Graphene-based mid-infrared photodetectors using metamaterials and related concepts

Graphene, a semi-metal with a gapless band structure, has been used in mid-infrared (MIR) photodetectors (PDs) for some time. However, these detectors often suffer from low responsivity due to the intrinsically low absorption and ultra-short carrier lifetime in graphene, large dark current, and low detectivity due to the semi-metallic nature of graphene. Over the past decade, much effort has been devoted to addressing these issues. A variety of metamaterials and related concepts has been employed to improve the detector responsivity by enhancing the graphene absorption and/or the carrier collection efficiency. Here, we provide an overview of the graphene MIR PDs with and without the use of approaches for responsivity enhancement. We focus our attention on the state-of-the-art graphene MIR PDs whose performance is improved by employing metamaterials and related concepts, including band structure engineering, the photogating effect, integration with plasmonic nanostructures and waveguides, the use of asymmetric plasmons, coupled plasmon-phonon polaritons, and small-twist-angle bilayer graphene. We conclude by providing possible directions for further performance improvement of graphene MIR PDs and a discussion on future applications of these detectors