# Dynamic and Active THz Graphene Metamaterial Devices
## 3 Applications of Graphene Metamaterial
### Terahertz Photodetectors  
Owing to its high-quality mechanical flexibility, good thermal conductivity, and ultra-high carrier mobility, graphene has excellent potential for wideband photodetection in the terahertz band. The core principle of the photodetector is to convert the incoming light signal into an electrical signal, which means that the conductivity of the material changes during the process. Several different mechanisms by which this can be accomplished in graphene have been reported, mainly including the photovoltaic effect (PVE) [85], the photo-thermoelectric effect (PTE) [86; 87], the bolometric effect (BE) [88; 89], and the plasma-wave-assisted mechanism in graphene FET [90; 91; 92; 93]. In order to improve the performance of detectors, most current research focuses on improving the absorption of electromagnetic waves by materials, reducing dark current, reducing response time, and broadening the working bandwidth as much as possible. We summarize several graphene-based methods to improve detection performance and to introduce the latest progress accordingly. It is worth noting that graphene shows high wave locality, and it supports the propagation and the dynamic tunability of surface waves. Therefore, graphene can be regarded as a very encouraging platform for integrated optoelectronics of terahertz antennas, such as resonant antennas [94; 95], hybrid plasmonic antennas [96; 97; 98], and leaky-wave and reflect array antennas [99; 100]. Combining single-layer or multilayer graphene antennas with photodetectors surpasses conventional detectors. In the past, integrated graphene antennas with various geometries have been investigated for detection purposes [101; 102; 103]. Additionally, employing graphene in an antenna integrated photodetector can improve signal coupling and other characteristics of the photodetector. [PERSON] et al. proposed a novel THz radiation detector using a dual-gated dipole antenna with a gap of \(\sim\)100 nm [104]. As shown in Figure 7a, the incoming radiation is intensely concentrated at the PN junction of the graphene channel above the antenna. This highly sensitive detector has an equivalent noise power of 80 \(\mathrm{pW}/\sqrt{\mathrm{Hz}}\), a response time of less than 30 ns, and a wide operating bandwidth (1.8-4.2 THz) at room temperature. Figure 7b,c show the specific performance of the device. Even more remarkable is that it achieves a combination of high performance that is not currently possible with advanced detectors. Tunnel field-effect transistors based on bilayer graphene (BLG) can also be used for sensitive THz detection [105]. They coupled the antenna with a tunnel junction created by the electrically tunable properties of BLG (Figure 7d), enabling low-noise (0.2 \(\mathrm{pW}/\sqrt{\mathrm{Hz}}\)) and high responsivity (\(>\)4 \(\mathrm{kV}/\mathrm{W}\)) detection, as shown in Figure 7e,f. A room temperature THz detector with a wide bandwidth (\(\sim\)100 GHz), high speed (several hundred ps), and high sensitivity (equivalent noise power \(\leq\) 120 \(\mathrm{pW}/\sqrt{\mathrm{Hz}}\)) at 3.4 THz were also achieved by using the antenna-coupled graphene FET [106]. Figure 7g-i shows how it works and its excellent detection performance. These excellent features open unique perspectives for high-speed applications in many research fields such as ultrafast nano-spectroscopy, high-speed communications, quantum science, and coherent control of quantum systems. Figure 7: (**a**) Schematic diagram of the antenna\(-\)integrated pn junction device; (**b**) Relation between the photo response and the applied voltage of the two branches of the antenna(VL and VR) when the incident frequency is 2.52 THz; (**c**) Noise-equivalent power (NEP); (Reprinted with permission from ref.