▎ 摘　　要

Researchers demonstrated a gate-tunable graphene photodetector with a bandwidth of up to 220 GHz. This was achieved by suppressing the 'RC' time constant using a resistive zinc oxide top gate. Optical-to-electrical conversion in graphene is a central phenomenon for realizing anticipated ultrafast and low-power-consumption information technologies. However, revealing its mechanism and intrinsic timescale require uncharted terahertz electronics and device architectures. Here we succeeded in resolving optical-to-electrical conversion processes in high-quality graphene via the on-chip electrical readout of an ultrafast photothermoelectric current. By suppressing the time constant of a resistor-capacitor circuit using a resistive zinc oxide top gate, we constructed a gate-tunable graphene photodetector with a bandwidth of up to 220 GHz. Measuring the non-local photocurrent dynamics, we found that the photocurrent extraction from the electrode is quasi-instantaneous without a measurable carrier transit time across several-micrometre-long graphene, following the Shockley-Ramo theorem. The time for photocurrent generation is exceptionally tunable from immediate to >4 ps, and its origin is identified as Fermi-level-dependent intraband carrier-carrier scattering. Our results bridge the gap between ultrafast optical science and device engineering, accelerating ultrafast graphene optoelectronic applications.