Dielectric nanospheres are important components for photonic applications, where interactions between whispering gallery modes (WGMs) can be used to construct photonic band structures. Using the electromagnetic fields generated by relativistic electrons in a monochromated 200 keV scanning transmission electron microscope, we record electron energy loss spectral features reflecting the density of states (DOS) of the electric-type WGMs in SiO2 nanospheres over an energy range extending into the far-ultraviolet regime. These results demonstrate possibilities for mapping the local DOS of photonic systems in an electron microscope with a spatial resolution of a few nanometers.
Bibliographical noteFunding Information:
This project has been supported by NSF DMR-0405195, EEC-0117770, and 0646547 and the Semiconductor Research Corporation and facilities MRSEC DMR-0520404 and DMR-0547976 funding. A. Yurtsever is thanked for useful discussions.