Formation of guided Cherenkov radiation in silicon-based nanocomposites

We use a monochromated 200 keV electron beam as a nanometer-resolution probe of the photonic density of states in bulk and nanoparticle Si/SiO(2) systems, observing infrared to ultraviolet waveguided Cherenkov modes in Si slabs, but none in SiO2. While isolated Si nanoparticles are too small to support Cherenkov radiation, we find high densities of Si nanoparticles in SiO2 support a damped form of the radiation, with the modes determined by the effective medium of the Si/SiO(2) mixture. The guided nature of the radiation is confirmed by the presence of a thickness-dependent cutoff.