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Editor Work2021 Vol. 10, No. 2
Previous Issue
2021, 10(2): 183-190.
Abstract
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Intelligent electromagnetic sensing, which is based on electromagnetic imaging, aims to realize the real-time and smart imaging and recognition of objects of interest. Thus, intelligent electromagnetic sensing has been applied in many areas, including science, engineering, and the military. Recently, we explored the unique capabilities of artificial intelligence and artificial materials in the flexible manipulation of electromagnetic information and electromagnetic wavefields, respectively. Further, we developed several interesting schemes for intelligent electromagnetic sensing by fully incorporating artificial intelligence with artificial materials, particularly information metamaterials. Thus, several intelligent electromagnetic sensing systems, which exhibit interesting properties, like low hardware cost and high efficiency, have been developed. The proposed sensing strategies are expected to pave the way for wireless communications, smart homes, and other future applications.
Intelligent electromagnetic sensing, which is based on electromagnetic imaging, aims to realize the real-time and smart imaging and recognition of objects of interest. Thus, intelligent electromagnetic sensing has been applied in many areas, including science, engineering, and the military. Recently, we explored the unique capabilities of artificial intelligence and artificial materials in the flexible manipulation of electromagnetic information and electromagnetic wavefields, respectively. Further, we developed several interesting schemes for intelligent electromagnetic sensing by fully incorporating artificial intelligence with artificial materials, particularly information metamaterials. Thus, several intelligent electromagnetic sensing systems, which exhibit interesting properties, like low hardware cost and high efficiency, have been developed. The proposed sensing strategies are expected to pave the way for wireless communications, smart homes, and other future applications.
2021, 10(2): 191-205.
As a two-dimensional metamaterial equivalent, the gradient metasurface has become a focus of intense research hotspot since it exhibits powerful ability in manipulating electromagnetic waves due to its planar architecture, flexible selection between anisotropic and isotropic structures, and its abrupt discontinues phase. Here, we first reviewed recent research progress in multifunctional metasurfaces based on the multiplexing concept from a new perspective of combining one, two and even more degrees of freedom of polarization, frequency, incident angles and directions (excitation information), and output-wave position information. Therein, we achieve a clear outline of a research program and technical approach to multifunctional metasurfaces. Second, we predict future routes of development of multifunctional metasurfaces, aiming to afford novel avenues to the realization of more sophisticated and larger-capacity integrated wavefront control and multifunctional devices with new physics, which are promising for highly-integrated and miniaturized future communication and radar devices.
As a two-dimensional metamaterial equivalent, the gradient metasurface has become a focus of intense research hotspot since it exhibits powerful ability in manipulating electromagnetic waves due to its planar architecture, flexible selection between anisotropic and isotropic structures, and its abrupt discontinues phase. Here, we first reviewed recent research progress in multifunctional metasurfaces based on the multiplexing concept from a new perspective of combining one, two and even more degrees of freedom of polarization, frequency, incident angles and directions (excitation information), and output-wave position information. Therein, we achieve a clear outline of a research program and technical approach to multifunctional metasurfaces. Second, we predict future routes of development of multifunctional metasurfaces, aiming to afford novel avenues to the realization of more sophisticated and larger-capacity integrated wavefront control and multifunctional devices with new physics, which are promising for highly-integrated and miniaturized future communication and radar devices.
2021, 10(2): 206-219.
Recently, reconfigurable metasurfaces have attracted intense attention in the field of electromagnetic metasurfaces. Compared with other metasurfaces, reconfigurable metasurfaces that uses steerable devices or materials to control the electromagnetic wave in real time are more versatile and show great promise in engineering applications. Our team has continuously explored advances of reconfigurable metasurfaces and also studied the microwave region from the perspectives of theory, technique and applications. This study reviews the research history of reconfigurable metasurfaces and summarizes some of our previous works, including a study on the amplitude, phase and polarization modulation of electromagnetic waves and their applications. Finally, the study discusses future challenges and possibilities for reconfigurable metasurfaces.
