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Magneto-mechanical metamaterials possess unique and tunable properties by adjusting their shape configurations in response to an external magnetic field. Their designs and functionalities are diverse and are utilized in a wide variety of applications, such as highly tunable elastic and electromagnetic wave filters and targeted shape morphing. In this perspective, we examine the general background of magneto-mechanical metamaterials and their diverse applications. The possible future directions in the field are also thoroughly discussed. [ABSTRACT FROM AUTHOR]

This work presents the utilization of square ring slots as resonators on active and passive metasurfaces operating within the K$_\text{a}$-band. Thereby, fully functional prototypes of reflectors and lenses based on a two-layer printed circuit board were fabricated and verified. The analytical model of the square ring slot structure is depicted extensively, which allows finding simulation optimized designs that can be applied to the passive prototypes. Additionally, the gained information on self-resonances is used for combinations with layers, activated through lumped varactor diodes. In the end, the reflection and transmission parameters of single active and passive designs as well as beam patterns of the lenses are measured and interpreted, resulting in a switching dynamic range of 15 dB at a bandwidth of 1 GHz for active reflectors and a maximum gain of 22 dBi at 26 GHz for the passive lens. While the active lens operates best approximately 1 GHz around the design frequency at 26 GHz, the passive one maintains a maximum gain higher than 20.8 dBi between 23 and 27 GHz.

To widen the attenuation bandwidth in a mass-in-mass metamaterial chain, active feedback control is employed within the unit cell. The optimal control via Linear Quadratic Regulator is used to ensure the stability of the solution as well as adaptive tuning of the system. The key concept is to obtain a wider bandwidth by reducing the displacement amplitude of the outer mass of each unit of the metamaterial. Transmittance for finite number of units and dispersion diagram for infinitely long metamaterial chain are obtained using the backward-substitution method and Bloch–Floquet's theorem, respectively. Results evidenced that an ultra-wide uninterrupted attenuation band starting from very low free wave frequency can be obtained. The dispersion diagram primarily depends on the frequency ratio of the resonating unit with the external main unit. The effect of the mass ratio is insignificant while active feedback control is implemented. This unique phenomenon implies that for a very low mass ratio also a wider bandwidth can be achieved. This enables to design a lightweight structure having wider bandwidth in low-frequency regime. [ABSTRACT FROM AUTHOR]

Metamaterials are an artificial electromagnetic material composed of periodic/non-periodic subwavelength micro-/nanostructures, i.e. meta-atoms. The meta-atom interacts with the incident electromagnetic wave and introduces electromagnetic resonance, which makes the metamaterial exhibit the desired electromagnetic characteristics. Therefore, the electromagnetic wave can be controlled by changing the geometry, configuration and distribution of the meta-atoms. Due to their flexible electromagnetic manipulation ability, metamaterials have attracted great interest in many fields, such as super-resolution imaging, high-sensitive detection, aerocraft stealth and laser-machining. A planar metamaterial with one or a few layers of meta-atoms is called a metasurface. The metasurface can not only manipulate the amplitude, phase and polarization of the electromagnetic waves, but also has the advantages of being ultra-thin, ultra-light and easy to process. In the terahertz (THz) region, more and more devices based on metasurfaces have been proposed for spectrum modulation and wavefront shaping, which has contributed to the rapid development of THz technology. This paper reviews the design principles and research progress of metamaterials/metasurfaces for spectrum modulation, wavefront shaping, polarization conversion and surface wave manipulation in the THz region. Active metamaterials can be used to manipulate electromagnetic waves dynamically, and this will become a research field with great application potential. In this review, the implementation schemes and research results of various active THz metamaterial devices are reviewed in detail. Furthermore, the potential applications of metamaterials/metasurfaces in security, high-capacity communication, biomedicine and other fields are analyzed. Finally, we discuss the future developments and challenges of THz metamaterials. [ABSTRACT FROM AUTHOR]

