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Metamaterials hold significant promise for enhancing the imaging capabilities of magnetic resonance imaging (MRI) machines as an additive technology, due to their unique ability to enhance local magnetic fields. However, despite their potential, the metamaterials reported in the context of MRI applications have often been impractical. This impracticality arises from their predominantly flat configurations and their susceptibility to shifts in resonance frequencies, preventing them from realizing their optimal performance. Here, a computational method for designing wearable and tunable metamaterials via freeform auxetics is introduced. The proposed computational‐design tools yield an approach to solving the complex circle packing problems in an interactive and efficient manner, thus facilitating the development of deployable metamaterials configured in freeform shapes. With such tools, the developed metamaterials may readily conform to a patient's knee, ankle, head, or any part of the body in need of imaging, and while ensuring an optimal resonance frequency, thereby paving the way for the widespread adoption of metamaterials in clinical MRI applications. [ABSTRACT FROM AUTHOR]

The diversity and practicability of terahertz metamaterials have experienced rapid development in the past decade due to the increasing demand for various devices. This topic has attracted significant interest from researchers. Among the key functional devices in terahertz metamaterial systems, the active control ability of terahertz metamaterials is highly valuable and captivating. This implies that the electromagnetic properties of metamaterials can be modulated over a wide dynamic range by external stimuli. This review categorizes the different types of tunable terahertz metamaterials based on the external stimuli to which they respond, namely, mechanical modulation, electrical modulation, magnetic modulation, and optical modulation. Mechanically modulated devices offer simple yet efficient modulation, while electrical and magnetic modulation provide effective active modulation through electrical mechanisms. Optical modulation, in contrast, focuses on incorporating various materials to achieve active modulation. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO2) is a promising candidate for electronic and optical switching applications in the terahertz frequency range due to the metal to insulator transition (MIT). In this study, the use of VO2 patterned as a metamaterial surface or coupled as a homogeneous layer with a metallic metamaterial surface on top is investigated in terms of performance. High-quality VO2 thin films were deposited on c-cut sapphire substrates by using the dc magnetron sputtering technique. A change in resistivity by a factor of 104 MIT in VO2 was observed allowing to investigate its use as a controllable metamaterial. The layer was patterned using a unique geometry (four-cross shaped) that operates in the THz frequency range. To understand its performance as a tunable THz filter, the four-cross structure fabricated from VO2 is compared to one fabricated from Au on VO2 bare film using UV lithography and ion beam etching techniques. The spectral performances of metamaterials were assessed using THz-Time Domain Spectroscopy (THz-TDS) and results were compared with simulations based on CST Microwave Studio. Absence of the resonant effects in the purely developed VO2 device, while clear observation of the MIT behavior shows the strong dependency of the inductive and/or capacitive effects of the four-cross structure on conductivity of the surface metamaterial, which is clearly observable for the Au-based device. In the latter case, the resonant transmittance of the filter can be effectively modulated by change in temperature. [ABSTRACT FROM AUTHOR]

Abstract Metamaterials hold significant promise for enhancing the imaging capabilities of magnetic resonance imaging (MRI) machines as an additive technology, due to their unique ability to enhance local magnetic fields. However, despite their potential, the metamaterials reported in the context of MRI applications have often been impractical. This impracticality arises from their predominantly flat configurations and their susceptibility to shifts in resonance frequencies, preventing them from realizing their optimal performance. Here, a computational method for designing wearable and tunable metamaterials via freeform auxetics is introduced. The proposed computational‐design tools yield an approach to solving the complex circle packing problems in an interactive and efficient manner, thus facilitating the development of deployable metamaterials configured in freeform shapes. With such tools, the developed metamaterials may readily conform to a patient's knee, ankle, head, or any part of the body in need of imaging, and while ensuring an optimal resonance frequency, thereby paving the way for the widespread adoption of metamaterials in clinical MRI applications.

