Fig. 1: Design concept and main results of metasurface-mediated graphene photodetectors.
a Illustration of the designed metasurface-mediated graphene photodetector, which consists of non-centrosymmetric sub-wavelength metallic nanoantennas as meta-atoms on top of graphene flakes. Under uniform illumination at 4 µm wavelength, global directional photocurrents are generated from each meta-atom at zero external bias (Vd = Vg = 0 V), mimicking the shift current in the bulk photovoltaic effect (BPVE). Importantly, due to the gapless nature of graphene, the local photoresponse from nanoantennas can efficiently contribute to the external circuit in a nonlocal manner, enabling a cascaded total photocurrent. From bottom to top: Si (gray), SiO2 (blue), graphene (black honeycomb), Pd/Au for nanoantennas, and electrodes (yellow). Inset illustrates the excitation of electrons at one edge of nanoantennas and the following directional transport. Black line denotes the band diagram of graphene. Yellow area is the graphene region that is covered and doped by metal. b Schematic of the experimental setup with control of the linear polarization state via rotation of half-wave plate (HWP). The focused laser beam has a beam diameter around 400 μm, which is much larger than the size of our device. The inset shows the scanning electron microscopy (SEM) image of our device in false colors: graphene in dark red, nanoantennas, and electrodes in yellow, substrate in dark blue. c Simulated near-field distribution and predicted vectorial photocurrent in a unit cell at different polarization angles of incident light (Pol). |E|2 represents the intensity of local electrical field. Yellow wave arrows indicate the flow of photocarriers generated at the nanoantenna–graphene interfaces. White arrows illustrate the resultant vectorial photocurrents (Iph). d Polar plot of measured Iph, which is the scalar projection of Iph on the orientation of drain–source electrodes. Red and blue areas mark the positive and negative signs of Iph.
