## 1 Introduction  
The fifth-generation mobile communication technology (5G) has been initially realized and gradually began to be commercialized in the world. Therefore, how to accelerate the basic research of the next generation mobile communication technology, namely the sixth-generation (6G) [1], is the most concerned problem of current researchers. The mainstream frequency band of 5G is 3 GHz-6 GHz, belonging to millimeter wave frequency range. However, to improve the transmission rate, communication bandwidth and information capacity of the system, it has been reported that the working frequency band of 6G network may be elevated to the higher terahertz (THz) region. Due to its high frequency, short pulse, strong penetrability, and low energy, THz wave has been widely used in communication, imaging, sensing and other fields. It is worth mentioning that THz spectrum has not been widely occupied so far, which is very suitable for future wireless communications [2]. Thus, in order to effectively promote the development and application of 6G technology in the future, researchers are committed to developing new THz radiation sources, detectors, and functional devices [3]. At present, THz functional devices are still relatively scarce, and the main limiting factor is the lack of natural reaction materials. It is generally known that the polar insulating material is transparent to THz waves while the metal electron completely shields THz waves, so the most natural materials hardly respond to THz radiation. The emergence of artificial composite materials called metamaterials (MMs) with periodically or randomly arranged subwavelength resonance unit has greatly solved this problem [4]. Different from natural materials, the dielectric constant and permeability of MMscan be arbitrarily changed by adjusting the micro structure units and their arrangements, so as to achieve effective control of THz waves, such as amplitude, phase, polarization, and wavelength. The unique physical properties of MMs usually originate from the resonance effect of structural elements. Resonance response mainly has three essential characteristics including resonant frequency, modulation depth (or resonant intensity) and Q-factor. To achieve sharp resonance with high Q-factor is the most effective strategy to greatly improve the performance of metadevices. It is of great significance in the fields of hypersensitive biochemical sensors [5], high-performance filtering [6], plasmon lasers [7], and slow light devices [8]. For example, using high Q-factor resonance with very narrow linewidth can realize ultra-sensitive real-time chemical and biomolecular detection, and resonance with large modulation depth will improve the detection sensitivity [9; 10]. However, due to the limitation of ohmic loss and radiation loss, the conventional metal MM resonance usually has a lower Q-factor (less than 10), which seriously limits the performance of metadevices. To achieve a high Q-factor resonance response, it is necessary to suppress both radiation loss and ohmic loss. In the THz band, most precious metals have very high conductivity, which can be regarded as perfect conductors. Thus, the ohmic loss of the MMs constructed by them is generally very low while the radiation loss is dominant. To obtain the Fano resonance with high Q-factor, it is very important to minimize the radiation loss of MMs. The theoretical and experimental results have confirmed that by introducing the asymmetric structure into the MMs, the high Q-factor Fano resonance response can be achieved [11; 12]. Fano resonance is a linear asymmetric resonance produced by the elimination or interference of a wide continuous spectrum and a narrow discrete resonance [13]. Its physical mechanism is to drive the anti-phase oscillation current to decrease the radiation loss.