The diversity and practicability of terahertz metamaterials have experienced rapid development in the past decade due to the increasing demand for various devices. This topic has attracted significant interest from researchers. Among the key functional devices in terahertz metamaterial systems, the active control ability of terahertz metamaterials is highly valuable and captivating. This implies that the electromagnetic properties of metamaterials can be modulated over a wide dynamic range by external stimuli. This review categorizes the different types of tunable terahertz metamaterials based on the external stimuli to which they respond, namely, mechanical modulation, electrical modulation, magnetic modulation, and optical modulation. Mechanically modulated devices offer simple yet efficient modulation, while electrical and magnetic modulation provide effective active modulation through electrical mechanisms. Optical modulation, in contrast, focuses on incorporating various materials to achieve active modulation.

Dielectric metamaterials with low ohmic losses and resonating in the local magnetic mode are preferable for enhancing material non-linearity. Here, we propose and experimentally demonstrate broadband extraordinary electromagnetic transmission (EET) behavior, which is induced by the coupling of magnetic modes of two ceramic cuboids. It is shown that extraordinary electromagnetic transmission behavior through a perforated metal sheet with a subwavelength aperture can be achieved by exciting the first-order magnetic mode Mie resonant coupling of these cuboids. Our findings indicate that the transmission bandwidth and amplitude are dependent on the strength of coupling between the two ceramic cuboids. Additionally, we utilized non-linear effects within the dielectric cuboids to achieve tunable extraordinary electromagnetic transmission behavior. Our results are promising for developing non-linearly tunable microwave devices such as filters and modulators of their strong light-matter interactions.

Electro-tunable metamaterials opened new doors to realize spatial light modulation at nanoscale by engineering meta-atoms at sub-wavelength scale. In this study, we propose a novel design of polarization-insensitive spatial light modulator that can operate in both transmission and reflection modes. We leverage GMR (guided-mode resonance)-effect of grating waveguide and ENZ (epsilon-near-zero) phenomenon of ITO (indium-tin-oxide) to realize an efficient electro-optic modulation. The proposed design consists of a two-dimensional array of silicon-nitride nanograting on top of ITO-integrated silicon-on-insulator platform. Effective medium theory combined with temporal coupled-mode analysis is used to examine the excitation of waveguide mode via coupling of spatial light. Upon applying external bias-voltage when ITO exhibits ENZ-effect by accumulating free-carriers at ITO–Al2O3 interface, the resonance feature disappears due to large modal loss. A modulation depth of ~0.96 (~0.85) in reflection (transmission)-mode is achieved for normally incident plane wave with random polarization angle at $1.55 ~\mu \text{m}$ wavelength. In addition, the operating wavelength can be shifted by utilizing frequency-scaling property of the proposed design. With ~774 Mbps modulation speed, the proposed scalable design paves the way toward the realization of tunable flat-optical components for integrated nanophotonic systems. [ABSTRACT FROM AUTHOR]

Terahertz metamaterial plays a significant role in the development of imaging, sensing, and communications. The function of conventional terahertz metamaterials was fixed after fabrication. They can only achieve a single function and do not have adjustable characteristics, which greatly limits the scalability and practical application of metamaterial. Here, we propose a vanadium dioxide-based terahertz metamaterial device, which is switchable between being a transmitter and an absorber. The transmission and absorption characteristics and temperature tunable properties of phase change metamaterials in the terahertz band were investigated. As the temperature of vanadium dioxide is varied between 20 °C and 80 °C, the device can switch between transmission and quad-band resonance absorption at the terahertz frequency range, with a high transmission rate of over 80% and a peak absorbance of 98.3%, respectively. In addition, when the device acts as an absorber, the proposed metamaterial device is tunable, and the modulation amplitude can reach 94.3%; while the device is used as a transmissive device, the modulation amplitude of the transmission peak at 81%. The results indicate that the proposed metamaterial device can promote the applications of terahertz devices, such as switching, modulation, and sensing. [ABSTRACT FROM AUTHOR]

