High performance grating couplers based on enhanced virtual directional couplers incorporating hybrid quasi-bound states in the continuum.

Grating couplers are critical fiber-to-chip interfaces in silicon photonics, but conventional designs suffer from inherent trade-offs between efficiency, bandwidth, and fabrication complexity. Here, we numerically demonstrate a high-performance uniform-period mirrorless grating coupler on a standard 400-nm silicon nitride (Si3N4) platform, combining an enhanced virtual directional coupler with hybrid quasi-bound state in the continuum (hybrid q-BIC) gratings consisting of titanium dioxide (TiO2) gratings, a silicon overlay, and nanoscale notches. Interlayer interference naturally forms a near-Gaussian radiation profile, achieving over 96% mode matching efficiency. The q-BIC gratings synergize with destructive interference from the silicon overlay, boosting directionality to near 97%. Finite-difference time-domain (FDTD) simulations yield a coupling efficiency of -0.34 dB (92.4%) at 1550 nm, along with a 1-dB bandwidth of 65 nm, without requiring distributed Bragg reflectors (DBRs), metallic back reflectors, or apodized gratings. This work offers a fabrication-friendly, high-performance solution for energy-efficient optical interconnects.