Population dynamics of the F4 and H4 levels in highly-doped Tm:ZB(L)AN glasses

Population dynamics of the ³F₄ and ³H₄ levels in Tm³⁺ doped ZB(L)AN glasses was studied for Tm³⁺ concentrations from 0.5 to 12 mol%. Fluorescence waveforms from these levels were measured at 1.8 μm (³F₄) and 800 nm (³H₄) with both direct and indirect pumping. Decay from the ³F₄ level was found to be exponential with non-radiative decay rates proportional to the square of the Tm concentration. This indicated a process of energy migration by diffusion within the excited Tm³⁺ ions followed by quenching at sites to which the ions could migrate. The decay of the directly pumped ³H₄ level exhibited both exponential and non-exponential behavior depending on the concentration. For the lowest concentration (0.5 mol%) the decay was exponential, but at concentrations of 1, 2, 4 and 6 mol% the decay waveforms were distinctly non-exponential. The non-exponential waveforms could be fitted by the Yokota-Tanimoto model for diffusion of excited donors and dipole-dipole interactions with acceptors. This model produced values for C_DD and C_DA, the donor-donor and donor-acceptor energy transfer parameters, respectively. At the higher concentrations (8, 10, 12 mol%) the waveforms were exponential with decay rates from which the cross-relaxation parameter for the process ³H₄, ³H₆ → ³F₄, ³F₄ was obtained. When the ³F₄ level is pumped at 1660 nm, the decay of the ³H₄ level confirmed the influence of the up-conversion energy transfer process ³F₄, ³F₄ → ³H₄, ³H₆.