This paper presents a textile-based C-shaped split-ring resonators (SRR) metamaterial (MTM) unit cells with an electrical tunability function. The proposed MTM was composed of two symmetrical C-shaped SRR combined with a central diagonal metal bar, whereas the RF varactor diode is placed on the backside of the splitted ground plane. Stopband behavior of single and array MTM unit cells were analyzed while the achieved negative index physical characteristics were widely studies. Though four different MTM arrays (i.e., 1 × 1, 1 × 2, 2 × 1, and 2 × 2) were analyzed in simulation, a 2 × 2-unit cell array was chosen to fabricate, and it was further undergone experimental validation. This proposed tunable MTM exhibits double negative (DNG)/left-handed properties with an average bandwidth of more than 2.8 GHz. Furthermore, attainable negative permittivity and negative permeability are within 2.66 to 9.59 GHz and within 2.77 to 15 GHz, respectively, at the frequency of interest (between 1 and 15 GHz). Moreover, the proposed tunable MTM also showed tunable transmission coefficient characteristics. The proposed electrically tunable textile MTM might function in a dynamic mode, making it suitable for a variety of microwave-wearable applications. A satisfactory agreement between simulations and experiments were achieved, demonstrating that the proposed MTM can operate over a wide bandwidth. [ABSTRACT FROM AUTHOR]

At present, optically controlled terahertz band metamaterials are mainly realized by changing the carrier concentration of semiconductor materials with laser. The composite lifetime of semiconductor materials is generally in nanosecond order, which limits the controlling time. Compared with semiconductor materials, CS2 has a fast light response time of about 1.68 ps and large optical nonlinearity. Based on the structure of sub-wavelength periodic metal block arrays, a strategy to adjust terahertz transmission of metamaterials using CS2 is proposed. Specifically, the control of terahertz wave transmission in this structure is studied by finite difference time domain(FDTD).

Terahertz metamaterial plays a significant role in the development of imaging, sensing, and communications. The function of conventional terahertz metamaterials was fixed after fabrication. They can only achieve a single function and do not have adjustable characteristics, which greatly limits the scalability and practical application of metamaterial. Here, we propose a vanadium dioxide-based terahertz metamaterial device, which is switchable between being a transmitter and an absorber. The transmission and absorption characteristics and temperature tunable properties of phase change metamaterials in the terahertz band were investigated. As the temperature of vanadium dioxide is varied between 20 °C and 80 °C, the device can switch between transmission and quad-band resonance absorption at the terahertz frequency range, with a high transmission rate of over 80% and a peak absorbance of 98.3%, respectively. In addition, when the device acts as an absorber, the proposed metamaterial device is tunable, and the modulation amplitude can reach 94.3%; while the device is used as a transmissive device, the modulation amplitude of the transmission peak at 81%. The results indicate that the proposed metamaterial device can promote the applications of terahertz devices, such as switching, modulation, and sensing.

The performance of metamaterial is limited to a designed narrow band due to its resonant nature, it is highly desirable to incorporate active inclusions in metamaterials to extend the operation bandwidth. This review summarizes the development in realizing the tunability of electromagnetic response in metamaterials incorporated with nematic liquid crystal (LC). From rigorous comparison, it is found that the anisotropic property of nematic LC is essential in predicting the influence of LC molecular director orientation on the resonant frequency of metamaterials. By carefully designing the metamaterials and properly infiltrating LC, the operation frequency of single/double negative parameters of metamaterials can be dynamically modulated with remarkable red/blue-shift, depending on the LC molecular orientation angle. Moreover, the recent liquid crystal-based developments and novel applications are investigated and highlighted.

A band-splicing strategy is proposed to greatly extend the tunable bandwidth of metamaterial absorbers (MMAs) based on circular split-rings (CSRs) containing varactors. In this strategy, the CSRs are split into multiple segments with the absorption frequency continuously tunable in a pre-set frequency range by adjusting varactors that are bridged by PIN diodes, enabling the splicing of the separately tunable frequency bands with the PIN diodes On/Off. By optimizing the parameters of the PIN diodes and varactors following a circuit model, the designed MMA experimentally demonstrated a significantly extended tunable frequency band of 2.8–4.95 GHz, which is spliced by the two tuning bands of 2.8–3.9 GHz (at PIN On) and 3.9–4.95 GHz (at PIN Off). To further show the expandability of this strategy, we also present another tunable MMA which in simulation performs a much wider bandwidth of 2.9–8.12 GHz.

