Imaging Photonic Modes of a TiO₂ Nanorod Metasurface via Photoelectron Emission Microscopy

Metasurfaces, photonic crystals, etc. are deployed in sensing, imaging, and nonlinear optics, among others. In designing dielectric metasurfaces, there is great importance in precise, nanoscale control over light-matter interactions that are enhanced by tight confinement of electromagnetic fields within sub-optical wavelength volumes. Because dielectric systems and their modes are designed to exploit symmetry as well as symmetry breaking, visualization of the field symmetries of the modes and their dependence on polarization of the excitation light source is important. Here, we present a study of optical resonances in a dielectric metasurface consisting of a square lattice of TiO₂ nanorods, imaged via photoemission electron microscopy (PEEM) and illuminated in the ultraviolet-visible wavelength range. This approach involves true far-field photonic excitation by normal incidence illumination and allows for near-field imaging at a micron-scale field of view at sub-wavelength spatial resolution. The metasurface is designed to support two closely overlapping resonances of different symmetries such that their individual contributions to the overall electromagnetic field distribution can be investigated as a function of excitation wavelength and polarization. The resonances are chosen to occur at 450 nm, where two-photon photoemission (2PPE) avoids electronic transitions over the electric band gap (3.0-3.2 eV) of TiO₂. Around this wavelength, 2PPE is influenced purely by the photonic modes, which enables a direct, nonlinear map of the field intensity at and near photonic resonance. To verify the relation between photoemission intensity and the electromagnetic field strength, we perform finite-difference time-domain simulations of the metasurface. By comparing the simulations that reproduce the field patterns and the switching between the two photonic modes with differing symmetries to the PEEM images as a function of the excitation wavelength, we estimate the electron inelastic mean free path (IMFP) to be 35 nm. This estimated IMFP is significantly larger than that of deep ultraviolet or x-ray excited photoelectrons ranging from a few Angstroms to nanometers. Our work demonstrates that the PEEM visualizes the true field distribution of photonic volume modes within nanostructures as deep as the inelastic mean free path of the photoelectron and show that the PEEM is suitable for metrology of dielectric metasurfaces in the visible spectrum range, complementing other near-field microscopy characterizations.<br/><br/>The work at Sandia National Laboratories was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (grant BES 20-017574) and, in part, by Sandia’s LDRD program. This work was performed in part at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy, Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly-owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.