Recently, reconfigurable metasurfaces have attracted intense attention in the field of electromagnetic metasurfaces. Compared with other metasurfaces, reconfigurable metasurfaces that uses steerable devices or materials to control the electromagnetic wave in real time are more versatile and show great promise in engineering applications. Our team has continuously explored advances of reconfigurable metasurfaces and also studied the microwave region from the perspectives of theory, technique and applications. This study reviews the research history of reconfigurable metasurfaces and summarizes some of our previous works, including a study on the amplitude, phase and polarization modulation of electromagnetic waves and their applications. Finally, the study discusses future challenges and possibilities for reconfigurable metasurfaces.
2021, 10(2): 220-239.
At present, research on metamaterials is continuously advancing to engineering applications, and great progress is being achieved in the areas of physical mechanisms and effects, design theory and methods, and fabrication and measurement. However, traditional metamaterials design mainly relies on artificial design and optimization. In the face of large-scale engineering applications, it is impossible to realize the rapid overall design of a large number of metamaterial structural units. In recent years, the proportion of intelligent algorithms covering traditional heuristic algorithms and neural network algorithms in metamaterials design has increased gradually. Metamaterials design based on intelligent algorithms can surpass the limitation of traditional methods in different substrate systems, frequency variation, and different performance indicators, offering the unique advantages of rapid design and architectural innovation. This paper summarizes the application of several typical intelligent algorithms, including the genetic algorithm, Hopfield network algorithm, and deep learning algorithm in metamaterials design, which include forward designs and an inverse design. The use of intelligent algorithms can achieve the rapid design of frequency selective surfaces under different performance indexes, multi-mechanisms composite absorber metamaterials, flat focusing, and abnormal reflection metasurfaces, providing the necessary support for design methods while promoting the engineering applications of metamaterials.
At present, research on metamaterials is continuously advancing to engineering applications, and great progress is being achieved in the areas of physical mechanisms and effects, design theory and methods, and fabrication and measurement. However, traditional metamaterials design mainly relies on artificial design and optimization. In the face of large-scale engineering applications, it is impossible to realize the rapid overall design of a large number of metamaterial structural units. In recent years, the proportion of intelligent algorithms covering traditional heuristic algorithms and neural network algorithms in metamaterials design has increased gradually. Metamaterials design based on intelligent algorithms can surpass the limitation of traditional methods in different substrate systems, frequency variation, and different performance indicators, offering the unique advantages of rapid design and architectural innovation. This paper summarizes the application of several typical intelligent algorithms, including the genetic algorithm, Hopfield network algorithm, and deep learning algorithm in metamaterials design, which include forward designs and an inverse design. The use of intelligent algorithms can achieve the rapid design of frequency selective surfaces under different performance indexes, multi-mechanisms composite absorber metamaterials, flat focusing, and abnormal reflection metasurfaces, providing the necessary support for design methods while promoting the engineering applications of metamaterials.
2021, 10(2): 240-258.
Electromagnetic metamaterials are artificial structures composed of a periodic or aperiodic arrangement of subwavelength unit cells and can regulate the physical characteristics of electromagnetic waves, such as their frequency, amplitude, phase, and polarization. Metamaterials overcome many limitations of traditional materials and can be used to realize interesting physical phenomena and applications that do not occur in nature. Over the past two decades, metamaterials have become a focus in the fields of physics and electronics owing to their powerful electromagnetic regulation ability. However, passive metamaterials have limitations in electromagnetic wave regulation, such as fixed operating frequency and single function. As such, increasing attention is being paid to tunable and active metamaterials. By introducing active elements, the functions of metamaterials can be dynamically regulated by external excitation signals, which is highly significant for practical applications. At present, commonly used control methods include electrical, temperature, light, and mechanical controls, among which light control has the advantages of remote and noncontact control, a fast modulation speed, and a simple structure. In this study, we summarize the latest progress in light-controlled electromagnetic metamaterial research, and introduce recent work on light-controlled metamaterials and metasurfaces in direct currents, microwaves, terahertz waves, and optics. We focus primarily on relevant operational mechanisms and application scenarios and discuss future prospects.
