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

Optical complex materials offer unprecedented opportunity to engineer fundamental band dispersion, which enables novel optoelectronic functionality and devices. Exploration of the 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 the zero-index lens is large enough. Here, we designed and fabricated a prototype of a zero-refractive-index lens by using a large-area silicon nanopillar array with a plane-concave profile. The near-zero refractive index was quantitatively measured near 1550 nm through the anomalous focusing effect, predictable by effective medium theory. The zero-index lens was also demonstrated to have ultralow longitudinal spherical aberration. Such an integrated-circuit-compatible device provides a new route to integrate all-silicon zero-index materials into optical communication, sensing, and modulation and to study fundamental physics in the emergent fields of topological photonics and valley photonics.