Ultra-low-loss on-chip zero-index materials.

Light travels in a zero-index medium without accumulating a spatial phase, resulting in perfect spatial coherence. Such coherence brings several potential applications, including arbitrarily shaped waveguides, phase-mismatch-free nonlinear propagation, large-area single-mode lasers, and extended superradiance. A promising platform to achieve these applications is an integrated Dirac-cone material that features an impedance-matched zero index. Although an integrated Dirac-cone material eliminates ohmic losses via its purely dielectric structure, it still entails out-of-plane radiation loss, limiting its applications to a small scale. We design an ultra-low-loss integrated Dirac cone material by achieving destructive interference above and below the material. The material consists of a square array of low-aspect-ratio silicon pillars embedded in silicon dioxide, featuring easy fabrication using a standard planar process. This design paves the way for leveraging the perfect spatial coherence of large-area zero-index materials in linear, nonlinear, and quantum optics. Calculations suggest that a new design of engineered medium can simultaneously yield a refractive index of zero and low optical loss at the telecommunications wavelength of 1550 nm. The development could lead to applications in nonlinear and quantum optics benefiting from an infinite coherence length. The approach of Tian Dong and coworkers from China and the US is to embed an array of silicon pillars (about 180 nm in radius and 1100 nm high) into a matrix of silicon dioxide to create a Dirac-cone photonic-crystal slab. Importantly, if the pillar height is chosen correctly any upwards and downwards radiation out of the slab can be made to destructively interfere thus reducing propagation loss to a level of 0.15 dB/mm. The slab should be possible to fabricate using standard processes.