▎ 摘　　要

Graphene-based photonic devices, such as ultrafast photodetectors, optical modulators and tunable surface plasmon polariton devices, have experienced rapid development in recent years(1-6) because they benefit greatly from graphene's strong field-controlled optical response(7,8). Here, we demonstrate a graphene/silicon-heterostructure photodiode formed by integrating graphene onto a silicon optical waveguide on a siliconon-insulator (SOI) with a near to mid-infrared operational range. The waveguide enables absorption of evanescent light that propagates parallel to the graphene sheet, which results in a responsivity as high as 0.13 A W-1 at a 1.5 V bias for 2.75 mu m light at room temperature. A photocurrent dependence on bias polarity was observed and attributed to two distinct mechanisms for optical absorption, that is, direct and indirect transitions in graphene at 1.55 mu m and 2.75 mu m, respectively. Our result demonstrates the use of in-plane absorption in a graphene-monolayer structure and the feasibility of exploiting indirect transitions in graphene/silicon-heterostructure waveguides for mid-infrared detection.