Metamaterial is a kind of material/structure that is artificially designed and has exceptional electromagnetic properties and/or other physical properties, not found in nature. A class of electromagnetic metamaterial with only one or a few layers of periodic or aperiodic arranged cell structures in the direction of electromagnetic waves propagation can be referred to as a metasurface. Metasurface can be considered as a two-dimensional representation of metamaterial and can realize the controlling of the amplitude, phase, and polarization state/direction of the incident electromagnetic wave. According to the novel electromagnetic characteristics of metasurface and its big advantages, a series of new planar devices and systems based on metasurface can be developed. The goal of this review article is firstly to provide introductions for basic metasurface, its significance properties, and application principles. Meanwhile, the main research progresses of regular metasurfaces and the newly developed reconfigurable metasurfaces are analyzed, focusing on the types of amplitude modulation, phase modulation, polarization modulation, and multidimensional modulation. Finally, the research significances of metasurface development trend and important engineering practical applications are analyzed in the end.
Achieving a high Q-factor metamaterial unit for a precision sensing application is highly demanded in recent years, and most of the developed high-performance sensors based on the high-Q metamaterial units are due to the dielectric/magnetic property changes of the substrate/superstrate. In this paper, we propose a completely different sensing metamaterial unit configuration, with good sensing sensitivity and precision properties, based on the thermally tunable liquid metals. Specifically, a basic thermally tunable metamaterial unit, the mercury-inspired split ring resonator (SRR), is firstly presented to theoretically show the magnetic resonance and negative permeability frequency band shift properties under different background temperatures. Then, considering the radiation loss mechanism of the conventional SRR metamaterial unit and based on the physically reliable ability of liquid metals, the modified mercury-inspired Fano and toroidal resonators with a large frequency tuning range and high Q-factor are developed and discussed. The numerical demonstrations have shown that the designed Fano and toroidal resonators have much better sensing precision performances compared to the conventional SRR for the temperature sensing application. The experimental demonstrations have also been used to verify the proposed mercury-based toroidal resonators, and good agreements are achieved.
High-performance temperature sensing is a key technique in modern Internet of Things. However, it is hard to attain a high precision while achieving a compact size for wireless sensing. Recently, metamaterials have been proposed to design a microwave, wireless temperature sensor, but precision is still an unsolved problem. By combining the high-quality factor (Q-factor) feature of a EIT-like metamaterial unit and the large temperature-sensing sensitivity performance of liquid metals, this paper designs and experimentally investigates an Hg-EIT-like metamaterial unit block for high figure-of-merit (FOM) temperature-sensing applications. A measured FOM of about 0.68 is realized, which is larger than most of the reported metamaterial-inspired temperature sensors.
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