Beam steering is one of the prevailing functions performed by electromagnetic metasurfaces. Its efficiency depends on a large number of physical parameters associated with resonant elements comprising the metasurface and is thus notoriously difficult to optimize. Here we formulate a theoretical model for evaluating the diffraction efficiency of an array of lossy resonant elements whose spectral response is dominated by the coupling between a leaky eigenmode and a single incoming/outgoing channel. We use it to deduce a formula for the maximum attainable diffraction efficiency and the gradient parameter profile for which it is achieved. The optimization procedure is demonstrated on the example of an electrically tunable liquid-crystal terahertz beam steering metasurface. Finally, the proposed model is benchmarked against rigorous metasurface simulations. [ABSTRACT FROM AUTHOR]

A tunable dual‐split‐disk resonator (TDSDR) composed of top and bottom Au split‐disk resonators (SDRs) in the terahertz (THz) wave is presented. TDSDR exhibits electromagnetically induced transparency (EIT) characteristic by changing the rotation angle of top SDR (θ) and distance between top and bottom SDRs (h). The bright and dark modes are defined by the resonances with θ = 90° and 0°, respectively. The resonances could also be controlled by changing h values. It can generate a tunable sharp resonance by moving the top SDR to change h value. Furthermore, the amplitude of EIT resonance could be tuned and switched on/off state, i.e., bright and dark modes by rotating top SDR with specific h value. When h value is 1 µm, the free spectra range (FSR) could be tuned 0.24 THz and EIT characteristic is not clear. By continuously changing h value to 3 µm, EIT characteristic could be tuned with a modulation depth of 75.58%. The average quality factor (Q‐factor) of resonances is 11.54 and average figure of merit (FOM) is 7.06. When h value is enlarged to 5 µm, EIT characteristic could be tuned with a modulation depth of 62.73%, while the average Q‐factor is 14.50 and average FOM is 9.35. [ABSTRACT FROM AUTHOR]

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A diversity of fascinating hollow and porous micro and nanoparticles are being developed for drug delivery purposes; however, their applications in other systems are barely touched. When filled by an active material, they form a tunable photonic metamaterial. Here, a small concentration of organic hollow cylinders called cochleates embedded into liquid crystals (LCs) is demonstrated to form a new switchable composite to control light scattering with voltage and temperature. The biocompatible cochleates, which are hollow, tube‐like, and hydrophilic in nature, can trap the liquid crystal molecules thus altering the system behavior. A switching device as thin as 6 µm with 12 wt% cochleates concentration is shown to be adequate to reveal a switchable privacy window. Possible molecular arrangement of the liquid crystal within the cochleate particles is suggested, and unexpected increase of the scattering with temperature depending on the applied voltage is observed as a result of change in the cochleate aggregation depending on the LC molecule orientation. [ABSTRACT FROM AUTHOR]

• Stress-stiffening in local resonances of rotating acoustic metamaterials can be captured using a stress stiffening function. • Rotating acoustic metamaterials with beam-tip-mass resonators support band gaps that depend linearly on the rotational speed. • Resonator parameters can be chosen such that band gaps of the metamaterial self-adjust to the synchronous vibration frequency, resulting in attenuation over a wide range of frequencies and thus rotational speeds. The rotation of mechanical systems can greatly change their modal and wave propagation response. In the classic example of the Foucault pendulum, the Coriolis force from the Earth's rotation causes the pendulum path to appear deflected to an observer on Earth. More recently, Coriolis forces and gyroscopic effects from rotation have been shown to break time-reversal symmetry, induce non-reciprocity, and change the band structure of phononic crystals. However, rotation also introduces centrifugal forces that can considerably affect the vibration response through stress stiffening and spin softening effects. Although this is well studied in the field of rotor dynamics, the effects of centrifugal forces on the dynamic behavior of acoustic metamaterials (AMs) are still unexplored. In this paper, we study the dependence of torsional band gaps on rotational speed in a rotating shaft with attached local resonances. Different from previous studies, we explicitly consider the effects of stress stiffening in local resonances by introducing a stress stiffening function in a reduced order model of the rotating AM. We then calculate the stress stiffening function for beam-tip-mass resonators and show that these resonators have a resonant frequency that scales linearly with the rotational speed in the asymptotic limit, which in turn causes the band gaps of the AM to depend linearly on the rotational speed. Motivated by the presence of synchronous vibrations in rotating machinery, in which frequency is also linearly dependent on rotational speed, we show that this rotating AM supports band gaps that self-adjust to the synchronous vibration frequency, resulting in attenuation over a wide range of frequencies and thus rotational speeds. Finally, we validate the reduced order model of the rotating AM using 3D finite element analysis. This work contributes a foundational understanding of the effects of rotation in AMs and builds a connection between the AM and rotor dynamics communities, with the potential to introduce novel vibration control in rotor dynamics problems. [ABSTRACT FROM AUTHOR]

