Selective Probing of Longitudinal and Transverse Plasmon Modes with Electron Phase-Matching

• Published in: Journal of physical chemistry. C 2023-11, Vol.127 (45), p.22252-22264
• Main Authors: Aguilar, Franck, Lourenço-Martins, Hugo, Montero, Damián, Li, Xiaoyan, Kociak, Mathieu, Campos, Alfredo
• Format: Article
• Language: English
• Subjects: C: Physical Properties of Materials and Interfaces; Physics
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.3c03998

The optical properties of metallic nanoparticles are dominated by localized surface plasmons (LSPs). Their properties depend on the constituting material, the size, and shape of the nano-object as well as its surrounding medium. In anisotropic structures, such as metallic nanorods, two families of modes generally exist, transverse and longitudinal. Their spectral and spatial overlaps usually impede their separate measurements in electron energy loss spectroscopy (EELS). In this work, we propose three different strategies to overcome this difficulty and selectively probe longitudinal and transverse modes. The first strategy is numeric and relies on morphing of nanostructures, rooted in the geometrical nature of LSPs. The two other strategies exploit the relativistic and wave nature of the electrons in an EELS experiment. The first one is the phase-matching between the electron and the plasmon excitation to enhance their coupling by either tilting the sample or modifying the electron kinetic energy. The second onepolarized EELS (pEELS)exploits the wave nature of electrons to mimic selection rules analogous to the one existing in light spectroscopies. The abovementioned strategies are exemplifiedeither experimentally or numericallyon a canonical plasmonic toy model: the nanorod. The goal of the paper is to bring together the state-of-the-art concepts of EELS for plasmonics to tackle a pedestrian problem in this field.