# Dynamic and Active THz Graphene Metamaterial Devices
## 3 Applications of Graphene Metamaterial
### Terahertz Photodetectors
[104] Copyright 2019, American Chemical Society) (**d**) Schematic diagram of dual-gated BLG transistor in hBN package; (**e**) Relationship between photo response and gated voltage (\(\mathrm{V_{bg}}\) and \(\mathrm{V_{tg}}\)) when the incident frequency is 0.13 THz; (**f**) Noise\(-\)equivalent power (NEP) at given \(\mathrm{V_{bg}}\); (Reprinted with permission from ref. [105]. Copyright 2021, Nature Publishing Group) (**g**) Schematic diagram of antenna coupled GEFT; (**h**) Photo response represented by \(\Delta\)u; (**i**) Noise\(-\)equivalent power (NEP) calculated as a function of \(\mathrm{V_{g}}\). (Reprinted with permission from ref. [106]. Copyright 2021, Nanophotonics). In addition to the coupling efficiency mentioned above, detectors made of single materials tend to have powerful dark currents, resulting in a large NEP and a low detection rate. Graphene combines with other 2D materials to form an interlayer van der Waals heterostructure. The high-energy potential barrier formed can effectively inhibit the disorderly motion of photoelectrons to reduce dark noise. Related work has made remarkable progress across a broader spectrum. [PERSON] et al. proposed a photodetector with a few-layer MoS\({}_{2}\)/glass-to-graphene heterostructure synthesized by a layer-by-layer transfer technique [107]. The ohmic contact formed between MoS\({}_{2}\) and graphene significantly improved the charge transfer efficiency in the device. Meanwhile, the results showed that the band arrangement of layered heterostructures can be controlled by lattice engineering of 2D nanosheets to improve the photoelectric properties. Similarly, [PERSON] et al. showed that heterojunction formation by doping graphene could improve the light-detection capability of detections in their device based on ferroelectric polarization of \(LiNbO_{3}\)[108]. The detectors' performance, such as detectivity and responsivity, improved by several orders of magnitude compared to previously reported devices. Although advances have focused on visible, near-infrared, and mid-infrared wavelengths, it is exciting that theoretical calculations have already shown that graphene can be extended to THz waves by altering the band gap of graphene [109; 110]. [PERSON] al. achieved a broad photo response from VIS (532 nm) up to the THz region (2.52 THz) in the reduced graphene oxide (RGO) and Si nanowire (SiNW) array heterojunction photodetector [111]. Other methods, such as defect engineering [112], improvement of substrate materials, and introduction of quantum dot structures in graphene field-effect transistors [113; 114; 115], can effectively improve the detection performance of graphene detectors. Graphene is not the only two-dimensional material that efficient terahertz photodetectors rely on. Others such as black phosphorus (BP), bismuth and telluride also perform well in detectors. However, due to its broad spectral properties (UV to THz), graphene's potential applications in photodetectors cover the mid-infrared and the terahertz bands, making it attractive to researchers in many different fields. Although the performance improvement methods mentioned in this section can enhance the performance of THz detectors by several orders of magnitude, the intrinsic properties of the materials themselves are still the fundamental factors. Table 3 presents the figure-of-merits of photodetectors based on graphene and some representative 2D materials. It can be seen that graphene devices perform well even in broadband bands, especially in terms of the response time, which is still improved for wider commercial applications. Thanks to these excellent detection properties, graphene-based sensors are widely used to detect material properties and for biomedical and chemical detection [116; 117; 118; 119; 120].