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.

石墨烯中的光电转换是实现预期的超快和低功耗信息技术的核心现象。然而，揭示其机制和内在时间尺度需要未知的太赫兹电子设备和设备架构。在这里，我们通过超快光热电流的片上电读出成功地解决了高质量石墨烯中的光电转换过程。通过使用电阻氧化锌顶栅抑制电阻-电容电路的时间常数，我们构建了一个带宽高达 220 GHz 的栅极可调石墨烯光电探测器。测量非局部光电流动力学，我们发现，根据 Shockley-Ramo 定理，从电极中提取的光电流是准瞬时的，没有可测量的载流子在几微米长的石墨烯上的传输时间。光电流产生的时间可以从立即调整到 > 4 ps，其起源被确定为费米能级相关的带内载流子散射。我们的结果弥合了超快光学科学和器件工程之间的差距，加速了超快石墨烯光电应用。