The exotic wave phenomena in metastructures and their wide applications are one of the most researched subjects in electromagnetic waves and photonics, from radio frequencies to optics, and extend into several other disciplines, such as acoustics, mechanics, thermodynamics, materials science, condensed matter, etc. One of the most exciting opportunities for wave engineering using metastructures consists in the manipulation of wave–matter interactions involving nonlinear optics, semiconductor physics, 2D materials, soft matter, quantum matter, etc. What we have learned from the physics of metastructures, since the start of research in metamaterials and metasurfaces,1–5 has been stimulating a much broader set of topics than originally thought; the excellent papers published within this Special Topic in Applied Physics Letters are a testament of that.6–52 Despite focusing on different topics, all the papers within this Special Topic collection share the same point of view: we can push the boundaries of wave physics for a broad range of applications, and we can engineer novel and on-demand physical properties to enhance current devices and even conceive new functionalities. Indeed, an important aspect of all research activities spanning the general area of metamaterials is its interdisciplinary nature, covering a wide range of expertise, in fundamental and applied physics, engineering, materials science, and beyond.
The idea of this Special Topic originated in the context of the 2020 International Congress on Artificial Materials for Novel Wave Phenomena, which, indeed, has been bringing together a wide range of scientists and expertise to facilitate these interdisciplinary discussions and interactions. The area of metastructures is much broader than it was years ago, and it has been growing since researchers in various disciplines are now mastering nanofabrication and applying concepts from quantum mechanics and condensed matter physics to conceive newer wave phenomena. Indeed, a large portion of the research presented in this Special Topic would not have happened without the growing progress in nanofabrication techniques, allowing the control of materials and, hence, field manipulation at nanoscale. Even electrically large structures like metasurfaces benefit from nanoscale fabrication methods because of the control of the field and its gradient at nanoscale.
This Special Topic in Applied Physics Letters collects recent advances in the broad area of metastructures. Though it is difficult to address the specific achievements, in particular, individual areas, since the field is broad, we will provide here highlights of some of the papers in this collection. We list them here, grouped within broad areas of interest.
Metasurfaces and gradient metastructures have been showing extreme opportunities for the manipulation of the reflected/transmitted wavefront, to realize ultrathin photonic devices and very flexible and versatile radio frequency reflectors. The contributed papers are Refs. 6–27.
As exemplary results within this topic, Chen and Capasso12 talk about metasurface-based components consisting of regular arrays of sub-wavelength dielectric nanostructures, providing also an outlook on future trends. They discuss that metasurface concepts are not only leading to a reduction in size and complexity of optical components, but also bring new functionalities, like a full-Stokes metasurface camera and a metasurface depth sensor. Zhang et al.13 investigate the effects of using a hyperuniform disordered distribution of metasurface elements to enlarge the operating bandwidth, with a significant reduction in radar cross section of the surface. This research shows that moving away from strictly periodic unit cells (with a gradient distribution) is a promising direction for possible future investigations. The paper by Kuznetsov et al.16 elaborates and experimentally demonstrates on a single-layer metasurface whose elements are based on pairs of complementary structures to manipulate circular wavefronts, yielding an effective approach to manipulate the optical wavefront. The paper by Faenzi et al.18 discusses metasurfaces as radiators, which has been a topic of intense investigation in the past decade. In this paper, the authors describe the generation of directive beams at two different frequencies within the same metasurface antenna based on surface wave excitation. Previously, metasurface antennas could generate tailored beams of radiation at a single frequency. The paper by Lundgren et al.20 discusses and demonstrates the concept of a metasurface absorbing a radio frequency field generated by an external device, that is then imaged using an infrared camera, hence using heat to image electromagnetic fields. Vabishchevich et al.22 demonstrate ultrafast all-optical on/off switching using semiconductor metasurfaces made of resonators that support both dipolar and quadrupolar Mie resonances. The authors show that multipole engineering with lattice diffraction opens design pathways for tunable metasurface-based integrated devices. Moreno-Peñarrubia et al.23 demonstrate an ultrathin and high-efficiency Pancharatnam–Berry phase metalens for millimeter waves using meta-atoms made of H shaped (i.e., dogbone shaped) pairs of conductors. The metalens focuses the wavefront of a circularly polarized incident wave and converts its handedness within an ultrathin profile composed of just two layers of patterned metals.
