Metamaterials and Their Devices, Second Edition

Over the past two decades, metamaterials (MMs) have led a revolution in new material science through the artificial arrangement of electric and magnetic resonance structures (meta-atoms) at subwavelength scale. In particular, they have enriched the fundamental rules of matter–light interactions, such as slow light, super resolution, super-lensing, and electromagnetic (EM) cloaking. The main reason for the attention paid to MMs is that they are very close in appearance to real life materials, such as perfect absorbers. EM MMs reveal remarkable responses to the incident EM wave, such as negative-refraction index, extraordinary optical transmission, electromagnetically induced transparency-like effects, and ultra-thin and broadband absorbers. The designed structures, the structural parameters, and the properties of used materials yield the effective electric permittivity (εeff(ω)) and the effective magnetic permeability (μeff(ω)) of overall MMs, based on the effective-medium theory. Studies on the control of EM response and its spatial distribution and dispersion are ripe and lead to potential and almost-realized applications. There are emerging fields in MM research, such as nonlinear, switchable, gain-assisted, sensor, quantum, and coding MMs, all representing a variety of MM applications. This Special Issue on “Metamaterials and their devices, Second Edition” aims to cover a broad range of topics, including MMs, lattice MMs, crystal materials and structures, plasmonic and dielectric MMs, photonic crystals, phononic crystals, metasurfaces, relevant fundamental issues, emerging fields for MMs, electromagnetic response, magnetic resonance, electric resonance, numerical methods, and applications. This Special Issue contains a mixture of review articles and original contributions.

A long-wave infrared circularly polarized photodetector based on an array of trapezoidal silicon pillars was investigated by Cheng et al. [1]. Integrating metasurface-based polarizing filters atop photodetectors enables the expansion of detection capabilities from intensity to polarization, offering significant potential for applications requiring high-precision discrimination in scientific, industrial, and defense sectors. However, such metasurfaces often introduce optical efficiency losses. They present a long-wave infrared (8.6 μm) circularly polarized photodetector capable of direct chiral discrimination, eliminating the need for additional optical components. The polarization selectivity arises from guided-mode resonances excited by two horizontally offset right-trapezoidal unit cells within a chiral metasurface. This design exhibits a pronounced transmittance contrast (~100%) between left circularly polarized light and right circularly polarized light while maintaining fabrication simplicity via a conventional single-step lithographic process. The proposed detector is expected to achieve high-dimensional physical characterization by resolving polarization-encoded vectorial information, demonstrating enhanced performance in complex environments.

Wavelength conversion in photonic crystal fibers via multiple Raman redshifts and soliton spectral tunneling was studied by Deng et al. [2]. The soliton spectral tunneling effect in photonic crystal fibers with three zero-dispersion wavelengths is an effective way to realize pulse wavelength conversion. However, due to the limitation of the soliton splitting mechanism, forming a tunneling soliton with higher energy and greater width by increasing the number of Raman redshifts remains a key challenge. Airy prime pulses generate polychromatic Raman solitons with small truncation coefficients, and they converge into a stable soliton after the soliton tunneling effect, which provides a new possibility for addressing this problem. This paper discussed how to control the energy, width, and central wavelength characteristics of the tunneling solitons generated in the photonic crystal fiber with three zero-dispersion wavelengths by adjusting the truncation coefficient α, peak power P, and central wavelength λ of the initial Airy prime pulse. The results showed that the smaller the truncation coefficient α, the greater the number of Raman self-frequency shifts and the greater the energy of the formed tunneling soliton. The increase in initial power P₀ will lead to an increase in tunneling soliton width. The larger initial center wavelength λ significantly increases both the width and the center-wavelength position of the tunneling soliton. These findings provide a theoretical basis for the application of Airy prime pulses in ultrafast optical wavelength control and new light source development.

