Harnessing subwavelength metamaterials for low-loss grating couplers in silicon nitride waveguides

High-performance photonic interconnects call upon novel solutions for efficient input/output optical interfaces. However, obtaining low-loss coupling between conventional optical fibers and photonic integrated circuits (PICs) remains a critical challenge due to material, modal, and geometrical discrepancies. In this work, we report advanced design approaches, leveraging subwavelength grating (SWG) metamaterials in silicon nitride (Si3N4) substrates. Harnessing unique properties of metamaterials within moderate index contrast SiN platform brings an extra degree of design freedom to seamlessly tailor light flow in grating couplers, improving both radiation efficiency and grating-tofiber field matching. In turn, this enhances fiber-chip coupling performance, while maintaining single-etch step manufacturing. SWG-engineered uniform Si3N4 couplers realized on low-pressure chemical vapor deposited (LPCVD) wafers showed experimental efficiency of about -4.5dB at 1550nm wavelength. Moreover, hybrid Si3N4 grating coupler designs with SWG metamaterials, operated at datacom range around 1310nm, boost the fiber-chip coupling efficiency to -2.4dB and -1.3dB levels for uniform and apodized structures, respectively. These results highlight the promising potential of metamaterial-engineered off-chip waveguide couplers in future innovations of complex SiN PICs, demanding low-loss interconnection with the outside world and cost-effective foundry-compatible fabrication.