Imaging Photonic Resonances within an All-Dielectric Metasurface via Photoelectron Emission Microscopy

Dielectric metasurfaces, through volume-type photonic resonances, enable precise control of light-matter interactions for applications including imaging, holography, and sensing. The application space of dielectric metasurfaces has extended from infrared to visible wavelengths by incorporating high refractive index materials, such as titanium dioxide (TiO₂). Understanding the fundamental and fabrication limits for these applications requires metrology with nanoscale resolution, sensitivity to electromagnetic fields within the meta-atom volume, and far-field excitation. In this work, photoelectron emission microscopy (PEEM) is used to image field distributions of photonic resonances in a TiO₂ metasurface excited with far-field, visible-wavelength illumination. The local volumetric field variations within the meta-atoms are analyzed as a function of illumination angle and polarization by comparing photoelectron images to finite-difference time-domain simulations. This study determines the inelastic mean free path of very low-energy (<1 eV) photoelectrons to be 35 ± 10 nm, which is comparable to the meta-atom height thereby highlighting PEEM sensitivity to resonances within the volume. Additionally, the simulations reveal high sensitivity of PEEM images to an in-plane component of the illumination k-vector. These results demonstrate that photoelectron imaging with subwavelength resolution offers unique advantages for examining light-matter interactions in volume-type (as opposed to surface) photonic modes within dielectric nanophotonic structures.