Line-defect phononic crystal structure for directional enhancement detection of weak acoustic signals was examined by Zhang et al. [3]. Effective detection of acoustic signals plays a crucial role in numerous fields, including industrial equipment fault prediction and environmental monitoring. Acoustic sensing technology, owing to its substantial information carrying capacity and non-contact measurement advantages, has garnered widespread attention in relevant applications. However, the effective detection of weak target acoustic signals amidst strong noise interference remains a critical challenge in this field. The core bottleneck lies in the difficulty of traditional detection methods to simultaneously achieve both high sensitivity and high directionality. To address this limitation, this work proposed a line-defect phononic crystal structure that enabled directional enhancement and detection of weak target signals under intense spatial noise interference by coupling defect-state localization characteristics with anisotropy mechanisms. Through theoretical derivation and finite element numerical simulation, the directional enhancement properties of this structure were systematically validated. Furthermore, numerical simulations were conducted to validate the detection of weak harmonic signals and weak bearing fault signals under strong spatial noise interference. The results demonstrated that this line-defect phononic crystal structure exhibited high feasibility and outstanding performance in detecting weak acoustic signals. This work provides novel insights for developing new acoustic detection methods combining high sensitivity with high directivity, showcasing unique advantages and broad application prospects in acoustic signal sensing, enhancement, and localization.

Tandem neural network based design of acoustic MMs for low-frequency vibration reduction in automobiles was also investigated by Deng et al. [4]. Automotive NVH (noise, vibration, and harshness) performance significantly impacts driving comfort and traffic safety. Vehicles exhibiting superior NVH characteristics are more likely to achieve consumer acceptance and enhance competitiveness in the marketplace. In the development of automotive NVH performance, traditional vibration reduction methods have proven to be mature and widely implemented. However, due to constraints related to size and weight, these methods typically address only high-frequency vibration control. Consequently, they struggle to effectively mitigate vehicle body and component vibration noise at frequencies below 200 Hz. In recent years, acoustic metamaterials (AMMs) have emerged as a promising solution for suppressing low-frequency vibrations. This development offers a novel approach for low-frequency vibration control. Nevertheless, conventional design methodologies for AMMs predominantly rely on empirical knowledge and necessitate continuous parameter adjustments to achieve desired bandgap characteristics—an endeavor that entails extensive calculations and considerable time investment. With advancements in machine learning technology, more efficient design strategies have become feasible. This paper presented a tandem neural network specifically developed for the design of AMMs. The trained neural network was capable of deriving both the bandgap characteristics from the design parameters of AMMs as well as deducing requisite design parameters based on specified bandgap targets. Focusing on addressing low-frequency vibrations in the back frame of automobile seats, this method facilitated the determination of necessary AMM design parameters. Experimental results demonstrated that this approach could effectively guide AMM designs with both speed and accuracy, and the designed AMMs achieved an impressive vibration attenuation rate of 63.6%.

Cheng et al. [5] elucidated the design of multifunctional polarization waveplates based on thermal phase-change metasurfaces. The switching function of traditional waveplates necessitates mechanical replacement or the superimposition of multiple waveplates, which gives rise to a complex system and a large volume. They have devised a multifunctional micro-waveplate based on the COMSOL simulation platform (v5.6), which concurrently integrated the compact nature of metasurfaces and the dynamic regulatory features of phase-change materials. When the phase-change material is in the crystalline phase, the metasurface possesses the functionality of a half-waveplate and is capable of performing chirality inversion of circularly polarized light within the wavelength ranges of 1.45 to 1.52 μm and 1.56 to 1.61 μm. When the phase-change material is in the amorphous phase, the metasurface serves as a quarter-waveplate and can achieve the conversion between linear and circular polarization through a 90° phase delay. The phase-change metasurface breaks through the constraint of fixed functions of traditional optical waveplates, facilitating the development of optical systems towards miniaturization, intelligence, and low power consumption, and providing a crucial technical route for the next generation of photonic integration and dynamic optical applications.

Evolution of phonon spectral energy density in superlattice structures was examined by Nasiri and Wang [6]. Superlattices are a distinctive class of artificial nanostructures formed by the periodic stacking of two or more materials. The high density of interfaces in these structures often gives rise to exotic physical properties. In the context of thermal transport, it is well established that such interfaces can significantly scatter particle-like phonons while also inducing constructive or destructive interference in wave-like phonons, depending on the relationship between the phonons’ coherence lengths and the superlattice’s period thickness. In this work, they systematically investigated the effect of temperature on the spectral energy density of phonon modes in superlattices. Additionally, we examine how variations in superlattice period thickness influence phonon lifetimes and energy density. Our findings provide critical insights into the spectral phonon properties of superlattices, particularly in terms of their coherence and lifetimes.

