Anapole Resonances Facilitated by High-Index Contrast between Substrate and Dielectric Nanodisk Enhance Vacuum Ultraviolet Generation

Anapole modes are known to be excited only in high-index nanoparticles. Here we show that the nanodisks made of dielectric materials with much lower refractive indices, such as LiNbO3, can generate anapole modes, provided that they are supported by a metallic or near-zero-index substrate. The important parameter for exciting an anapole mode is the index contrast between the nanodisk and substrate, rather than the refractive index of the nanodisk. We numerically demonstrate highly efficient coherent generation of vacuum ultraviolet with an efficiency higher than 10(-2) % based on second-harmonic generation via the excitation of anapole modes in LiNbO3 nanodisks supported by an aluminum substrate. The highly efficient vacuum ultraviolet nanosource proposed in this work can find important applications for spectroscopic and sensing purposes at the nanoscale.