2025/10/21 by Sacchi, Luca, Palmieri, Alfonso, Mishra, Vitthal +3 · 1 citation
#Applied Physics (physics.app-ph) #FOS: Physical sciences #Optics (physics.optics)
paper · doi:10.48550/arxiv.2510.21816
Metasurfaces -- planar arrays of subwavelength nanostructures -- are typically realized with high-index dielectrics, while low-index platforms are often dismissed for their weaker contrast. Here, we identify and experimentally verify regimes where a low-index platform (SiO2) surpasses a high-index counterpart (TiO2). We demonstrate that a low index suppresses higher-order Bloch modes, enabling the design of efficient devices with relaxed feature sizes. Low-index metasurfaces also offer two intrinsic advantages: a broad, well-behaved chromatic response without the need for explicit dispersion engineering, and strong tolerance to fabrication errors. We validate these features experimentally with silica metagratings, metalenses, and structured-light phase plates at λ=632 nm. The metagratings reach ≥50% absolute diffraction efficiency over a 200 nm bandwidth, the metalenses deliver 75% absolute diffraction efficiency with diffraction-limited performance, and the vortex phase plates achieve 80% conversion efficiency at the design wavelength and 60% with 100 nm wavelength detuning. These results delineate conditions where low-index metasurfaces outperform high-index designs, suggesting a route to scalable, broadband, fabrication error-resilient flat optics.