Relaxation between closely spaced electronic levels of rare-earth ions doped in nanocrystals embedded in glass

The relaxation between closely spaced (separation \ensuremath{\sim}10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) electronic levels of rare-earth ions doped in insulating nanocrystals (size 10--25 nm) embedded in glass is studied by time-resolved fluorescence and fluorescence line narrowing techniques. For particle sizes below 25 nm the relaxation rate increases with a decrease in particle size. The increased relaxation rate is explained by the interaction of the electronic states of rare-earth ions in nanocrystals with the vibrational excitations of the surrounding glass medium and with mixed vibrational modes of the nanocrystallites and the glass. The possibility for the use of rare-earth doped nanocrystals as a probe of the dynamical processes in glass is discussed.