# Dynamic and Active THz Graphene Metamaterial Devices
## 3 Applications of Graphene Metamaterial
### THz Generation
(**c**) Tunability of HHG efficiency as a function of gating voltage, Inset: the driving terahertz signal; (Reprinted with permission from ref. [26] Copyright 2021, American Association for the Advancement of Science) (**d**) Graphene on silicon nitride waveguide (GSiNW) architecture, Inset: Dirac cone structure of the DFG process. (**e**) Tuning of the graphene terahertz plasmon signal. (**f**) DFG signal intensity increases linearly with the pump rising; (Reprinted with permission from ref. [137]. Copyright 2018, Nature Publishing Group) (**g**) Graphene-nanoribbon structure of THz source. (**h**) Tunability of plasmonic emission of the device. (**i**) Pump power dependence of plasmonic emission from graphene nanoribbons. (Reprinted with permission from ref. [139] Copyright 2019, American Chemical Society). gratings [27; 147], and split-ring resonators [148; 149]. Thus, the combination of metamaterial and graphene could significantly improve the HHG efficiency of graphene. Furthermore, there are many studies for HHG enhancement with graphene metasurfaces [141; 150]. For example, [PERSON] et al. designed a graphene-covered groove grating structure to generate third-order harmonics [151]. The grating groove structure localized and strongly enhanced the electric field in the groove, which improved the nonlinear optical response. Compared with the ordinary flat substrate covered by graphene, the generated third harmonic power increased by nearly 20 times. The conversion rate of the third harmonics increased with the enhancement of the incident THz field. Later, [PERSON] et al. reported a graphene-metal grating platform to provide field enhancement experimentally. Compared with the device without metal grating, the nonlinear efficiency of the graphene-metal grating structure improved by 50 times and by three orders of magnitude for the third harmonic [152]. Compared with HHG, the THz generation based on the graphene DFG does not need the incident of other THz fields. It is achieved using laser sources at near-infrared frequencies, with the energy conversion from the input optical energy to the THz signal [153]. In 2014, [PERSON] et al. proposed a scheme to generate THz plasmons with DFG. The theoretical result showed that the efficient generation of THz plasmon could be achieved through the DFG process of graphene, and the generated THz plasmon could be changed by adjusting the incident angle and the graphene Fermi level. In addition to the case of the free-space pump, they also discussed the composite structure of graphene waveguides, demonstrating the feasibility of this method for on-chip generation of THz plasmons [53]. Later, [PERSON] et al. proved experimentally the all-optical generation of THz plasmons. Two visible light pulses and a generated THz plasmon matched both the wavevector and the energy. The result showed that the excited THz plasmon could have a defined wavevector and direction across an extensive frequency range, with a conversion efficiency of \(10^{-5}\)[135]. However, free-space pump-based THz generation is not suitable for integration. In 2013, [PERSON] et al. proposed a method to generate THz plasmons with the graphene composite waveguide. Unlike the THz plasmon generation by DFG, the FWM as a third-order nonlinear excitation of the THz plasmon was presented in their work. The numerical calculation showed that the generated THz plasmon power reached 67 W when the input pump power was 1 kW. Nevertheless, other factors in the numerical calculation were not considered, such as the low third-order nonlinear conversion efficiency [134], other FWM processes, or the graphene Fermi level, so the actual conversion efficiency could be much lower. Until recently, experimental results for graphene FWM based THz plasmon had not been reported. However, in 2018, both theoretical and experimental results for THz plasmon generation on a graphene composite waveguide were reported: [PERSON] et al.