  Perfectly vertical grating couplers leveraging metamaterials can achieve both
high coupling efficiency and minimal back reflection. The fabricability of
these designs, with segmentations in both the longitudinal and transverse
dimensions, hinges on the minimum feature size offered by cutting-edge
fabrication technologies. In this work we present both numerical and
experimental evidence that high performance devices can be obtained while using
large transverse segmentation periods of up to 650 nm, thereby increasing the
critical feature sizes. For single-step etched couplers produced on the 220 nm
silicon-on-insulator platform, we demonstrate coupling efficiencies of nearly
50% in the C-band and remarkably low back reflections of -22 dB at zero-degree
incidence angle. Notably, the duty cycles used in our optimized designs deviate
significantly from those predicted by traditional effective medium models, even
for small periods. Our findings promise to expand the range of optical
properties achievable in metamaterials and offer fresh insights into the
fine-tuning of nanophotonic devices.