Electromagnetic metamaterials are artificial structures composed of a periodic or aperiodic arrangement of subwavelength unit cells and can regulate the physical characteristics of electromagnetic waves, such as their frequency, amplitude, phase, and polarization. Metamaterials overcome many limitations of traditional materials and can be used to realize interesting physical phenomena and applications that do not occur in nature. Over the past two decades, metamaterials have become a focus in the fields of physics and electronics owing to their powerful electromagnetic regulation ability. However, passive metamaterials have limitations in electromagnetic wave regulation, such as fixed operating frequency and single function. As such, increasing attention is being paid to tunable and active metamaterials. By introducing active elements, the functions of metamaterials can be dynamically regulated by external excitation signals, which is highly significant for practical applications. At present, commonly used control methods include electrical, temperature, light, and mechanical controls, among which light control has the advantages of remote and noncontact control, a fast modulation speed, and a simple structure. In this study, we summarize the latest progress in light-controlled electromagnetic metamaterial research, and introduce recent work on light-controlled metamaterials and metasurfaces in direct currents, microwaves, terahertz waves, and optics. We focus primarily on relevant operational mechanisms and application scenarios and discuss future prospects.
2021, 10(2): 259-266.
The Programmable Metasurface (PM) can flexibly manipulate electromagnetic waves in real time using loading active devices on the meta-element. Calculating the radiation fields of the PM with complex structures using full-wave simulation software is time-consuming, which results in design efficiency. To accurately and efficiently solve the mapping relationship from coding schemes to radiation fields, an Auto-Measuring System (AMS) of radiation patterns is designed. A few Code-to-Pattern (C-P) data are measured via the AMS. Then, a forward Deep Neural Network (DNN) is proposed, the DNN is trained by the measured data, and an accurate and efficient prediction of C-P is realized. More C-P data are generated based on the proposed forward neural network, and the data are used to train another proposed inverse DNN and realize the accurate prediction of codes when given patterns in real time. This method provides a new alternative scheme for radar beamforming and has application values in intelligent radar beamforming and microwave imaging.
The Programmable Metasurface (PM) can flexibly manipulate electromagnetic waves in real time using loading active devices on the meta-element. Calculating the radiation fields of the PM with complex structures using full-wave simulation software is time-consuming, which results in design efficiency. To accurately and efficiently solve the mapping relationship from coding schemes to radiation fields, an Auto-Measuring System (AMS) of radiation patterns is designed. A few Code-to-Pattern (C-P) data are measured via the AMS. Then, a forward Deep Neural Network (DNN) is proposed, the DNN is trained by the measured data, and an accurate and efficient prediction of C-P is realized. More C-P data are generated based on the proposed forward neural network, and the data are used to train another proposed inverse DNN and realize the accurate prediction of codes when given patterns in real time. This method provides a new alternative scheme for radar beamforming and has application values in intelligent radar beamforming and microwave imaging.
2021, 10(2): 267-273.
A quasi-Bessel beam is a type of nondiffracted beam commonly used in microwave and optical fields. Although numerous methods have been proposed for quasi-Bessel beam generation, they are valid only in linear systems, indicating that the generation of nonlinear quasi-Bessel beams remains a major challenge. Thus, we propose a new approach to produce quasi-Bessel beams at high-order harmonics based on the time-domain digital-coding metasurface, which is utilized to achieve accurate control of the phase profile at the nonlinear frequencies via proper coding strategies. The effect of phase discretization is also analyzed in detail. The simulation results confirm the validity of the proposed method, which provides a new approach for nonlinear beam manipulation.
