We present the design of a metal grating coupler compatible with membrane photonic circuit platforms, consisting of a buried metal grating and a metal mirror. A nonapodized design provides a fiber-to-chip coupling efficiency at 1.55 µm up to 73%, whereas apodized designs show theoretical efficiencies as high as 89%, with a 3 dB bandwidth of 61 and 78 nm, respectively. An important advantage is that the coupling efficiency is independent from the underlying layer stack, enabling its use in diverse applications. For example, a thin buffer layer is required to achieve optical coupling for the heterogeneous integration of III–V and silicon photonics, whereas a thick buffer is of interest for thermal isolation between photonic membranes and CMOS circuits.
Dolores Calzadilla, V. M., Heiss, D., & Smit, M. K. (2014). Highly efficient metal grating coupler for membrane-based integrated photonics. Optics Letters, 39(9), 2786-2789. https://doi.org/10.1364/OL.39.002786