This study investigates the role of pitch size in achieving high numerical aperture (NA) and focusing efficiency in metalens design, while demonstrating how high refractive index materials contribute to performance enhancement by enabling smaller pitch sizes through reduced filling ratios. Silicon-rich nitride (SRN) was chosen as the material platform due to its high refractive index, CMOS compatibility, and cost-effective fabrication. Two SRN-based metalenses were designed: a geometric phase metalens (GPM) and a propagation phase metalens (PPM), each evaluated at aspect ratios of 10:1 and 4:1. The GPM achieved an NA of 0.99 with focusing efficiencies of 26.6% (10:1) and 17.6% (4:1), while the PPM achieved NAs of 0.95 (10:1) and 0.97 (4:1) with focusing efficiencies of 65.6% and 32.1%, respectively. Our findings demonstrate that smaller pitch sizes reduce spatial discretization errors, enhancing phase fidelity and focusing efficiency, which is applicable to various applications such as imaging and polarimetry.
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