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 Sienriched 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 alummophosphate 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.