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Perfectly vertical grating couplers leveraging metamaterials can achieve bothhigh coupling efficiency and minimal back reflection. The fabricability ofthese designs, with segmentations in both the longitudinal and transversedimensions, hinges on the minimum feature size offered by cutting-edgefabrication technologies. In this work we present both numerical andexperimental evidence that high performance devices can be obtained while usinglarge transverse segmentation periods of up to 650 nm, thereby increasing thecritical feature sizes. For single-step etched couplers produced on the 220 nmsilicon-on-insulator platform, we demonstrate coupling efficiencies of nearly50% in the C-band and remarkably low back reflections of -22 dB at zero-degreeincidence angle. Notably, the duty cycles used in our optimized designs deviatesignificantly from those predicted by traditional effective medium models, evenfor small periods. Our findings promise to expand the range of opticalproperties achievable in metamaterials and offer fresh insights into thefine-tuning of nanophotonic devices.

Perfectly vertical silicon metamaterial grating couplers with large segmentation periods up to 650 nm