Qianlang Liu1 Steven Quillin2 David Masiello2 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States
2, University of Washington, Seattle, Washington, United States

Controlling the nanolocalization of light through engineering the optical responses of dielectric structures has drawn ever increasing interest, which relies on a fundamental understandings of how a material’s composition, size and morphology affect its optical behaviors. STEM EELS provides a unique way of detecting local optical responses, as the probing electron or the so-called fast electron in a STEM can be viewed as an evanescent source of supercontinuum light, which is often referred to as the virtual photons. Under certain circumstances, a target material may exhibit resonance excitations when interacting with this supercontinuum light source, causing energy losses of the fast electrons which can then be detected using EELS. Because this work focuses on studying resonant modes which are nanoparticle geometry specific, they are noted as “cavity modes”. Two oxides (TiO2 nanoparticles and CeO2 nanocubes) with similar refractive indices but different geometries are employed here as the target materials, and complicated spectral peaks within the bandgap regions of these two oxides are detected using monochromated EELS in the aloof beam mode. In this mode, the electron probe is positioned only a few nanometers away from the particle surfaces and signals are generated from delocalized electron-solid interactions [1]. Since energy losses smaller than the bandgap energies are detected, these signals are not the result of electronic excitations (significant bandgap states are not expected to present in these materials) but rather from the excitations of optical-frequency geometric modes in these oxides. Simulations based on classical electrodynamics are performed to interpret the complex spectral features. Combination of experiments and simulations reveal that many factors influence the energy and strength of the cavity modes, including materials’ refractive indices, particle aggregation and fast electron velocity. Analytical Mie analysis also reveals that these geometric cavity modes are encoded in the scattering properties of the oxide particles when they are exposed to light or electron irradiation.
[1] P.A. Crozier, Ultramicroscopy 180 (2017), 104.
[2] Funding from DOE (DE-SC0004954) and NSF CHE-1253775 and the use of NION microscope at John M. Cowley Center for Microscopy at Arizona State University are greatly acknowledged.