Decoupling light redistribution from absorption via waveguide-integrated bilayer metagratings

Integrated optical systems, encompassing microwave photonics, optical communication receivers, and optical sensing, require photodetectors with high saturation power and responsivity. Achieving uniform and efficient light absorption is essential for realizing these properties. However, traditional waveguide-coupled photodetectors struggle with a trade-off between absorption efficiency and uniformity due to exponential intensity decay in the waveguide direction. In this work, we propose waveguide-integrated bilayer metagratings that decouple light redistribution from absorption, achieving uniform and efficient absorption simultaneously. By elucidating the fundamental physics governing resonant mode coupling between the upper silicon metagrating (Si MG) and waveguide, we demonstrate that the Si MG couples out the waveguide mode and redistributes its intensity uniformly with 84% coupling efficiency. The lower germanium metagrating (Ge MG), serving as the absorber, achieves 90% absorption efficiency via guided-mode resonance. Compared with traditional evanescent coupling, this design enhances light intensity uniformity in the germanium absorption region while maintaining >70% overall efficiency. This approach holds promise for resolving the uniformity-efficiency trade-off in waveguide-coupled photodetectors, while extending beyond germanium systems to provide a strategy for III–V/Si hybrid integration in applications ranging from LIDAR receivers to quantum photonic processors.