Vectorial Near-Field Imaging of Silicon Heterostructure Cavities in Air-Slot Waveguides

Air-slot photonic nanocavities have many relevant advantages for hybrid integration of external light emitters/absorbers in Si-based optoelectronic devices. Still an experimental major drawback is the difficulty of performing near-field polarization mapping of the photonic modes localized in air-regions. Here, we demonstrate that near-field resonant transmission can achieve high-fidelity vectorial imaging of the electric-field distribution of a nanocavity mode in the challenging case of a silicon heterostructure photonic-crystal cavity realized in an air-slot waveguide where the mode is localized both in air and dielectric regions. We discern and control the in-plane independent polarization components of the electric field of the photonic mode by exploiting the Fano imaging technique. The use of metallic coated aperture near-field probes allows to map the two polarizations with a deep sub-wavelength imaging ( $\lambda $ /15) resolution.