A quasi-Bessel beam is a type of nondiffracted beam commonly used in microwave and optical fields. Although numerous methods have been proposed for quasi-Bessel beam generation, they are valid only in linear systems, indicating that the generation of nonlinear quasi-Bessel beams remains a major challenge. Thus, we propose a new approach to produce quasi-Bessel beams at high-order harmonics based on the time-domain digital-coding metasurface, which is utilized to achieve accurate control of the phase profile at the nonlinear frequencies via proper coding strategies. The effect of phase discretization is also analyzed in detail. The simulation results confirm the validity of the proposed method, which provides a new approach for nonlinear beam manipulation.
2021, 10(2): 274-280.
Most previous circuit analog absorbers only considered absorption performance under normal incidences, leading to bad absorption for large incident angles, particularly those > 30°. With the advancement in modern bistatic radar detection technology, radar electromagnetic waves may come from different spatial directions, thereby necessitating radar absorbers with high absorption performance under normal and oblique incidences. Thus, in this paper, we present a novel wideband absorber comprising a conductive square-loop array embedded with lumped resistors and a well-designed Wide-Angle Impedance Matching (WAIM) layer. Results show that the WAIM layer can significantly improve absorption under oblique incidences. To make the absorber design clear and simple, an Equivalent Circuit (EC) and strict calculating formulas are proposed under normal and oblique incidences. Fractional bandwidth is increased into 137.1% through measurement under normal incidence, and the structure has a common fractional bandwidth of at least 110.5% for at least 10 dB reflection reduction when the incidence angle < 45°. The similarity among EC calculated, simulated, and measured results proves the validity of the designed absorber.
Most previous circuit analog absorbers only considered absorption performance under normal incidences, leading to bad absorption for large incident angles, particularly those > 30°. With the advancement in modern bistatic radar detection technology, radar electromagnetic waves may come from different spatial directions, thereby necessitating radar absorbers with high absorption performance under normal and oblique incidences. Thus, in this paper, we present a novel wideband absorber comprising a conductive square-loop array embedded with lumped resistors and a well-designed Wide-Angle Impedance Matching (WAIM) layer. Results show that the WAIM layer can significantly improve absorption under oblique incidences. To make the absorber design clear and simple, an Equivalent Circuit (EC) and strict calculating formulas are proposed under normal and oblique incidences. Fractional bandwidth is increased into 137.1% through measurement under normal incidence, and the structure has a common fractional bandwidth of at least 110.5% for at least 10 dB reflection reduction when the incidence angle < 45°. The similarity among EC calculated, simulated, and measured results proves the validity of the designed absorber.
2021, 10(2): 281-287.
This paper introduces a W-band phased electromagnetic surface radar system. This phased electromagnetic surface works at 92～96 GHz and is manufactured with general PCB technology and processing accuracy requirements. The appropriate unit design makes the DC bias voltage of each PIN diode on the phased electromagnetic surface controlled for the current reversal purpose at a low cost and portability. A 180° phase shift of the unit cell can be provided as the current direction on the unit cell is reversed. Beam scanning in different directions can be formed when inputting the right spatial codes. This transmission type phased electromagnetic surface with the ability of beam scanning is used as the receiving antenna of the radar system. This paper presents a W-band phased electromagnetic surface radar system and its manufacture and measurement results, which are fundamental to further research of precision guidance, target identification, and imaging.
This paper introduces a W-band phased electromagnetic surface radar system. This phased electromagnetic surface works at 92～96 GHz and is manufactured with general PCB technology and processing accuracy requirements. The appropriate unit design makes the DC bias voltage of each PIN diode on the phased electromagnetic surface controlled for the current reversal purpose at a low cost and portability. A 180° phase shift of the unit cell can be provided as the current direction on the unit cell is reversed. Beam scanning in different directions can be formed when inputting the right spatial codes. This transmission type phased electromagnetic surface with the ability of beam scanning is used as the receiving antenna of the radar system. This paper presents a W-band phased electromagnetic surface radar system and its manufacture and measurement results, which are fundamental to further research of precision guidance, target identification, and imaging.
