Nanometre-resolution three-dimensional tomographic and vectorial near-field imaging in dielectric optical resonators

All-dielectric optical nano-resonators have emerged as low-loss, versatile and highly adaptable components in nanophotonic structures for manipulating electromagnetic waves and enhancing light-matter interactions. However, achieving full three-dimensional characterization of near fields within dielectric nano-resonators poses great experimental challenges. Here we develop a technique to image near-field wave patterns inside dielectric optical nano-resonators using high-order sideband generation. By exploiting the phase sensitivity of various harmonic orders, which enables the detection of near-field distributions at distinct depths, we achieve three-dimensional tomographic and near-field imaging with a transverse resolution of ~920 nm and a longitudinal resolution of ~130 nm inside a micrometre-thick silicon anapole resonator. Our method offers high-contrast polarization sensitivity and phase-resolving capabilities, providing comprehensive vectorial near-field information and could be applied to diverse dielectric metamaterials.