Functional Metamaterials and Metadevices

Metamaterials are originally structured as an artificial electromagnetic media, which has developed as a novel approach for engineering the electromagnetic response of passive micro- and nanostructured materials by engaging resonance excitations. This concept is gradually extended to optical, acoustic, thermal, mechanical metasystems, and so on. In each metasystem, the application target is shifting toward achieving tunable, switchable, nonlinear, and sensing functionalities to form relevant metasurfaces and metadevices. This chapter will give a brief review about the concept of evolution from metamaterials to metadevices, focusing on photonic, terahertz, and microwave electromagnetic metamaterials as well as relevant metadevices with functionalities attained through the exploitation of phase-change media, semiconductors, graphene, carbon nanotubes, and liquid crystals. The metadevices encompass microelectromechanical ones and others engaging the nonlinear and quantum response of superconductors, electrostatic, and optomechanical forces, along with nonlinear metadevices incorporating lumped nonlinear components.

More and more metamaterials and metadevices are developed through a general approach from modeling and design, materials selection and synthesis, fabrication processing and optimization, to validation and certification. Great many design approaches and fabrication processes have been explored for each kind of metamaterials including electromagnetic metamaterials, acoustic metamaterials, mechanical metamaterials, and more others. Moreover, Metamaterials are being applied to the development and construction of many new metadevices throughout the electromagnetic spectrum of the electromagnetic metamaterials, for example. Limitations posed by the electromagnetic metamaterial operational bandwidth and losses can be effectively mitigated through the incorporation of tunable elements into the metadevices. This chapter will give a generic review for the commonly used design approaches and tuning methods as well as conventional fabrication methods and additive manufacturing for various scales of 2D and 3D metamaterials and metadevices.

Electromagnetic metamaterials are engineered materials that exhibit controllable and tunable electromagnetic properties within a desired frequency range. They are usually made of periodic metallic resonant inclusions with dimensions much smaller than the operational wavelength. Since their introduction, many applications have been found from the radio (RF) and microwave frequency range up to the terahertz and optical ranges. One key advantage of electromagnetic metamaterial lies in their subwavelength resonators making them suitable for miniaturization of RF circuits and components. This chapter mainly addresses the electromagnetic metamaterials applied in the RF and the microwave frequency ranges, covering background theory, single- and double-negative metamaterials, magneto-dielectrics and zero-index metamaterials, LC-loaded transmission line metamaterials, electromagnetic bandgap, bi-isotropic and bi-anisotropic metamaterials, as well as microwave metamaterial-inspired metadevices.

Terahertz metamaterials are designed to interact at terahertz (THz) frequencies, which is usually defined as 0.1–10 THz. Terahertz regime is located at the interface of electronics and photonics where technologies directly translated from microwave and optical regime generally fail to operate. This is so-called terahertz gap that is caused mainly by weak or nonexistent material response at terahertz frequencies. Coupled with various passive and active media, terahertz metamaterials offer a solution to fill the terahertz gap. Moreover, there is a possibility that terahertz metadevices will become the next enabling technological breakthrough in terahertz technologies, particularly desirable to raise the performance and functionality of terahertz systems in imaging, sensing, spectroscopy, and nondestructive evaluation. This chapter will give a brief review about manipulation technologies and applications of various terahertz metamaterials and metadevices, such as passive type, active type, and flexible metamaterials and metadevices.

Photonic metamaterials are artificially engineered materials containing nanostructures that interact with light, mainly covering infrared or visible wavelengths. Photonic metamaterials have revolutionarily altered the way for designing various optical metadevices with utilization of novel, spatially varying architectures of metamaterials where the electromagnetic properties of every position are carefully prescribed. They bring the promise of creating entirely new prospects for controlling and manipulating photons and provide potential benefits in related fields including optical sensing, miniature antennae, novel waveguides, subwavelength imaging, nanoscale photolithography, and photonic circuits. Compared with microwave and THz metamaterials, photonic metamaterials are generally more difficult to realize, but substantial progress in this area has been achieved. Current research and development are focusing on design optimization, new phenomena exploration, and metadevices postulation. This chapter will review the progress on photonic metamaterials and metadevices, covering nanoscale photonic crystals, transformation optics, light emission and absorption control, as well as hyperbolic, optical dielectric, superconducting and quantum, and nanomechanical photonic metamaterials.

