Realization of Zero-Refractive-Index Lens with Ultralow Spherical Aberration

TL;DR

This paper reports the design and fabrication of a silicon-based zero-refractive-index lens that achieves anomalous focusing with ultralow spherical aberration, enabling advanced integrated photonic applications.

Contribution

The authors demonstrate a large-area silicon nanopillar zero-index lens with measured near-zero refractive index and ultralow spherical aberration, advancing all-dielectric zero-index photonics.

Findings

Near-zero refractive index measured at 1.55 μm

Achieved anomalous focusing without spherical aberration

Device is compatible with integrated photonics platforms

Abstract

Optical complex materials offer unprecedented opportunity to engineer fundamental band dispersion which enables novel optoelectronic functionality and devices. Exploration of photonic Dirac cone at the center of momentum space has inspired an exceptional characteristic of zero-index, which is similar to zero effective mass in fermionic Dirac systems. Such all-dielectric zero-index photonic crystals provide an in-plane mechanism such that the energy of the propagating waves can be well confined along the chip direction. A straightforward example is to achieve the anomalous focusing effect without longitudinal spherical aberration, when the size of zero-index lens is large enough. Here, we designed and fabricated a prototype of zero-refractive-index lens by comprising large-area silicon nanopillar array with plane-concave profile. Near-zero refractive index was quantitatively measured near 1.55 um through anomalous focusing effect, predictable by effective medium theory. The zero-index lens was also demonstrated to perform ultralow longitudinal spherical aberration. Such IC compatible device provides a new route to integrate all-silicon zero-index materials into optical communication, sensing, and modulation, and to study fundamental physics on the emergent fields of topological photonics and valley photonics.