[10a-W621-1~13] 3.12 Nanoscale optical science and near-field optics

[10a-W621-2] Cathodoluminescence observation of single Ag nanoparticles coupled to silver substrates

〇Takumi Sannomiya^1,2, Taeko Matsukata¹, Takayuki Okamoto³, Naoki Yamamoto¹ (1.Tokyo Tech., 2.JST PRESTO, 3.Riken)

Keywords:nanoparticle, cathodoluminescence, coupled mode

Metallic nanoparticles can couple to the flat metal surface. We investigate this coupling mode using scanning transmission electron microscopy combined with cathodoluminescence (STEM-CL). Silver particles are deposited on a silver surface covered with 100 nm silica. Silver particles were deposited by thermal evaporation in argon atmosphere, which can produce almost perfectly spherical particles. Without the silver surface, we observed dipole and quadrupole modes well matching the Mie theory. With the silver surface, the dipole modes split into two resonance peaks when the particle diameter exceeds 100 nm. This can be understood as coupled modes generated from the localized surface plasmon resonance of the nanoparticle and Fabry-Perot cavity resonance generated within the silica spacer layer between the silver particle and the silver surface.
Thanks to this strong coupling of the particle to the substrate, we are able to optically excite the nanoparticle through surface plasmon polariton (SPP) on the silver surface. By electron beam, SPP can be directly generated simply by hitting the metal surface by focused electron beam at desired positions. At the same time as SPP excitation, transition radiation is coherently induced by accelerated electrons. Since all these processes are coherent, the generated propagating light can interfere each other. We try to extract the phase of the coupled resonance modes by using this interference. We can also map the SPP interference field with phase, by angle- and polarization- resolved STEM-CL.