# Dynamic and Active THz Graphene Metamaterial Devices
## 3 Applications of Graphene Metamaterial
### THz Generation  
Compared with the graphene THz devices mentioned above, graphene-based THz generation is still in a much earlier stage. Because the traditional THz radiation equipment is complicated and expensive, the generation of THz is significantly more challenging than generating more extended wavelength sources (microwaves). Table 4 shows the recent evolution of THz generation, and we could find that different structures combined with different mechanisms result in different characteristics and performances of THz generators. In the following content, we briefly discuss the recent progress of THz generation based on different mechanisms, including the generation and the tunability of THz sources based on different mechanisms. High harmonic generation could convert the incident THz signals into signals with a much higher frequency, which is critical for generating THz sources with a specific frequency [141]. Figure 8a shows the schematic of the odd-order harmonics emitted from the monolayer graphene [26]. The transmission rate of the fundamental wave is more than 90% for the generated third, fifth, and seventh harmonics; the transmission rate is slightly lower than \(10^{-1}\), \(10^{-2}\), and \(10^{-3}\), respectively, in the normalized spectral amplitude for 0-2.5 THz (Figure 8b). As the pump power increases, the HHG efficiency tends to saturate, caused by the dissipative nature of the interaction between the THz field and the carriers (Figure 8c). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Work** & **Structure** & **Mechanism** & **Efficiency** & **Ref.** \\ \hline [PERSON] et al. & SiO\({}_{2}\) substrate & HHG & \(10^{-3}\) & 2018 [25] \\ [PERSON] et al. & metasurface & THG & \(10^{-2}\) & 2022 [133] \\ [PERSON] et al. & gate-tunable graphene & HHG & \(\sim\)10\({}^{-2}\) & 2021 [26] \\ [PERSON] et al. & waveguide & FWM & 6.7\% & 2014 [134] \\ [PERSON] et al. & SiO\({}_{2}\) substrate & DFG & \(6\times 10^{-6}\) & 2016 [135] \\ [PERSON] et al. & graphene/AlGaAs waveguide & DFG & \(4\times 10^{-6}\) & 2018 [136] \\ [PERSON] et al. & gate-tunable graphene waveguide & DFG & \(\sim\)0.6 \(\times 10^{-4}\) & 2018 [137] \\ [PERSON] et al. & gate-tunable graphene & current-driven & - & 2019 [138] \\ [PERSON] et al. & gate-tunable graphene nanoribbons & current-driven & \(\sim\)2 \(\times 10^{-8}\) & 2019 [139] \\ [PERSON] et al. & gate-tunable graphene & current-driven & - & 2021 [140] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of THz generation. Besides, the improvement of HHG efficiency is also significant. Because of the graphene tunability [142; 143; 144], the HHG efficiency could be improved by tuning the Fermi level of graphene. For example, [PERSON] et al. designed a gated graphene device for tuning the high harmonic generation [26]; graphene was grown by CVD with a single layer and placed on a 1 mm thick substrate. Two metal electrodes were connected at both sides of the graphene as source and drain, and the gating was controlled by another pair of electrodes, as shown in Figure 8a. In the experiment, there was a multicycle terahertz field with a central frequency of 0.3 THz incidents on the sample, the peak electric field was as high as 80 kV/cm. In Figure 8b, the frequency-domain spectra of the THz field that transmitted through the graphene was detected. The spectra contains odd-order harmonics of the fundamental frequency (0.3 THz) up to the seventh order; the HHG efficiency could be tuned by the Fermi level of graphene, as shown in Figure 8b. By tuning the Fermi level of graphene and achieving a THG efficiency enhancement of approximately two orders by using electrical gating of graphene (Figure 8c), the resonant effects of metamaterials induced by their geometry could enhance light-matter interactions, including hole arrays [145; 146],  
Figure 8: THz generation. (**a**) Gated graphene sample device for tuning the high harmonic generation. (**b**) Amplitude spectra of terahertz wave and high harmonics transmitted through the graphene.