<b>High Efficiency Si</b>_{<strong>3</strong>}<b>N</b>_{<strong>4</strong>}<b> Grating Couplers with Hybrid Al</b><b>₂</b><b>O</b><b>₃</b><b>/Metal-Mirror Structure for Back-Reflection Suppression</b>

We present a high-performance one-dimensional silicon nitride grating coupler optimized via chirped duty cycle and period engineering for superior mode matching. A tailored multilayer dielectric stack above the grating suppresses back reflection and enhances index matching, while a metal reflector beneath redirects leakage light to boost coupling efficiency. Three-dimensional finite-difference time-domain simulations yield a record 95% peak efficiency and a 57 nm 1 dB bandwidth, combining high performance with broadband operation. This approach provides a compact, fabrication-ready optical interface for silicon photonic integrated circuits, advancing the performance frontier of one-dimensional silicon nitride grating couplers.