Deactivation effects of the lowest excited states of Er3+ and Ho3+ introduced by Nd3+ ions in LiYF4 crystals

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Abstract
The deactivation effects of the lowest excited states of Er3+ and Ho3+ introduced in Er-or Ho-LiYF4 (YLF) crystals, codoped with Nd ions, were observed by the activator’s fluorescence decays. In the case of Er:Nd:YLF, the I13/24→4I15/2 and I11/24→4I13/2 transitions at 1.5 and 2.7 μm, respectively, were analyzed. The I75→5I8 and I65→5I7 transitions at 2.1 and 2.9 μm, respectively, were investigated for the Ho:Nd:YLF system. Laser excitations generated by a tunable optical parametric oscillator were used in this investigation. The use of a resonant laser excitation to induce the fluorescence allowed accurate measurements of the donor fluorescence decay by a time-resolved infrared spectroscopic system with a time resolution of 0.5 μs. As a result, a general criterion for the migration mechanism, involved in the donor to acceptor energy transfer, was proposed and depends on a parameter R. This parameter was defined as the ratio between the transfer rate obtained from the best fit of the fluorescence decay and the theoretical transfer rate predicted by the diffusion model. It was observed that the donor to acceptor transfer is always dominated by a diffusion migration (R∼1) if the donor is in the second excited state, despite a great variation of the CDD/CDA ratio (from ∼1 to 371). Nevertheless, a discrete energy migration was found to dominate in the [Ho, Er]→Nd energy transfer when the first excited state of the activator is involved. In this case, the experimental value of the transfer rate is smaller than expected according to the hopping model. Introducing a finite trapping efficiency of an exciton migration in the hopping model, all the observed experimental results were explained. The presence of Nd ions, in addition to decreasing the lifetime of the first excited state of Er3+ and Ho3+ in YLF, also depopulates the second excited state (partially), depending on the Nd concentration used.