2021, 10(2): 288-295.
In this paper, we propose a detailed architectural design, principle of operation, and modeling analysis of a high-performance microwave computational imaging system based on information metamaterials. We use the excellent electromagnetic wave manipulation capabilities of information metamaterials, combined with compressive sampling theory, and elaborate the design approaches of stray beam generation and high-performance radiation. Furthermore, we develop a numeric model for describing this imaging system and propose a high-performance frequency-diverse information metamaterial element whose band stop characteristic can cover the X-band. Based on the element, a high-performance information metamaterial lens is designed, which achieved 75% radiation efficiency, which is three times compared with existing metamaterial apertures over the imaging region. Finally, on the basis of the proposed numeric model, we compute and reconstruct ideal scattering objects using the high-performance information metamaterial lens, validating the ability of image restoration. Study on the high-performance microwave computation imaging system in this work laid the foundation for solid theory and prospective exploration, which can be applied for imaging radar, security monitoring, and medical testing.
In this paper, we propose a detailed architectural design, principle of operation, and modeling analysis of a high-performance microwave computational imaging system based on information metamaterials. We use the excellent electromagnetic wave manipulation capabilities of information metamaterials, combined with compressive sampling theory, and elaborate the design approaches of stray beam generation and high-performance radiation. Furthermore, we develop a numeric model for describing this imaging system and propose a high-performance frequency-diverse information metamaterial element whose band stop characteristic can cover the X-band. Based on the element, a high-performance information metamaterial lens is designed, which achieved 75% radiation efficiency, which is three times compared with existing metamaterial apertures over the imaging region. Finally, on the basis of the proposed numeric model, we compute and reconstruct ideal scattering objects using the high-performance information metamaterial lens, validating the ability of image restoration. Study on the high-performance microwave computation imaging system in this work laid the foundation for solid theory and prospective exploration, which can be applied for imaging radar, security monitoring, and medical testing.
2021, 10(2): 296-303.
The coincidence imaging system based on a metamaterial surface solves the problem of low detection efficiency. Nevertheless, the number of effective imaging points is limited owing to the lack of the detection mode. To solve this problem, based on the first-order statistical characteristics of a reference-radiation field, a correlation-imaging signal model based on a randomly-modulated metamaterial surface is established, and the imaging error is analyzed. This study presents a robust coincidence imaging called Differential Coincidence Imaging (DCI), which uses the differential of different modes to form a new detection mode. The DCI analysis proves that it can improve the imaging quality. At the same time, the resolution of a special DCI method called the Gradient Coincidence Imaging (GCI) method is analyzed, which effectively improves the ability of extracting a target edge. With the special design of a metasurface unit, the edge information of a target can be extracted directly in the imaging process without obtaining an image. Finally, the proposed methods are validated through simulation experiments.
The coincidence imaging system based on a metamaterial surface solves the problem of low detection efficiency. Nevertheless, the number of effective imaging points is limited owing to the lack of the detection mode. To solve this problem, based on the first-order statistical characteristics of a reference-radiation field, a correlation-imaging signal model based on a randomly-modulated metamaterial surface is established, and the imaging error is analyzed. This study presents a robust coincidence imaging called Differential Coincidence Imaging (DCI), which uses the differential of different modes to form a new detection mode. The DCI analysis proves that it can improve the imaging quality. At the same time, the resolution of a special DCI method called the Gradient Coincidence Imaging (GCI) method is analyzed, which effectively improves the ability of extracting a target edge. With the special design of a metasurface unit, the edge information of a target can be extracted directly in the imaging process without obtaining an image. Finally, the proposed methods are validated through simulation experiments.
2021, 10(2): 304-312.
