfigurable transmitarrays and reconfigurable reflectarrays. The fabricated unit cell was characterized in a waveguide simulator. The return loss in the reflection mode and insertion loss in the transmission mode are smaller than 1.8 dB for all states at 10.63 GHz, and a 1-bit phase shift for both modes is achieved within 180° ± 10°. When compared to full-wave electromagnetic simulation results, the proposed unit cell shows good results and is thus verified.

Space-Time-Coding Digital Metasurfaces: Principles and Applications

Space-time-modulated metastructures characterized by spatiotemporally varying properties have recently attracted great interest and become one of the most fascinating and promising research fields. In the meantime, space-time-coding digital metasurfaces with inherently programmable natures emerge as powerful and versatile platforms for implementing the spatiotemporal modulations, which have been successfully realized and used to manipulate the electromagnetic waves in both the spectral and spatial domains. In this article, we systematically introduce the general concepts and working principles of space-time-coding digital metasurfaces and provide a comprehensive survey of recent advances and representative applications in this field. Specifically, we illustrate the examples of complicated wave manipulations, including harmonic beam control and programmable nonreciprocal effect. The fascinating strategy of space-time-coding opens the door to exciting scenarios for information systems, with abundant applications ranging from wireless communications to imaging and radars. We summarize this review by presenting the perspectives on the existing challenges and future directions in this fast-growing research field.

Broadband bifunctional Luneburg-Fisheye lens based on anisotropic metasurface

Luneburg lenses and Maxwell fisheye lenses possess distinct properties of focusing, well beyond conventional lenses made of uniform materials. In this paper, a planar broadband bifunctional Luneburg-fisheye lens synthesized by gradient anisotropic metasurface is proposed. The proposed anisotropic metasurface is formed by non-resonant anisotropic cells, so that it can independently realize the equivalent gradient refractive indexes of Luneburg lens and Maxwell fisheye lens along orthogonal directions in a broad band, respectively. To verify the performance of the bifunctional lens, a prototype associated with a feeding log-periodic dipole antenna has been fabricated. Experimental results show that the proposed lens functions well over a wide frequency range with high efficiency and low profile, which coincides well with theoretical predictions and simulated results. It is expected that the proposed design will facilitate the applications of multifunctional metadevices in microwave and optical ranges.

Spatial wave control using a self-biased nonlinear metasurface at microwave frequencies

Recently, investigation of metasurfaces has been extended to wave control through exploiting nonlinearity. Among all of the ways to achieve tunable metasurfaces with multiplexed performances, nonlinearity is one of the promising choices. Although several proposals have been reported to obtain nonlinear architectures at visible frequencies, the area of incorporating nonlinearity in form of passive-designing at microwave metasurfaces is open for investigation. In this paper, a passive wideband nonlinear metasurface is manifested, which is composed of embedded L-shape and Γ -shape meta-atoms with PIN-diode elements. The proposed self-biased nonlinear metasurface has two operational states: at low power intensities, it acts as a Quarter Wave Plate (QWP) in the frequency range from 13.24 GHz to 16.38 GHz with an Axial Ratio (AR) of over 21.2%. In contrast, at high power intensities, by using the polarization conversion property of the proposed PIN-diode based meta-atoms, the metasurface can act as a digital metasurface. It means that by arranging the meta-atoms with a certain coding pattern, the metasurface can manipulate the scattered beams and synthesize well-known patterns such as diffusion-like and chessboard patterns at an ultra-wide frequency range from 8.12 GHz to 19.27 GHz (BW=81.4%). Full-wave and nonlinear simulations are carried out to justify the performance of the wideband nonlinear metasurface. We expect the proposed self-biased nonlinear metasurface at microwave frequencies reveals excellent opportunities to design limiter metasurfaces and compact reconfigurable imaging systems.

