Transient Charge Trapping in Eu3+ -Doped Nanocomposites by Ultrafast Laser Excitation

Eu3+ was incorporated as probe ions into nanocomposite glasses, such as metal silver doped phosphate glass and crystalline nanocomposites Si-enriched SiO2 glass. Luminescence studies show that the doped europium ions are present as trivalent Eu3+, and are distributed in the matrix on the boundary surface of nanoparticles. The valence switching from Eu3+ to Eu2+ was observed in Si-SiO2 nanocomposites when ultrafast laser excitation was applied. In silver metal nanoparticles embedded aluminophosphate glass, a time-resolved ultrafast degenerate-four-wave-mixing (DFWM) experiment shows enhanced third-order nonlinearity at zero-delay time, followed by a bell-like signal buildup. It is attributed to the creation of electronic polaron and vibronic Wannier-Mott exciton (WME). In europium codoped sample, however, the bell-like signal is depressed. All the above observations are interpreted as the result of a strong Coulomb interaction between conduction electrons produced inside the nanoparticles by laser excitation and Eu3+ ions residing near the boundary surface. The trivalent europium ions play the role as positive charges attracting electrons. This results in temporary formation of Eu2+, and blocks the resonant tunneling transition in the silver-glass system to avoid creating large radius polaron.