  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.
