High‐Responsivity Mid‐Infrared Black Phosphorus Slow Light Waveguide Photodetector

Abstract Black phosphorus (BP) offers unique opportunities for mid‐infrared (MIR) waveguide photodetectors due to its narrow direct bandgap and layered lattice structure. Further miniaturization of the photodetector will improve operation speed, signal‐to‐noise ratio, and internal quantum efficiency. However, it is challenging to maintain high responsivities in miniaturized BP waveguide photodetectors because of reduced light–matter interaction lengths. To address this issue, a method utilizing the slow light effect in photonic crystal waveguides (PhCWGs) is proposed and experimentally demonstrated. A shared‐BP photonic system is proposed and utilized to fairly and precisely characterize the slow light enhancement. Close to the band edge around 3.8 µm, the responsivity is enhanced by more than tenfold in the BP photodetector on a 10 µm long PhCWG as compared with the counterpart on a subwavelength grating waveguide. At a 0.5 V bias, the BP PhCWG photodetector achieves a 11.31 A W⁻¹ responsivity and a 0.012 nW Hz^−1/2 noise equivalent power. The trap‐induced photoconductive gain is validated as both the dominant photoresponse mechanism and the major limiting factor of the response speed. The BP slow light waveguide photodetector is envisioned to realize miniaturized high‐performance on‐chip MIR systems for widespread applications including environmental monitoring, industrial process control, and medical diagnostics.