The design and processing of metamaterials was reviewed by Matei et al. [7]. Metamaterials represent artificially structured materials that exhibit unusual properties, such as a negative refractive index, negative permeability and permittivity, negative Poisson’s ratios, and unique optical effects, which are inaccessible in natural materials. According to recent developments, novel devices and tools based on MMs are attracting great interest as they offer improved performance, functionality, sensitivity, biocompatibility, complex structures, and design freedom. Leveraging numerical design approaches, such as finite element analysis and finite difference time domain methods, researchers have tailored metamaterials to meet specific requirements in various areas through a range of manufacturing techniques. These materials can be broadly classified into optical, mechanical, thermal, electromagnetic, and acoustic categories based on their properties and intended use. The choice of fabrication method depends heavily on the specific application, the desired scale, and the complexity of the metamaterial design. These manufacturing methods can be broadly divided into top-down and bottom-up approaches, while each of them has advantages and limitations and offers valuable pathways for the development of the final product. This review offered a basic overview of metamaterials, covering their fundamental principles, fabrication and characterization techniques, and current design methodologies. It also explored their diverse applications, including specific case studies in medicine, while addressing existing limitations and challenges. Finally, this review highlights future perspectives, emphasizing the need for continued innovation in fabrication and characterization to unlock the full potential of MMs.

Soroosh et al. [8] studied a compact and fast resonant cavity-based encoder on a photonic crystal platform. The soliton spectral tunneling effect in photonic crystal fibers with three zero-dispersion wavelengths is an effective way to realize pulse wavelength conversion. However, due to the limitation of the soliton splitting mechanism, forming a tunneling soliton with higher energy and greater width by increasing the number of Raman redshifts remains a key challenge. Airy prime pulses generate polychromatic Raman solitons with small truncation coefficients, and they converge into a stable soliton after the soliton tunneling effect, which provides a new possibility for addressing this problem. This paper discussed how to control the energy, width, and central wavelength characteristics of the tunneling solitons generated in the photonic crystal fiber with three zero-dispersion wavelengths by adjusting the truncation coefficient α, peak power P, and central wavelength λ of the initial Airy prime pulse. The results showed that the smaller the truncation coefficient α, the greater the number of Raman self-frequency shifts and the greater the energy of the formed tunneling soliton. The increase in initial power P₀ led to an increase in tunneling soliton width. The larger initial center wavelength λ significantly increased the width and center-wavelength position of the tunneling soliton. These findings provide a theoretical basis for the application of Airy prime pulses in ultrafast optical wavelength control and new light source development.

Sun et al. [9] examined an embedded rough-neck Helmholtz resonator low-frequency acoustic attenuator. Effective detection of acoustic signals plays a crucial role in numerous fields, including industrial equipment fault prediction and environmental monitoring. Acoustic sensing technology, owing to its substantial information-carrying capacity and non-contact measurement advantages, has garnered widespread attention in relevant applications. However, the effective detection of weak target acoustic signals amidst strong noise interference remains a critical challenge in this field. The core bottleneck lies in the difficulty of traditional detection methods to simultaneously achieve both high sensitivity and high directionality. To address this limitation, this work proposed a line-defect phononic crystal structure that enabled directional enhancement and detection of weak target signals under intense spatial noise interference by coupling defect-state localization characteristics with anisotropy mechanisms. Through theoretical derivation and finite element numerical simulation, the directional enhancement properties of this structure were systematically validated. Furthermore, numerical simulations were conducted to validate the detection of weak harmonic signals and weak bearing fault signals under strong spatial noise interference. The results demonstrated that this line-defect phononic crystal structure exhibited high feasibility and outstanding performance in detecting weak acoustic signals. This work provides novel insights for developing new acoustic detection methods combining high sensitivity with high directivity, showcasing unique advantages and broad application prospects in acoustic signal sensing, enhancement, and localization.

The present Special Issue on “Metamaterials and Their Devices, Second Edition” can be considered a status report reviewing the progress achieved over the past two decades in several subject areas affected by metamaterials and their devices.