Efficient high-contrast metastructure vertical to in-plane silicon nitride waveguide coupler for silicon photonics integration

We report the first high-contrast metastructure (HCM) coupler for an optical fiber with surface-normal incidence to an inplane silicon nitride (Si₃N₄) waveguide on a silicon photonics platform. The device enables efficient free-space-to-waveguide and fiber-to-waveguide coupling by utilizing a subwavelength HCM resonator on top of a Si₃N₄ waveguide with a matched propagation constant for evanescent mode coupling. Through a global optimization process using a genetic algorithm, the HCM parameters were selected to achieve high coupling efficiency. The device is a ridge waveguide with Si₃N₄ core and cladded by SiO₂ layers, on top of which an a-Si layer is deposited and etched to form HCM resonator designed at 1060 nm wavelength. We demonstrate a coupling efficiency of –2.2 dB, with a deviation of 0.2 dB from the simulated result, confirming the excellent agreement between design and experiment. The structure shows robust tolerance to variations in grating duty cycle and air-gap width, ensuring reliable performance under practical lithography and etching conditions. The demonstrated efficient coupling at 1060 nm highlights the capability of Si₃N₄ HCM couplers beyond conventional telecom bands, especially for near-infrared photonic integration. This approach provides a scalable foundation for multi-wavelength operation, and can be further extended to realize compact wavelength-division multiplexing (DWDM) interfaces in future photonic integrated circuits (PICs).