We propose a general scheme to manipulate fundamental and harmonic frequencies simultaneously in a nonlinear fashion based on time-varying polarization-converting programmable metasurface. Co-polarization and cross-polarization reflection can be switched dynamically at an operating frequency of 2.4 GHz by loading metasurface with PIN (p-i-n) diodes. As a result, by adjusting a duty cycle and frequency of square-wave-type time-varying signals used for time-varying modulation, energy distribution and frequency shift in the frequency domain can be manipulated. To verify this, we fabricated a sample and conducted experiments, and the results agreed well with the theoretical prediction, confirming the design principle. Furthermore, we propose a wireless communication system based on binary amplitude-shift keying as an example of its practical application. The proposed one eliminates the need for complex device components on the emitter because the information is directly modulated onto the metasurface, greatly simplifying the traditional system. The proposed system can achieve a maximum transmission data rate of up to 625 kbps in experiments. The proposed metasurface paves a new way for time-varying manipulation of microwave and can have potential in real-world applications, such as next-generation communication and high-resolution imaging.
We propose a general scheme to manipulate fundamental and harmonic frequencies simultaneously in a nonlinear fashion based on time-varying polarization-converting programmable metasurface. Co-polarization and cross-polarization reflection can be switched dynamically at an operating frequency of 2.4 GHz by loading metasurface with PIN (p-i-n) diodes. As a result, by adjusting a duty cycle and frequency of square-wave-type time-varying signals used for time-varying modulation, energy distribution and frequency shift in the frequency domain can be manipulated. To verify this, we fabricated a sample and conducted experiments, and the results agreed well with the theoretical prediction, confirming the design principle. Furthermore, we propose a wireless communication system based on binary amplitude-shift keying as an example of its practical application. The proposed one eliminates the need for complex device components on the emitter because the information is directly modulated onto the metasurface, greatly simplifying the traditional system. The proposed system can achieve a maximum transmission data rate of up to 625 kbps in experiments. The proposed metasurface paves a new way for time-varying manipulation of microwave and can have potential in real-world applications, such as next-generation communication and high-resolution imaging.
2021, 10(2): 313-325.
In this paper, we propose the utilization of a programmable metasurface for flexibly manipulating ambient Wi-Fi signals. First, we propose a new and efficient optimization algorithm CWGS (Complex Weighted Gerchberg-Saxton), which is based on an electromagnetic scattering model of the metasurface. The proposed algorithm quickly redesigns the complex amplitude distribution of the Wi-Fi field bounced off the programmable metasurface to enhance the Wi-Fi signals at desired locations significantly. Second, we fabricated a large-scale programmable metasurface that operates at the 2.4 GHz frequency band. We conducted several experiments using the fabricated metasurface to verify the proposed optimization algorithm’s feasibility and effectiveness. Both the theoretical and experimental results show that the programmable metasurface can dynamically boost Wi-Fi signals at multiple locations. Besides, we experimentally verified that using the developed strategy could improve the Wi-Fi signals by 23.5 dB. The results of our work improve the usability and practicality of the programmable metasurface in real-world applications and pave the way for wireless communications, future smart homes, and other applications.
In this paper, we propose the utilization of a programmable metasurface for flexibly manipulating ambient Wi-Fi signals. First, we propose a new and efficient optimization algorithm CWGS (Complex Weighted Gerchberg-Saxton), which is based on an electromagnetic scattering model of the metasurface. The proposed algorithm quickly redesigns the complex amplitude distribution of the Wi-Fi field bounced off the programmable metasurface to enhance the Wi-Fi signals at desired locations significantly. Second, we fabricated a large-scale programmable metasurface that operates at the 2.4 GHz frequency band. We conducted several experiments using the fabricated metasurface to verify the proposed optimization algorithm’s feasibility and effectiveness. Both the theoretical and experimental results show that the programmable metasurface can dynamically boost Wi-Fi signals at multiple locations. Besides, we experimentally verified that using the developed strategy could improve the Wi-Fi signals by 23.5 dB. The results of our work improve the usability and practicality of the programmable metasurface in real-world applications and pave the way for wireless communications, future smart homes, and other applications.
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