# Dynamic and Active THz Graphene Metamaterial Devices
## 4 Conclusions and Future Perspective  
This review has summarized the fundamental aspects of graphene in terms of linear and nonlinear properties and the optical modulation mechanism of graphene metamaterial. Furthermore, it has reviewed the applications of graphene metamaterial in the THz region, including terahertz amplitude and phase modulators and photodetectors and terahertz generation. The metamaterial structure provides an effective way to enhance light-graphene interactions in a tunable and a feasible fashion, with the advantages of high efficiency, integration, multi-functionality, and low energy consumption. However, limited by the complex graphene fabrication technology and the incompatibility with a wide range of substrates, there are still challenges for the practical utilization of graphene terahertz devices.  
Advanced research on graphene metamaterials in terahertz is developing toward intelligence and integration, and we further note some potential directions. In addition to graphene, other new materials have also been extensively investigated for THz devices, such as 2D transition metal dichalcogenides (TMDCs) [157], BP [158], and topological insulators. These new materials provide new light-matter interactions and mechanism platforms by combining with metamaterial structures, leading to an integrated THz meta-device, and even new THz phenomena. In structural design, artificial intelligence and machine learning are expected to reshape the smart design of meta-devices. Many recent studies have been devoted to metastructure design methods and emerging material platforms, demonstrating ANN-assisted predictions for topological transitions and an inverse design of 2D-topological insulators [159; 160]. Compact and highly tunable THz devices are vital for developing miniaturized and integrated THz technology. Chip-scale integrated devices for terahertz generation and phase modulation were recently demonstrated [137; 161]. They are all based on the waveguide, which could be easily integrated into chips of THz electronics. The combination of two-dimensional materials or 2-DEG and waveguides provides much potential for THz chip-scale integrated devices. It is expected to achieve fully integrated on-chip THz devices, which will significantly promote the miniaturization of THz devices and the development of THz communication devices.  
Conceptualization, L.W. and N.A.; writing--original draft preparation, L.W. and N.A.; writing--review and editing, X.H., X.Z. and A.Z.; supervision, Y.Z. and B.Y.; funding acquisition, Y.Z. and B.Y. All authors have read and agreed to the published version of the manuscript.  
acknowledges the support of the National Key Research and Development Program of China (2021 YFA1401000), the China Postdoctoral Science Foundation (2021M/00706), and the National Science Foundation of China (61901093). B.C.Y. acknowledges the support of the National Science Foundation of China (61975025, U2130106), the National Key Research and Development Program (2021 YFB2800602), and the Key project of Zhejiang Laboratory (2020 KFY00562).  
Not applicable.  
The authors declare no conflict of interest.