Optical Dark-Field and Electron Energy Loss Imaging and Spectroscopy of Symmetry-Forbidden Modes in Loaded Nanogap Antennas

We have produced large numbers of hybrid metal–semiconductor nanogap antennas using a scalable electrochemical approach and systematically characterized the spectral and spatial character of their plasmonic modes with optical dark-field scattering, electron energy loss spectroscopy with principal component analysis, and full wave simulations. The coordination of these techniques reveal that these nanostructures support degenerate transverse modes which split due to substrate interactions, a longitudinal mode which scales with antenna length, and a symmetry-forbidden <i>gap-localized transverse</i> mode. This gap-localized transverse mode arises from mode splitting of transverse resonances supported on both antenna arms and is confined to the gap load enabling (i) delivery of substantial energy to the gap material and (ii) the possibility of tuning the antenna resonance <i>via</i> active modulation of the gap material’s optical properties. The resonant position of this symmetry-forbidden mode is sensitive to gap size, dielectric strength of the gap material, and is highly suppressed in air-gapped structures which may explain its absence from the literature to date. Understanding the complex modal structure supported on hybrid nanosystems is necessary to enable the multifunctional components many seek

Strain Effects in Epitaxial VO₂ Thin Films on Columnar Buffer-Layer TiO₂/Al₂O₃ Virtual Substrates

Epitaxial VO₂/TiO₂ thin film heterostructures were grown on (100) (m-cut) Al₂O₃ substrates via pulsed laser deposition. We have demonstrated the ability to reduce the semiconductor–metal transition (SMT) temperature of VO₂ to ∼44 °C while retaining a 4 order of magnitude SMT using the TiO₂ buffer layer. A combination of electrical transport and X-ray diffraction reciprocal space mapping studies help examine the specific strain states of VO₂/TiO₂/Al₂O₃ heterostructures as a function of TiO₂ film growth temperatures. Atomic force microscopy and transmission electron microscopy analyses show that the columnar microstructure present in TiO₂ buffer films is responsible for the partially strained VO₂ film behavior and subsequently favorable transport characteristics with a lower SMT temperature. Such findings are of crucial importance for both the technological implementation of the VO₂ system, where reduction of its SMT temperature is widely sought, as well as the broader complex oxide community, where greater understanding of the evolution of microstructure, strain, and functional properties is a high priority