Silica meta-optics: When high-performance does not need a high-index

TL;DR

This paper demonstrates that low-index silica metasurfaces can outperform high-index counterparts in efficiency, bandwidth, and fabrication tolerance, challenging the conventional preference for high-index materials in metasurface design.

Contribution

It identifies and experimentally verifies regimes where low-index SiO₂ metasurfaces surpass high-index TiO₂ in performance, broadening the design space for flat optics.

Findings

Low-index metasurfaces suppress higher-order Bloch modes.

Silica metasurfaces achieve high diffraction efficiency over broad bandwidths.

Low-index designs exhibit strong fabrication error tolerance.

Abstract

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 (SiO$_2$) surpasses a high-index counterpart (TiO$_2$). 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 $\lambda=632\ nm$. The metagratings reach $\geq$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.