Optical resonances in silica-nanosphere dimers and trimers probed with high-energy electrons at the nanoscale

Optical dielectric resonances have the ability to modulate optical energy, and by coupling multiple resonators into photonic molecules, new opportunities emerge in spectral engineering. Using electron-beam spectroscopy, in a 300-keV scanning transmission electron microscope, we probe optical modes of photonic molecules at the nanometer scale. Monochromated electron energy-loss spectra of 400nm-silica-sphere monomers, dimers and trimers display series of sharp peaks in the ultraviolet region. For monomers, the peaks are assigned to low angular number optical whispering gallery modes. For dimers and trimers, experimental observations, supported by simulations, reveal energy shifts and peak broadening resulting from coupling between spherical modes. Variations in spectra for different electron probe positions demonstrate the possibility to extract information on local photonic density of states at the nanometer scale.