Chiral metamaterials generally consist of arrays of planar metallic or dielectric gammadions on a substrate, where, if linearly polarized electromagnetic wave such as GHz, THz, or visible light is incident on the array, it becomes elliptically polarized upon interaction with the gammadions with the same handedness as the gammadion itself. While the conventional metamaterials require negative permittivity and permeability simultaneously to achieve negative refraction, chiral metamaterials offer an alternative and simpler route to realize negative refraction with a strong chirality while neither negative permittivity nor permeability is required. Furthermore, in combination with strong resonances, optical tuning, and hyperbolicity, chirality can lead to a new route to negative refraction, switchable chirality, photonic topological insulators, and other chiral metadevices. In addition, fabricate approaches such as chiral top-down, bottom-up, direct laser writing, 3D printing, and block copolymer self-assembly methods have been explored for forming various chiral metamaterials. Nevertheless, a combination of smart designs and material property control methods such as (quantum) gain and nonlinearities will be needed for providing a new foundation for many practical applications of chirality. This chapter will briefly review the developments on chiral metamaterials, including fundamental principles, various chiral metamaterials realized by different nanofabrication approaches, and the applications and future prospects of this emerging field.

A plasmonic metamaterial uses surface plasmons to achieve optical properties not seen in nature, while metasurfaces can be utilized to control the electromagnetic waves within one infinitely thin layer, permitting substantial advantages, such as easy fabrication, low cost, and high degree of integration. By patterning plasmonic nanostructures and engineering the spatial phase distribution within the plane, exotic optical phenomena and optical components can be achieved, including negative refraction or reflection, as well as ultrathin focusing or diverging lenses. In other words, metasurfaces tailor the in-plane phase front using an extremely thin slab consisting of judiciously designed plasmonic structures. To some extent, metasurface can be considered as the optical counterpart of frequency selective surface, which introduce prescribed phase distribution within deep subwavelength thickness, leading to exceptional changes of light propagation characteristics. This chapter will review the fundamentals, recent advances, and future perspectives in the emerging field of plasmonic metamaterials and metasurfaces as well as their applications relating to the frequency response, phase shift, and polarization state control, aiming to open up new exciting opportunities for nanoscience and nanotechnology.

Frequency selective surfaces (FSSs) are traditionally formed by two-dimensional periodic arrangement of metallic elements on a dielectric substrate. Depending on the geometry and arrangement of the metallic unit cell, the array might show different functionalities such as band-pass or band-stop spatial filter, absorber, reflect array, and so on. Metamaterials inspired frequency selective surfaces operate based on a different principle that allows superior performance over the traditional structures. For instance, instead of using fully resonant elements as the unit cell of the FSS, nonresonant unit cells with small dimensions are used. The electrical size of the unit cells is decreased to less than l/4 and even in some cases smaller than l/10. These miniaturized elements act as lumped capacitors or inductors and are arranged in a way that they couple to the incident electromagnetic wave. An advantage of this type of FSS is that its frequency behavior can be accurately modelled using lumped element circuit model. Therefore, FSSs with specified functionalities can be designed by the aid of standard circuit-based filter theory. Furthermore, other metamaterials inspired FSSs with different improved functionalities and tunability have also been designed and implemented, such as low-profile second-order band-pass FSS, dual band FSSs with close band spacing, FSS with quasi-elliptical frequency response, and FSSs for high-power microwave and terahertz applications. This chapter will review the progress of the metamaterials inspired FSSs.