Ultrafast reprogrammable multifunctional vanadium-dioxide-assisted metasurface for dynamic THz wavefront engineering

In this paper, for the first time, a new generation of ultrafast reprogrammable multi-mission bias encoded metasurface is proposed for dynamic terahertz wavefront engineering by employing VO2 reversible and fast monoclinic to tetragonal phase transition. The multi-functionality of our designed VO2 based coding metasurface (VBCM) was guaranteed by elaborately designed meta-atom comprising three-patterned VO2 thin films whose operational statuses can be dynamically tuned among four states of "00"-"11" by merely changing the biasing voltage controlled by an external Field-programmable gate array platform. Capitalizing on such meta-atom design and by driving VBCM with different spiral-like and spiral-parabola-like coding sequences, single vortex beam and focused vortex beam with interchangeable orbital angular momentum modes were satisfactorily generated respectively. Additionally, by adopting superposition theorem and convolution operation, symmetric/asymmetric multiple beams and arbitrarily-oriented multiple vortex beams in pre-demined directions with different topological charges are realized. Several illustrative examples successfully have clarified that the proposed VBCM is a promising candidate for solving crucial terahertz challenges such as high data rate wireless communication where ultrafast switching between several missions is required.

Frequency-Domain and Spatial-Domain Reconfigurable Metasurface

The recently proposed digital reconfigurable metasurfaces make it possible to manipulate electromagnetic (EM) waves flexibly. However, most existing reconfigurable metasurfaces can only exhibit a relatively single performance in the spatial domain. Here, we propose a general frequency- and spatial-domain reconfigurable metasurface (FSRM) that can manipulate the EM waves and realize reconfigurable functions in multifrequency bands. In the frequency domain, FSRM can convert different linearly polarized (LP) incident waves into left- and right-hand circularly polarized reflected waves, in which PIN diodes are used to switch the polarization conversions in different frequency bands. When the polarization direction of the incident LP wave is 45° from the +x-axis, the FSRM modulates the incident waves as a 1-bit programmable metasurface in the spatial domain. Two-dimensional beam scanning, vortex beams with orbital angular momentums, and specific beams with desired transmission directions are demonstrated via real-time adjustment of the digital coding state. To validate the modulation methodology, an FSRM prototype is fabricated and measured, which could respond to different functions for different polarization incidences. The measured results agree well with the theoretical analyses. The proposed FSRM will provide new opportunities for smart material designs.

Intelligent Electromagnetic Sensing with Learnable Data Acquisition and Processing

Electromagnetic (EM) sensing is a widespread contactless examination technique with applications in areas such as health care and the internet of things. Most conventional sensing systems lack intelligence, which not only results in expensive hardware and complicated computational algorithms but also poses important challenges for real-time in situ sensing. To address this shortcoming, we propose the concept of intelligent sensing by designing a programmable metasurface for data-driven learnable data acquisition and integrating it into a data-driven learnable data-processing pipeline. Thereby, a measurement strategy can be learned jointly with a matching data post-processing scheme, optimally tailored to the specific sensing hardware, task, and scene, allowing us to perform high-quality imaging and high-accuracy recognition with a remarkably reduced number of measurements. We report the first experimental demonstration of "learned sensing" applied to microwave imaging and gesture recognition. Our results pave the way for learned EM sensing with low latency and computational burden.

Active Cloaking of a Non-Uniform Scatterer

An object illuminated by an electromagnetic wave can be actively cloaked using a surface conformal array of radiating sources to cancel out scattering. This method is promising as elementary antennas can be used as sources while its active nature can surpass passivity-based performance limitations. While this technique has been conceptually extended to accommodate complex geometries, experimental validation past simple uniform scatterers is lacking. To address this scarcity, the design and experimental demonstration of a low-profile, active cloak capable of concealing a complex, metallic, polygonal target is presented. This cloak is constructed with commercially available monopoles and enclosed within a parallel-plate waveguide-based apparatus to approximate a quasi-2D environment. Performance is then assessed when the target is illuminated at either frontal or oblique incidence by a 1.2 GHz cylindrical wave. Overall, the cloak reduces the target’s scattering cross-section by an average of 7.2 dB at frontal incidence and 8.6 dB at oblique incidence. These results demonstrate the feasibility of this kind of active cloaking for more complex scatterers containing flat surfaces and edges. Further analysis shows that the cloak possesses a functional bandwidth of 14% and can be reconfigured for single frequency operation over 0.8–1.8 GHz.

