Dynamical process of rare earth ions doped in nanocrystals embedded in amorphous matrices
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The optical dephasing processes of rare earth ions doped in nanocrystals embedded in glass are studied at low temperatures with both time and frequency domain coherent spectroscopic techniques – the two-pulse photon echo and spectral hole burning. The temperature and nanocrystal size dependence of the homogeneous linewidth and the optical dephasing time are investigated, and the spectral diffusion effects occurring in the time scale of the hole burning measurement is discussed. The quasi-linear temperature dependence of the homogeneous linewidth obtained with spectral hole burning at low temperatures for Eu3+ ions doped in nanocrystals embedded in amorphous matrices is similar in character to what is observed from the ions directly doped in glass. This suggests that the interaction of the Eu3+ ions doped in nanocrystals with the two- level system of the glass matrix dominate the optical dephasing of Eu3+ ions. For the Pr3+:LaF3 nanocrystals embedded in oxyfluoride glass, the results of the two-pulse photon echo and spectral hole burning further support this assertion that the interaction of the Pr3+ ions in nanocrystals with the two-level system of the glass dominates the optical dephasing at low temperatures. However, a higher power term appeared in the temperature dependence of the line width, which is observed only in the hole burning measurement at temperatures greater than 4 K, suggesting that another new mechanism begins to dominate the dephasing process instead of the interaction with the two-level system. Attempts to identify spectral diffusion are made by comparing the homogeneous broadening measured with the two-pulse photon echo and spectral hole burning. In addition to the homogeneous broadening, other effects on the excited state dynamics resulting from the confinement effect of the nanocrystals and from the interaction with the surrounding glass matrix are studied. These include the effects of the size and the surrounding medium on the fluorescence lifetime and the effect of confinement on the relaxation rate between two closely spaced electronic levels due to single phonon processes.