This paper provides an active metamaterial (MTM) glucose sensor with programmable functionalities to sensitively detect a large scale of glucose solution concentration up to 7000 mg/dL. The active sensor consists of periodical units that can be individually actuated by PIN diodes via Micro-Control-Unit for obtaining four tunable states presented as digital coding sequences 00/01/10/11. Hence, for the MTM with ${n}$ units, it possesses as many as $4^{n}$ programable resonances, which can be taken advantage of to enhance the sensitivity and expand the detecting range of concentration. Specifically, by investigating different programmable modes for different concentration ranges, it can obtain high sensitivity of 10−3-10−2 dB/(mg/dL) for detecting concentration up to 7000 mg/dL: for concentration of 0–200 mg/dL, the programmable state 1111 is utilized with obtaining the sensitivity of $3.243\times 10^{-3}{-}1.839\times 10^{-2}$ dB/(mg/dL); for concentration of 200–3000 mg/dL, the programmable state 1011 is selected to realize the sensitivity of $2.116\times 10^{-3}{-}1.124\times 10^{-2}$ dB/(mg/dL); for more higher concentration of 3000–7000 mg/dL, another programmable state 0010 is used for obtaining sensitivity of $2.611\times 10^{-4}{-}1.149\times 10^{-3}$ dB/(mg/dL). Particularly, the programmable state 0000 is chosen as the sensing map, so that the measured S11 magnitude in the mapping state can tell the glucose concentration range of the material under test and guide the active MTM sensor go to the appropriate programmable state. This work aims to study the advantage of active sensor with programmable functionalities and motivate the use of sensors with programmable modes to identify, classify, and characterize biochemical and chemical analytes. [ABSTRACT FROM AUTHOR]

Electromagnetic phase conjugation provides an appealing path toward communication and target localization in dynamic media such as the earth's atmosphere. However, realizing phase conjugation-based devices has proven difficult because of the inherent complexity of systems that employ receivers and transmitters working on the same band. In this letter, we propose the concept of out-of-band phase conjugation in which the frequency of the phase conjugated wave is an integer multiple of the incident wave frequency. Remarkably, the out-of-band phase conjugated wave maintains the autofocusing property that makes phase conjugation so powerful and opens the door to imaging technology effective in random and dynamic media. [ABSTRACT FROM AUTHOR]

The ability to create linear systems that manifest broadband nonreciprocal wave propagation would provide for exquisite control over acoustic signals for electronic filtering in communication and noise control. Acoustic nonreciprocity has predominately been achieved by approaches that introduce nonlinear interaction, mean-flow biasing, smart skins, and spatio-temporal parametric modulation into the system. Each approach suffers from at least one of the following drawbacks: the introduction of modulation tones, narrow band filtering, and the interruption of mean flow in fluid acoustics. We now show that an acoustic media that is non-local and active provides a new means to break reciprocity in a linear fashion without these deleterious effects. We realize this media using a distributed network of interlaced subwavelength sensor–actuator pairs with unidirectional signal transport. We exploit this new design space to create a stable metamaterial with non-even dispersion relations and electronically tunable nonreciprocal behavior over a broad range of frequencies.

In this paper, we shall demonstrate firstly that a normal incidence can be totally reflected from a slab made of active metamaterial with purely-imaginary-impedance. Then we shall predict that a localized steady state of electromagnetic wave dependent on initial input can exist in a double-slab structure, which relates to the non null solution of equations formed by electromagnetic field boundary conditions. These results may provide a feasible way to effectively treat loss and/or gain problems and thus store electromagnetic wave (light) permanently at room temperature. In addition, our work indicates that metamaterials with purely-imaginary-impedance may enable remarkable electromagnetic phenomena and merit further study. [ABSTRACT FROM AUTHOR]

Incorporation of active devices/media such as transistors for microwave and gain media for optics may be very attractive for enabling desired low loss and broadband metamaterials. Such metamaterials can even have gain which may very well lead to new and exciting physical phenomena. We investigate microwave composite right/left-handed transmission lines (CRLH-TL) incorporating ideal gain devices such as constant negative resistance. With realistic lumped element values, we have shown that the negative phase constant of this kind of transmission lines is maintained (i.e., left-handedness kept) while gain can be obtained (negative attenuation constant of transmission line) simultaneously. Possible implementation and challenging issues of the proposed active CRLH-TL are also discussed.

