A Correlative Study of Plasmonic Metasurfaces by Electron Energy-Loss Spectroscopy and Optical Spectroscopy: From Nanoscale Hotspots to Macroscale Optical Response.

We present a dual probe study of plasmonic metasurfaces using both electron energy-loss spectroscopy and optical spectroscopy. By fabricating plasmonic metasurfaces on ultrathin membranes, we enable both optical reflection and transmission spectroscopy, alongside electron transmission microscopy (TEM). This dual-probe technique provides a comprehensive insight into the energy absorption mechanisms and allows a precise spatial mapping of plasmonic hotspots at the metallic nanostructures. Finite element simulations were employed to optimize the metasurface geometry for obtaining high-intensity localized surface plasmon resonances (LSPRs) in the visible range, identify plasmonic hotspots, and elucidate experimentally observed resonance modes. By comparing the spectra obtained by the two methods with simulations, we identify the dominating dipole LSPR modes of Al nanodiscs placed on 200 nm tall SiO nanopillars in a hexagonal array with pitch 340 nm. The methodology here demonstrated, comprising optical transmission spectroscopy and TEM, can be readily extended to studies of other nanostructured samples.