3D Imaging of Optical Modes in Dielectric Photonic Nanocavities with Sub-wavelength Field Confinement

Dielectric optical cavities are emerging as viable platforms for efficiently concentrating light within extremely small volumes of sub-wavelength dimensions. This breaks with the notion that only plasmonic nanostructures can achieve this scale of confinement and enables strong light-matter interactions without the losses typically associated with metals. Here, we directly visualize the optical modes of a topology-optimized silicon bowtie nanocavity using multi-orientation electron energy-loss spectroscopy. Tomographic reconstruction of the resulting data sets reveals the three-dimensional profiles of several polarized optical modes in close agreement with simulations. A resonance near the telecom wavelength (∼1550 nm) is shown to be tightly localized at the bowtie bridge, confirming its deep sub-wavelength mode volume. These findings establish electron beam spectroscopy as a powerful tool for mapping three-dimensional field confinement in dielectric photonic cavities with potential applications in future photonic and quantum technologies.