Almost ten years have passed since the first experimental attempts of enhancing functionality of radiofrequency metamaterials by embedding active circuits that mimic behavior of hypothetical negative capacitors, negative inductors and negative resistors. While negative capacitors and negative inductors can compensate for dispersive behavior of ordinary passive metamaterials and provide wide operational bandwidth, negative resistors can compensate for inherent losses. Here, the evolution of aforementioned research field, together with the most important theoretical and experimental results is reviewed. In addition, some very recent efforts that go beyond idealistic impedance negation and make use of inherent non-ideality, instability, and non-linearity of realistic devices, are highlighted. Finally, a very fundamental, but still unsolved issue of common theoretical framework that connects causality, stability, and non-linearity of networks with negative elements is stressed.

• Tunable and directional guided wave control was realized by a metasurface. • An active metamaterial design was realized by shape memory alloy elements. • The ultrasonic radar allows emitting guided waves in desired directions. • Wave steering was made possible via an electrical-thermal control approach. This article presents a new metasurface made of shape memory alloy (SMA) unit cells for steering ultrasonic guided waves in desired emitting directions. The philosophy behind the controllable wave steering performance of the metamaterial system resides in the tunability of its stopband. Such a tunable feature originates from the reversible dramatic variation of the elastic modulus of the shape memory element under the thermal loads. The research starts with the investigation of the adjustable bandgap behavior of the metastructure at different statuses from the scrutiny of the wave dispersive characteristics. Additionally, a systematic parametric study is carried out to unfold the bandgap evolution principle by changing the SMA stub height. Subsequently, numerical modeling of the wave steering behavior is performed by shifting the stopband of one sector within the unit-cell-ring away from the excitation frequency. The wavefield images associated with the wave directionality plots demonstrate that the thermally-activated metasurface area can open up a transmission path for the ultrasonic waves, while the rest portion will not allow guided waves to penetrate. By rotating the designated sector, the metamaterial structure could work like a wave emission radar, realizing the steerable unidirectional wave radiation from a single piezoelectric transducer. In the end, the targeting phenomena are validated by the Scanning Laser Doppler Vibrometry (SLDV) experimental tests. The proposed active metasurface system with the controllable ultrasonic wave interrogating capability may find its application for directional guided wavefield generation in future Structural Health Monitoring (SHM) and Nondestructive Evaluation (NDE) systems. [ABSTRACT FROM AUTHOR]

Superconducting terahertz metamaterials have attracted significant interest due to low loss, efficient resonance switching and large-range frequency tunability. The super conductivity in the metamaterials dramatically reduces ohmic loss and absorption to levels suitable for novel devices over a broad range of electromagnetic spectrum. Most metamaterials utilize subwavelength-scale split-ring resonators as unit building blocks, which are proved to support fundamental inductive-capacitive resonance, to achieve unique resonance performance. We presented a review of terahertz superconducting metamaterials and their implementation in multifunctional devices. We began with the recent development of superconducting metamaterials and their potential applications in controlling and manipulating terahertz waves. Then we explored the tuning behaviors of resonance properties in several typical, actively controllable metamaterials through integrating active components. Finally, the ultrafast dynamic nonlinear response to high intensity terahertz field in the superconducting metamaterials was presented. [ABSTRACT FROM AUTHOR]

Programmable metamaterials are suitable for their dynamic and real-time control capabilities of electromagnetic (EM) functions in radars and antenna communications, but it remains a challenge to achieve dynamic modulation of arbitrary transmission phase with high transmission efficiency. Here, we propose a paradigm to tailor transmission phase shift in real time by switching modes between waveguide and SSPP based on the voltage-driven PIN diodes. Step-like phase shift is achieved by the "ON" and "OFF" states of PIN diodes, while continuous phase regulation is by the characteristic of the nonlinear region between those two states. As validations, three systems with programmable functionalities are implemented, including the multibeam generator, the dual-beam scanner, and the active phased-array antenna. The experimental results are consistent with simulation, which verify the feasibility of the proposed approach. Our work offers an alternative route for transmission full-phase modulation and provides unprecedented potential for high-gain, real-time, and multidimensional EM capabilities in applications such as active phased array radars, self-adaption radomes, smart beam shaping.