An electrically tunable, textile-based metamaterial (MTM) is presented in this work. The proposed MTM unit cell consists of a decagonal-shaped split-ring resonator and a slotted ground plane integrated with RF varactor diodes. The characteristics of the proposed MTM were first studied independently using a single unit cell, prior to different array combinations consisting of 1 × 2, 2 × 1, and 2 × 2 unit cells. Experimental validation was conducted for the fabricated 2 × 2 unit cell array format. The proposed tunable MTM array exhibits tunable left-handed characteristics for both simulation and measurement from 2.71 to 5.51 GHz and provides a tunable transmission coefficient of the MTM. Besides the left-handed properties within the frequency of interest (from 1 to 15 GHz), the proposed MTM also exhibits negative permittivity and permeability from 8.54 to 10.82 GHz and from 10.6 to 13.78 GHz, respectively. The proposed tunable MTM could operate in a dynamic mode using a feedback system for different microwave wearable applications.

• Demonstration of a THz graphene-integrated MMA that exhibit an exceptional absorption. • The proposed absorber allows significant transmission outside the absorption band. • The absorption strength is shown to be systematically tunable between 41% and 91%. • The tunability mechanism is explained by the response of the graphene's conductivity to the applied bias voltage. • The proposed absorber can function like a bidirectional switchable absorption window. THz metamaterials that exhibit excellent absorption properties with real-time tunability have been of great interest for various applications. In this paper, we combine symmetric metamaterials and patterned graphene to achieve an electrically-switchable bidirectional absorption window operating in terahertz frequencies. The proposed metamaterial can homogeneously absorb the incident waves while allowing significant transmission outside the absorption band. Our simulations show that the tunable electrical conductivity of the patterned graphene can result in a desired control of the absorptivity between 41% and 91%. Finite integration simulations, standard retrieval procedure, equivalent circuit model, and effective medium analysis are employed to elaborate our idea. [ABSTRACT FROM AUTHOR]

In this work, a tunable dual-band near-infrared perfect metamaterial absorber formed by combining a highly birefringent nematic liquid crystal with a nanoscale metamaterial cavity arranged in mirror symmetry is designed and numerically investigated. Electromagnetic simulations indicate that the absorbance greater than 99.4% may be achieved at 328 THz and 364 THz. Perfect absorbance results both form the use of highly lossy metal and the optimization of the metamaterial structure. In addition, absorbance of the metamaterial device can be substantially tuned both in terms of its magnitude and wavelength with the spectral tunability up to 8 THz by switching of liquid crystal alignment. The soft-matter-based metamaterial absorbers may pave a crucial role towards various active multifunctional systems working in the near-infrared range. [ABSTRACT FROM AUTHOR]

We propose metal–semiconductor–metal cavity arrays as active elements of electrically tunable metasurfaces operating in the terahertz spectrum. Their function is based on reverse biasing the Schottky junction formed between top metal strips and the n-type semiconductor buried beneath. A gate bias between the strips and a back metal reflector controls the electron depletion layer thickness thus tuning the Drude permittivity of the cavity array. Using a rigorous multiphysics framework which combines Maxwell equations for terahertz waves and the drift-diffusion model for describing the carrier behavior in the semiconductor, we find a theoretically infinite extinction ratio, insertion loss of around 10%, and picosecond intrinsic switching times at 1 THz, for a structure designed to enter the critical coupling regime once the depletion layer reaches the bottom metal contact. We also show that the proposed modulation concept can be used for devices operating at the higher end of the terahertz spectrum, discussing the limitations on their performance. [ABSTRACT FROM AUTHOR]

