## 4 High-Performance Photodetector Based on Graphene
A typical work based on PB effect is reported by [PERSON] employing a bowtie structure to reach a high responsivity of 0.5 A/W and a bandwidth of 110 GHz (Figure 4a) [62]. [PERSON] et al. demonstrated a coherent, plasmonic-structure-based graphene optical receiver (Figure 4b) which is capable of the reception of both a 200 Gbit/s quadrature phase-shift keying (QPSK) signal and a 240 Gbit/s 16 quadrature amplitude modulation (16 QAM) signal on a single-polarization carrier, thanks to the ultrahigh operation bandwidth of the graphene photodetector [58]. Finally, a plasmonic/silicon hybrid graphene photodetector covering 1.55 um and 2 um is reported by [PERSON] et al. where the responsivity is 0.07 A/W and 0.4 A/W at 1.55 um and 2 um respectively [63]. From the above-mentioned works on plasmonic graphene photodetectors, we can conclude that plasmonic structures can dramatically optimize both the responsivity and the bandwidth of graphene-based receivers. Metallic absorption remains an obstacle, acting to reduce responsivity, since its characteristic absorption does not contribute to the photocurrent. To address this issue, [PERSON] et al. proposed and demonstrated a double slot structure consisting of both the silicon slot waveguide and the plasmonic slot waveguide (Figure 4c) [59]. By optimizing the structural parameters, the metallic absorption is reduced to 0.2 dB/um and the responsivity increased to 0.6 A/W. Besides the PV and PB effect, the PTE effect has also recently been employed in silicon/graphene photodetectors. The PTE effect uses on the photon-induced electron temperature difference between two different graphene doping regions, which is normally achieved by external gating. Thus, the photocurrent is generated by an optically induced temperature gradient, which is proportional to the Seebeck coefficient. By optimizing the gate and source voltages simultaneously, a maximum responsivity of 0.36 A/W and a 3-dB operation bandwidth of 42 GHz is reached [64]. Figure 3: Graphene/silicon photodetector based on PV effect. (**a**) First graphene photodetector operating at 1550 nm. Reprinted with permission from _Nano Lett._ 2011, 11, 7, 2804–2808. Copyright 2011 American Chemical Society. (**b**) The graphene photodetector based on the silicon slot waveguide. Reproduced from Ref. [49] with permission from the Royal Society of Chemistry [56]. (**c**) The plasmonic enhanced high-performance graphene photodetector [57]. Moreover, photodetectors based on PTE effect can directly convert the optical signal to voltage signal, removing the need for a transimpedance amplifier. Various structures including photonic crystal waveguides [65], micro-ring resonators [66] and double layer graphene [67] have been employed to enhance the light-matter interaction of the PTE effect, reaching impressive performances, such as responsivity higher than 90 V/W and operation bandwidths larger than 65 GHz. Although the external gating function adds complexity to the fabrication process compared to the photodetectors based on PV and PB effect, the PTE graphene photodetector is appealing to industry since it supports the direct connection between the photodetector and the read-out electric circuit. The performances of a typical graphene/silicon photodetector and state-of-the-art photodetectors based on conventional bulk materials is compared in Table 1. Graphene/silicon photodetector can achieve high ultra-high bandwidth more than 110 GHz, thanks to the ultrahigh carrier mobility of graphene. The responsivity of graphene photodetector is comparable with the other material platforms and can be further improved with further optimization by enhancing the light-graphene overlap and interaction.