Abstract: The present work describes the design of three flat superlens structures for acoustic source imaging and explores an active acoustic metamaterial (AAM) to realise such a design. The first two lenses are constructed via the coordinate transform method (CTM), and their constituent materials are anisotropic. The third lens consists of a material that has both a negative density and a negative bulk modulus. In these lenses, the quality of the images is “clear” and sharp; thus, the diffraction limit of classical lenses is overcome. Finally, a multi-control strategy is developed to achieve the desired parameters and to eliminate coupling effects in the AAM. [Copyright &y& Elsevier]

• A nonlocal active metamaterial with feedback control can achieve a tunable bandgap, which can be broadened by using a symmetric feedback signal. • In a metamaterial with an asymmetric feedback signal, the spatial symmetries of the control force are broken by the signal. • The non-Hermitian skin effects are achieved by asymmetrical feedback signal, and the asymmetrical feedback control gain can change the topological properties with winding number. • The imaginary parts of the wavenumber are asymmetrically distributed in space, and the metamaterial can undergo nonreciprocal wave propagation and directional amplification across a broad frequency range. • Function switching between bandgap control and nonreciprocity can be obtained by changing the feedback control signal. In this paper, a nonlocal active metamaterial with feedback control is proposed. For a metamaterial with a symmetric feedback signal, the tunable bandgap and negative effective stiffness are discussed. Symmetric displacement feedback control significantly broadens the bandgap, and combined with velocity feedback control can rapidly attenuate the vibration amplitude. In the metamaterial with an asymmetric feedback signal, the spatial symmetries of the control force are broken by the asymmetrical feedback signal. The non-Hermitian skin effects are achieved by asymmetrical feedback signal. The modes of localized are determined by topological properties with winding number, and the asymmetrical feedback control gain can change the topological properties with winding number. The imaginary parts of the wavenumber are also asymmetrically distributed in space. Theoretical analysis shows that nonlocal active metamaterials with asymmetric feedback signals can achieve nonreciprocal wave propagation and directional amplification across a broad frequency range. Numerical results verify that this metamaterial possesses the characteristics of nonreciprocal wave propagation and directional amplification. The nonlocal active metamaterials in this paper can not only achieve superior bandgap tuning, but also nonreciprocal wave propagation and directional amplification. Function switching between bandgap control and nonreciprocity can be realized by switching the feedback control signal. [Display omitted]. [ABSTRACT FROM AUTHOR]

Incorporation of active devices/media such as transistors for microwave and gain media for optics may be very attractive for enabling desired low loss and broadband metamaterials. Such metamaterials can even have gain which may very well lead to new and exciting physical phenomena. We investigate microwave composite right/left-handed transmission lines (CRLH-TL) incorporating ideal gain devices such as constant negative resistance. With realistic lumped element values, we have shown that the negative phase constant of this kind of transmission lines is maintained (i.e., left-handedness kept) while gain can be obtained (negative attenuation constant of transmission line) simultaneously. Possible implementation and challenging issues of the proposed active CRLH-TL are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: The majority of the designs of electromagnetic cloaks presented in the current literature require a metamaterial, the relative permittivity and/or permeability of which are smaller than one. Due to basic dispersion constraints, such permittivity (or permeability) can exist only within some restricted frequency band. This fact causes a passive electromagnetic cloak to be an inherently narrowband device. Here, it is shown both numerically and experimentally that the basic dispersion constraints of any passive medium might be overcome by using an active transmission line-based metamaterial. This type of metamaterial appears promising for the design of broadband cloaking devices. [ABSTRACT FROM AUTHOR]

Incorporation of active devices/media such as transistors for microwave and gain media for optics may be very attractive for enabling desired low loss and broadband metamaterials. Such metamaterials can even have gain which may very well lead to new and exciting physical phenomena. We investigate microwave composite right/left-handed transmission lines (CRLH-TL) incorporating ideal gain devices such as constant negative resistance. With realistic lumped element values, we have shown that the negative phase constant of this kind of transmission lines is maintained (i.e., left-handedness kept) while gain can be obtained (negative attenuation constant of transmission line) simultaneously. Possible implementation and challenging issues of the proposed active CRLH-TL are also discussed.