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1. 3D metamaterials

   https://doi.org/10.1038/s42254-018-0018-y  

   Kadic, Muamer ; Milton, Graeme W. ; van Hecke, Martin ; Wegener, Martin ( March 2019 , Nature Reviews Physics)
2. Ballistic metamaterials

   https://doi.org/10.1364/OPTICA.402891  

   Li, Kun ; Simmons, Evan ; Briggs, Andrew_F ; Bank, Seth_R ; Wasserman, Daniel ; Podolskiy, Viktor_A ; Narimanov, Evgenii_E ( December 2020 , Optica)

   We report the theoretical prediction and experimental realization of the optical phenomenon of “ballistic resonance.” This resonance, resulting from the interplay between free charge motion in confining geometries and periodic driving electromagnetic fields, can be utilized to achieve negative permittivity at frequencies well above the bulk plasma frequency. As a proof of principle, we demonstrate all-semiconductor hyperbolic metamaterials operating at frequencies 60% above the plasma frequency of the constituent doped semiconductor “metallic” layer. Ballistic resonance will therefore enable the realization and deployment of various applications that rely on local field enhancement and emission modulation, typically associated with plasmonic materials, in new materials platforms.

    

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3. 2D Quantum Metamaterials

   Kirk, Wiley P ; Randall, John N ; Owen, James H ( September 2019 , World scientific research)

   Exciting developments in strategic areas of science and engineering makes for possible new engineered structures identified as quantum metamaterials. These new structures offer unusual properties that involve fundamental concepts such as entangled quantum states, superposition, quantum coherence, analog quantum simulation, etc., opening a new era of technological advancement. This manuscript presents the output of a recent workshop held at the National Institute of Standards and Technology in 2018. It covers the key scientific ideas, various technical approaches under investigation, and the potential technological outcomes in a new field of research. 

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4. Magnetoactive Acoustic Metamaterials

   https://doi.org/10.1002/adma.201706348  

   Yu, Kunhao ; Fang, Nicholas X. ; Huang, Guoliang ; Wang, Qiming ( April 2018 , Advanced Materials)

   Acoustic metamaterials with negative constitutive parameters (modulus and/or mass density) have shown great potential in diverse applications ranging from sonic cloaking, abnormal refraction and superlensing, to noise canceling. In conventional acoustic metamaterials, the negative constitutive parameters are engineered via tailored structures with fixed geometries; therefore, the relationships between constitutive parameters and acoustic frequencies are typically fixed to form a 2D phase space once the structures are fabricated. Here, by means of a model system of magnetoactive lattice structures, stimuli‐responsive acoustic metamaterials are demonstrated to be able to extend the 2D phase space to 3D through rapidly and repeatedly switching signs of constitutive parameters with remote magnetic fields. It is shown for the first time that effective modulus can be reversibly switched between positive and negative within controlled frequency regimes through lattice buckling modulated by theoretically predicted magnetic fields. The magnetically triggered negative‐modulus and cavity‐induced negative density are integrated to achieve flexible switching between single‐negative and double‐negative. This strategy opens promising avenues for remote, rapid, and reversible modulation of acoustic transportation, refraction, imaging, and focusing in subwavelength regimes.

    

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5. Topological hyperbolic metamaterials

   https://doi.org/10.1515/nanoph-2023-0768  

   Li, Zhitong ; Gu, Qing ( February 2024 , Nanophotonics)

   Hyperbolic metamaterial (HMM) is a unique type of anisotropic material that can exhibit metal and dielectric properties at the same time. This unique characteristic results in it having unbounded isofrequency surface contours, leading to exotic phenomena such as spontaneous emission enhancement and applications such as super-resolution imaging. However, at optical frequencies, HMM must be artificially engineered and always requires a metal constituent, whose intrinsic loss significantly limits the experimentally accessible wave vector values, thus negatively impacting the performance of these applications. The need to reduce loss in HMM stimulated the development of the second-generation HMM, termed active HMM, where gain materials are utilized to compensate for metal’s intrinsic loss. With the advent of topological photonics that allows robust light transportation immune to disorders and defects, research on HMM also entered the topological regime. Tremendous efforts have been dedicated to exploring the topological transition from elliptical to hyperbolic dispersion and topologically protected edge states in HMM, which also prompted the invention of lossless HMM formed by all-dielectric material. Furthermore, emerging twistronics can also provide a route to manipulate topological transitions in HMMs. In this review, we survey recent progress in topological effects in HMMs and provide prospects on possible future research directions.

    

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