A Novel Coding Metasurface for Wireless Power Transfer Applications

We propose and implement a novel 1-bit coding metasurface that is capable of focusing and steering beam for enhancing power transfer efficiency of the electromagnetic (EM) wave-based wireless power transfer systems. The proposed metasurface comprises 16 × 16 unit cells which are designed with a fractal structure and the operating frequency of 5.8 GHz. One PIN diode is incorporated within each unit cell and enables two states with 180 ° phase change of the reflected signal at the unit cell. The two states of the unit cell correspond to the ON and OFF states of the PIN diode or “0” and “1” coding in the metasurface. By appropriately handling the ON/OFF states of the coding metasurface, we can control the reflected EM wave impinged on the metasurface. To verify the working ability of the coding metasurface, a prototype metasurface with a control board has been fabricated and measured. The results showed that the coding metasurface is capable of focusing beam to desired direction. For practical scenarios, we propose an adaptive optimal phase control scheme for focusing the beam to a mobile target. Furthermore, we prove that the proposed adaptive optimal phase control scheme outperforms the random phase control and beam synthesis schemes.

Intelligent metasurface imager and recognizer

There is an increasing need to remotely monitor people in daily life using radio-frequency probe signals. However, conventional systems can hardly be deployed in real-world settings since they typically require objects to either deliberately cooperate or carry a wireless active device or identification tag. To accomplish complicated successive tasks using a single device in real time, we propose the simultaneous use of a smart metasurface imager and recognizer, empowered by a network of artificial neural networks (ANNs) for adaptively controlling data flow. Here, three ANNs are employed in an integrated hierarchy, transforming measured microwave data into images of the whole human body, classifying specifically designated spots (hand and chest) within the whole image, and recognizing human hand signs instantly at a Wi-Fi frequency of 2.4 GHz. Instantaneous in situ full-scene imaging and adaptive recognition of hand signs and vital signs of multiple non-cooperative people were experimentally demonstrated. We also show that the proposed intelligent metasurface system works well even when it is passively excited by stray Wi-Fi signals that ubiquitously exist in our daily lives. The reported strategy could open up a new avenue for future smart cities, smart homes, human-device interaction interfaces, health monitoring, and safety screening free of visual privacy issues.

Metasurface-based focus-tunable mirror

Varifocal mirrors, which have various applications in optical coherent tomography and three-dimensional displays, are traditionally based on the fluid pressure or mechanical pusher to deform the mirror. The limitations of conventional varifocal mirrors are obvious, such as the heavy size of the device and constraints of tunability, due to their mechanical pressure control elements. The reprogrammable metasurface, a new flat photonic device with multifunction in an ultrathin dimension, paves the way towards an ultrathin and lightweight mirror with precise phase profile. Here, an active reconfigurable metasurface is proposed to achieve the manipulation of the wavefront. The meta-atom in the metasurface is integrated with one varactor diode to manipulate the electromagnetic response. As the bias voltage increases from 0 to 20 V, the resonant frequency shifts from 5.5 to 6.0 GHz, which generates a broad tunable phase region, leading to 5 diopters (about 50%) change without any mechanical element and a broad tunable frequency band. In addition, the focus point can not only be steered in the axial line above the metasurface but also in the whole working plane. The proposed focus-tunable metasurface mirror may be a key in enabling future ultrathin reconfigurable optical devices with applications such as multiphoton microscopy, high speed imaging and confocal microscopy.

Shaping Electromagnetic Waves with Flexible and Continuous Control of the Beam Directions Using Holography and Convolution Theorem

In this article, several versatile electromagnetic (EM) waves are presented with predefined shapes and directions based on the holography and convolution theorem. Inspiring the holography theory, a reflective interferogram is characterized by interfering the near field distributions of the object and reference waves. In this regard, the interference pattern on the hologram could be viewed as the inverse Fourier transform of the object and reference waves. Therefore, the capability of steering the EM shaped beam is realized using the convolution theorem (as an interesting property of the Fourier transform), which makes a link between the hologram impedance-pattern and far-field pattern domains. The main advantage of incorporating the holography concept and convolution theorem is realizing arbitrary shaped-beam EM waves with the possibility of flexible manipulation of the beam directions without employing any optimization algorithm and mathematical computation. It is demonstrated that the method could implement a combination of simple beams (such as collimated beams) and complex beams (such as cosecant squared, flat top, isoflux beams, etc.) with each beam possessing arbitrary direction by the same design topology. To experimentally verify the concept, a prototype of the hologram with three separate beams including two tilted cosecant squared shaped beam and one broadside pencil beam is fabricated and measured. The measured results show a significant agreement between theoretical findings.