Photoresponse enhancement in a cavity–antenna-coupled graphene terahertz detector

The unique and excellent optoelectronic properties of graphene make it a promising material for THz detection. However, the huge gap between the atomic thickness of graphene and the long wavelength of THz radiation severely limits the efficiency of light absorption and the photoresponse. Although optical antennas are commonly used to concentrate THz waves into deep subwavelength regions, a large amount of power is re-radiated out as waste rather than being absorbed by the active material. Here, we propose a cavity-antenna hybrid structure to enhance the THz absorption in a graphene flake through interference manipulation and impedance matching. The photoresponse of the device under an impedance-matched condition is 15 times higher than that under an impedance-mismatched condition. The cavity-antenna-coupled graphene THz detector exhibits a responsivity of 10.2 mA W^-1 and a noise-equivalent power of 0.92 nW Hz^-0.5. The excellent performance of our device makes it one of the best room temperature sub-THz graphene detectors that we investigated. A theoretical model was established to analyze the interaction between the antenna-coupled graphene and the cavity structure. With a combination of both electrostatic doping and interference manipulation, the light coupling management by impedance matching can enhance the absorptance of graphene by more than two orders of magnitude. This work experimentally and theoretically revealed that the cavity-antenna hybrid structure can prominently enhance the responsivity of a tiny piece of graphene in the sub-THz regime. Meanwhile, this strategy can also be applied to enhance the absorptance of other low-dimensional or bulk materials.