Another topic of recent interest in the area of metastructures is non-Hermitian electromagnetics and exceptional points, concepts that have been leading to interesting new wave phenomena. In particular, it has been shown that the effect of a perturbation on the eigenvalues of a system is not linearly proportional to the amount of perturbation, but rather to its square root (for an exceptional point of order two), as discussed in Ref. 28. The concept of having a waveguide with distributed gain and radiating antennas, leading to a distributed coherent radiating oscillator operating at an exceptional point, is presented in Ref. 29.
Ramaccia et al.30 discuss the extension to temporal interfaces of filter engineering, realizing wave manipulation by time-switching a uniform material. This approach appears to be very exciting for the prospects of metamaterials, introducing a temporal dimension to metamaterial design. In this same broad area, Guenneau et al.31 discuss the temporal evolution of a metamaterial cloak response in the context of elastic waves. Also Fujii et al.34 discuss cloaking mechanisms for acoustic and elastic waves, opening to the next important topic featured in this Special Topic collection.
In the context of acoustic, elastic, and mechanical waves, our Special Topic features several exciting papers, since this is an area of growing interest in the community of engineered materials. Huang et al.32 as well as Mi et al.36 and Zaccherini et al.37 discuss various metamaterial geometries supporting broadband absorption exploiting sophisticated sound-matter interactions. Bilal et al.33 introduce a complex metamaterial geometry to realize multiplexing and de-multiplexing for sound.
Metastructures have been found very promising in the manipulation and enhancement of wave–matter interaction at classical and quantum levels. For example, Tonkaev et al.40 show how to accelerate photoluminescence with smart engineering of photonic density of states by depositing a perovskite film on a hyperbolic metamaterial. The authors experimentally confirm the acceleration of radiative recombination by almost three times. Liberal and Ziolkowski42 analyze the wealth of decay dynamics phenomena that can be observed in metamaterial waveguides that have a complex dispersion profile. They explore the nonperturbative decay dynamics of a quantum emitter coupled to a composite right-/left-handed transmission line, including a “mu-near-zero band edge” and an “epsilon-near-zero band edge.” The decay rates associated with the spectral features are related to branch cut singularities that contribute with fractional decay dynamics.
Another broad area of research interest in metamaterials focuses on topological phenomena. The main idea, borrowed from condensed matter physics, is that the nontrivial topological features of the band diagram of a metamaterial can be translated into an unusually robust boundary propagation of waves, of great interest for various applications from microwaves, photonics to acoustics. Several exciting works on this topic are featured in this collection. For instance, Yu et al.44 exploit resonant coupling to realize broadband slow light within a topological bandgap, with important implications for photonic technologies. Proctor et al.45 discuss a metamaterial geometry supporting higher-order topological states, which implies localization of states within a topological bandgap at higher dimensions. Kandil and Sievenpiper46 investigate spin-dependent unidirectional propagation in a waveguide consisting of C-shaped metallic particles characterized by extrinsic chirality and strong transverse spin, and the unidirectionality sensitivity to various defects in the chiral waveguide that flips the spin direction of the wave. Nora Rosa et al.48 demonstrate an elegant and simple implementation of quasi-periodic one-dimensional metamaterials using LEGO blocks, reporting nontrivial topological features.
These and several other papers in this Special Topic collection (we could not discuss all of them for the sake of brevity) demonstrate the rich opportunities emerging from applying interdisciplinary topics to the field of engineered materials.
This Special Topic collection provides an opportunity for the broad readership of Applied Physics Letters to get a glimpse of the recent advances in the area of metastructures and their applications. We hope that this selected collection of articles may serve as a platform to further stimulate researchers to expand their horizon beyond the classical fields of research in electromagnetics, photonics, and wave physics in general and, thus, further advance the potential advantages of metastructures for future applications.
The authors thank Lesley Cohen, Editor-in-Chief, for her constant guidance, Emma Nicholson Van Burns, Journal Manager, and Jessica Trudeau, Editorial Assistant, for their technical assistance with publishing.
DATA AVAILABILITY
Data sharing is not applicable to this article as no new data were created or analyzed in this study.
