Introduction

Metamaterials, artificially engineered structures with properties not found in nature, have garnered significant attention in recent years. This mini-review focuses on advancements in metamaterial design over the past five years (2020-2024), based on the provided list of papers. We will explore two major research agendas: (1) Tunable Metamaterial Absorbers and (2) Programmable Mechanical Metamaterials.

Tunable Metamaterial Absorbers

A significant portion of recent metamaterial research has focused on developing tunable absorbers, particularly in the terahertz (THz) and infrared regimes. These absorbers find applications in sensing, cloaking, and energy harvesting. Early work in this area explored various materials and designs to achieve broadband and multi-band absorption.

Terahertz Absorbers: Ben-Xin Wang's research group investigated dual-band THz metamaterial absorbers using square patches for sensing in 2020 (Ben‐Xin Wang et al., 2020, Nanoscale Advances). Later, Ben-Xin Wang's group designed multiple-frequency-band THz metamaterial absorbers with adjustable absorption peaks using toothed resonators (Ben‐Xin Wang et al., 2023, Materials & Design). In 2023, Ben-Xin Wang's group also demonstrated a triple-band electromagnetically induced transparency (EIT) THz metamaterial for sensing applications (Ben‐Xin Wang et al., 2023, Nanoscale). Yongzhi Cheng's group explored graphene-based THz absorbers, demonstrating a broadband tunable absorber based on a single-layer complementary gammadion-shaped graphene in 2020 (Fu Chen et al., 2020, Materials (Basel)). Kai-Da Xu's group presented a high Q-factor dual-band THz metamaterial absorber with sensing characteristics (Dongxu Wang et al., 2023, Nanoscale).

Tunable Materials: Vanadium dioxide (VO2) has emerged as a popular material for achieving tunability due to its temperature-dependent phase transition. Zao Yi's research group extensively investigated thermal tuning of THz metamaterial absorber properties using VO2 (Zhipeng Zheng et al., 2020, Physical Chemistry Chemical Physics, Zhipeng Zheng et al., 2021, Physical Chemistry Chemical Physics, Zhipeng Zheng et al., 2022, Physical Chemistry Chemical Physics). They also demonstrated a switchable THz device combining ultra-wideband absorption and reflection using VO2 (Zhipeng Zheng et al., 2021, Physical Chemistry Chemical Physics, Zhipeng Zheng et al., 2022, Physical Chemistry Chemical Physics). Bin Tang's group explored thermally switching between perfect absorption and asymmetric transmission using VO2-assisted metamaterials (Yi Ren et al., 2021, Optics Express) and designed an active tunable THz functional metamaterial based on hybrid-graphene vanadium dioxide (Haonan Qi et al., 2023, Physical Chemistry Chemical Physics). Shubo Cheng's group has also explored the use of Dirac semimetals for high sensitivity tunable metamaterial absorption devices (Shubo Cheng et al., 2024, Optics Communications).

Broadband and Ultra-wideband Absorbers: Yichun Liu's group focused on ultra-broadband metamaterial absorbers in the long to very long infrared regime (Yu Zhou et al., 2020, Light Science & Applications, Yu Zhou et al., 2021, Light Science & Applications). Shubo Cheng's group designed high absorptivity and ultra-wideband solar absorbers based on Ti-Al2O3 cross elliptical disk arrays (Hongjie Zhang et al., 2023, Coatings (Basel)).

Sensing Applications: Several papers highlight the application of metamaterial absorbers for sensing. Jin Zhang's group demonstrated highly sensitive detection of malignant glioma cells using a THz biosensor based on electromagnetically induced transparency (Jin Zhang et al., 2020, Biosensors and Bioelectronics, Jin Zhang et al., 2021, Biosensors and Bioelectronics). Md. Moniruzzaman's group designed a perfect metamaterial absorber for S, X and Ku band microwave sensing applications (M G Rabbani et al., 2024, Sci. rep. (Nat. Publ. Group)).

Programmable Mechanical Metamaterials

Another significant area of advancement is in programmable mechanical metamaterials, which can be designed to exhibit specific mechanical properties and functionalities. This includes the development of reprogrammable metamaterials, auxetic metamaterials (materials with a negative Poisson's ratio), and metamaterials for vibration suppression and energy absorption.

Reprogrammable Metamaterials: Pedro M. Reis's group demonstrated a reprogrammable mechanical metamaterial with stable memory (Tian Chen et al., 2020, Nature, Tian Chen et al., 2021, Nature). Chang Chen's group developed a mechanical metamaterial with reprogrammable logical functions (Mei Tie et al., 2021, Nature Communications) and deployable mechanical metamaterials with multistep programmable transformation (Zhiqiang Meng et al., 2022, Science Advances). Xin Fang's group introduced programmable gear-based mechanical metamaterials (Xin Fang et al., 2021, Nature Materials, Xin Fang et al., 2022, Nature Materials). Rui Zhu's group engineered zero modes in transformable mechanical metamaterials (Zhou Hu et al., 2023, Nature Communications).

Auxetic Metamaterials: Xin Ren's group has extensively studied auxetic metamaterials, exploring their mechanical properties and potential applications (Ting Huang et al., 2021, Engineering Structures, Xiang Yu Zhang et al., 2022, Thin-Walled Structures, Xian Cheng et al., 2022, International Journal of Mechanical Sciences, Tao Zhi et al., 2022, International Journal of Mechanical Sciences, Xing Chi Teng et al., 2022, International Journal of Mechanical Sciences). Krzysztof K. Dudek's group investigated micro-scale auxetic hierarchical mechanical metamaterials for shape morphing (Krzysztof K. Dudek et al., 2022, Advanced Materials) and micro-scale mechanical metamaterials with a controllable transition in the Poisson's ratio and band gap formation (Krzysztof K. Dudek et al., 2023, Advanced Materials).

Vibration Suppression and Energy Absorption: Andrew C. M. Austin's group explored metamaterial beams with graded local resonators for broadband vibration suppression (Guobiao Hu et al., 2020, Mechanical Systems and Signal Processing). Gengkai Hu's group tailored mechanical metamaterials with programmable quasi-zero-stiffness features for full-band vibration isolation (Quan Zhang et al., 2021, Advanced Functional Materials). Chunlei Li's group investigated hierarchical re-entrant honeycomb metamaterials for energy absorption and vibration insulation (Nanfang Ma et al., 2023, International Journal of Mechanical Sciences) and non-contact electromagnetic controlled metamaterial beams for low-frequency vibration suppression (Yu Sun et al., 2024, International Journal of Solids and Structures). Zhendong Li's group developed a ribbed strategy that disrupts conventional metamaterial deformation mechanisms for superior energy absorption (Xinxin Wang et al., 2024, Virtual and Physical Prototyping).

Conclusion

The past five years have witnessed significant progress in metamaterial design, particularly in the areas of tunable absorbers and programmable mechanical metamaterials. Research has focused on exploring new materials, designs, and functionalities, leading to advancements in sensing, cloaking, vibration suppression, and energy absorption. Future research directions may include exploring more complex metamaterial architectures, developing advanced manufacturing techniques, and integrating metamaterials into real-world applications.