An electrically-tunable metamaterial is herein designed for the active control of damped elastic waves. The periodic device is conceived including both elastic phases and a piezoelectric phase, shunted by a dissipative electric circuit whose impedance/admittance can be adjusted on demand. As a consequence, the frequency band structure of the metamaterial can be modified to meet design requirements, possibly changing over time. A significant issue is that in the presence of a dissipative circuit, the frequency spectra are obtained by solving eigen-problems with rational terms. This circumstance makes the problem particularly difficult to treat, either resorting to analytical or numerical techniques. In this context, a new derationalization strategy is proposed to overcome some limitations of standard approaches. The starting point is an infinite-dimensional rational eigen-problem, obtained by expanding in their Fourier series the periodic terms involved in the governing dynamic equations. A special derationalization is then applied to the truncated eigen-problem. The key idea is exploiting a LU factorization of the matrix collecting the rational terms. The method allows to considerably reduce the size of the problem to solve with respect to available techniques in literature. This strategy is successfully applied to the case of a three-phase metamaterial shunted by a series RLC circuit with rational admittance. • A new metamaterial is designed for the active control of damped elastic waves. • A periodic microstructured material with a shunted piezoelectric phase is conceived. • An advanced de-rationalization scheme is proposed to study damped wave propagation. • The characterization gives useful information for optimal design of acoustic filters. [ABSTRACT FROM AUTHOR]

We explore by theoretical means an extreme renormalisation of the eigenmodes of a dimer of dipolar meta-atoms due to strong light–matter interactions. Firstly, by tuning the height of an enclosing photonic cavity, we can lower the energy level of the symmetric ‘bright’ mode underneath that of the anti-symmetric ‘dark’ mode. This is possible due to the polaritonic nature of the symmetric mode, that shares simultaneously its excitation with the cavity and the dimer. For a heterogeneous dimer, we show that the polariton modes can be smoothly tuned from symmetric to anti-symmetric, resulting in a variable mode localisation from extended throughout the cavity to concentrated around the vicinity of the dimer. In addition, we reveal a critical point where one of the meta-atoms becomes ‘shrouded’, with no response to a driving electric field, and thus the field re-radiated by the dimer is only that of the other meta-atom. We provide an exact analytical description of the system from first principles, as well as full-wave electromagnetic simulations that show a strong quantitative agreement with the analytical model. Our description is relevant for any physical dimer where dipolar interactions are the dominant mechanism.

Abstract: The quest for unconventional optical materials finds natural answers in the field of plasmonics. Here, special composites can manifest singularities in their dielectric permittivity. The so‐called epsilon‐near‐zero (εNZ) condition is typically encountered in artificial materials called hyperbolic metamaterials (HMMs). Unfortunately, tuning the HMMs εNZ is still challenging. Here it is demonstrated how the εNZ frequency of an HMM can be reversibly tuned via thermally induced water absorption/desorption. The key element is a dielectric hygroscopic material, consisting of a blend of a polymer, a sol–gel unsintered TiO2, and an organic dye. Due to the hygroscopic nature of unsintered TiO2, an increase of temperature induces a reversible physical contraction of the thickness of the dielectric blend, as well as an increase of refractive index. This causes a remarkable 45 nm shift of the absorption peak of the embedded dye, acting as a chromatic label. When such a blend is embedded in an HMM, a reversible thermal tuning of the overall optical response, as well as an epsilon‐near‐zero wavelength shift by about 25 nm, is induced. The remarkable tuning range shown here, besides obvious HMM‐based temperature sensing applications, paves the way toward a plethora of new functions in which tunable εNZ materials are needed. [ABSTRACT FROM AUTHOR]

Conventional metamaterials are usually confined to planar settings due to the fabrication challenges at the nanoscale. Here, we explore the shape transformation of metasurfaces into three‐dimensional metamaterials to generate some on‐demand plasmon modes which would be otherwise unachievable in two dimensions. This method is inspired by the buckling‐induced folding of a layer of metal sheet bonded to a stretchable compliant substrate in some specified regions. Two prototypes of three‐dimensional metamaterials are obtained, demonstrating the enhancement of quality factors as well as the tunability. We find the occurrence of sharp Fano resonance and the resonance mode with toroidal dipole attributed to the structural asymmetry in depth. This work provides an effective approach to practical fabrication of nanoscale metamaterials in an extra dimension that would enable a wide range of applications in the optical spectrum. [ABSTRACT FROM AUTHOR]