Metamaterials have brought unique functionalities by allowing the engineering of the material parameters at the level of their elementary units (meta-atoms) to creating functional metadevices. One of the important developments in this field is the demonstration of many of the nonlinear effects known in nonlinear physics and nonlinear optics such as nonlinear self-action, parametric interactions, and frequency conversion, which will boost the development of various methods for achieving tunable, switchable, nonlinear, and sensing functionalities of metamaterials. The study of nonlinear effects in artificial media and engineering the nonlinear response of such media are crucially important for this progress. In the context of photonic integration, for instance, metamaterials promise pathways for light that are impossible in normal materials and offer new freedom in exploiting nonlinear processes. By incorporating nonlinear and tunable metamaterials, it will be possible to create functional metamaterials that display sensitive tuning and novel or enhanced nonlinear behavior. These materials will ultimately provide the basis of a revolutionary platform for optical processing. This chapter will give a brief review on the update progress of nonlinear metamaterials and inspired functional metadevices.

Acoustic metamaterials have expanded the capabilities of acoustic wave manipulation with diverse application potentials, such as negative refraction, superresolution, cloaking, enhanced absorption, nonreciprocity, active control, and material tunability. Acoustic metamaterials are also expected to affect ultrasonic acoustics, where countless applications, such as medical imagining, lie detection. Owing to the simplicity of the fabrication process—compared to those for electronic and display devices, for example, the acoustic metadevices may be commercialized, targeting old challenges such as noise abatement and selective perception in human audition. Moreover, many of the novel acoustic metamaterial structures have transcended the original definition of metamaterials as arising from the collective manifestations of constituent resonating units, but they continue to extend wave manipulation functionalities beyond those found in nature. The new ideas hatched in acoustic metamaterials research, coupled with the expanding technologies of computational simulation and additive manufacturing, will produce the next generation of acoustical materials and metadevices. This chapter will review the development of acoustic metamaterials from the initial findings of mass density and bulk modulus frequency dispersions in locally resonant structures to the diverse functionalities afforded by the perspective of negative constitutive parameter values and their implications for acoustic wave behaviors, including compact phase manipulation structures, superabsorption, and actively controllable metamaterials as well as the new directions on acoustic wave transport in moving fluid, elastic, and mechanical metamaterials, graphene-inspired metamaterials, and structures whose characteristics are best delineated by non-Hermitian Hamiltonians.

Building upon the success of electromagnetic and acoustic metamaterials, mechanical metamaterials have been developed for obtaining extraordinary or extreme elasticity tensors and mass-density tensors to thereby mold static stress fields or the flow of longitudinal/transverse elastic vibrations in unprecedented ways. With the advances in additive manufacturing techniques that have enabled fabricating materials with arbitrarily complex micro-/nano-architectures, the rationally designed micro-/nano-architecture of mechanical metamaterials gives rise to unprecedented or rare mechanical properties that could be exploited to create advanced materials with novel functionalities. For instance, extremal metamaterials are extremely stiff in certain modes of deformation, while they are extremely soft in other modes of deformation; proper micro- and nano-architectural control can allow for unique material performance such as ultra-lightweight, high stiffness and high strength materials, negative Poisson’s ratio, negative stiffness, and negative thermal expansion coefficient. This chapter will give a brief review focusing on recent advances and remaining challenges in this emerging field. Examples are auxetic, ultra-lightweight, negative mass density, negative modulus, penta-mode, dilational, anisotropic mass density, origami, nonlinear, bistable, reprogrammable, and seismic shielding mechanical metamaterials.

Leveraging structured materials systems to generate tailored response to a stimulus, metamaterials have grown to encompass research in optics, electromagnetics, acoustics, mechanics, and, increasingly, novel hybrid material responses. The field is shifting towards tunable, switchable, nonlinear and sensing functionalities of metamaterials and their applications, involving metasurfaces, metadevices and metasystems. For instance, optical metamaterials have redefined light behaviors from strong response to optical magnetic fields, negative refraction, fast and slow light propagation in zero index and trapping structures, to flat, thin and perfect lenses. This chapter will present emerging fronts and future trends of metamaterials and metadevices, focusing on emerging metamaterials capabilities and new concepts; manipulation of metasurface properties; research trends of nonlinear, active and tunable properties; future metadevices and applications; and prospective manufacturing and assembly technologies.