## FDTD simulation for GeSn photodetector with high absorption coefficient ### General design guidelines Although with good mode and propagation profile, due to the high absorption of $\ce{GeSn}$ (direct-bandgap $\ce{GeSn}$ could have a $k\approx 0.2$), the taper part should be adjusted to meet the carrier extraction requirement. The new design is based on evanescent coupling, the light is guided into a inverse taper, while becoming narrower, the light begin to leak into $\ce{GeSn}$ and surroundings. In ideal case, the light get absorbed by our active region, then carriers are extracted. ![[Drawing 2025-07-18 14.40.37.excalidraw.svg]] Top view of the inverse taper and $\ce{GeSn}$. ![[Drawing 2025-07-18 14.47.23.excalidraw.svg]]Side view of the inverse taper and $\ce{GeSn}$. For the primary design, we will adjust the position of $\ce{GeSn}$ layer, and see whether the light could couple in properly.