Upconversion, lifetime quenching, and ground-state bleaching in Nd³⁺:LiYF₄

Since the Nd3+:LiYF4 system has some advantage over Nd3+:YAG and Nd3+:YVO4 for high-power scaling of diode-end-pumping, this system has been investigated under strong excitation, in this case using a Ti:sapphire pump. The interionic processes responsible for fluorescence saturation have been determined, due allowance being taken for the significant ground-state bleaching under these conditions. Their temperature dependence, which is relevant to scaling consideration, has been investigated theoretically, and found to be rather small over a wide temperature range. By comparing the experimental data with finite-element rate-equation calculations, the influence of interionic upconversion is determined quantitatively, and a published value of the upconversion parameter is confirmed. The spatial dependence of ground-state bleaching and quenching of the fluorescence lifetime is calculated. Analytical expressions are derived, including the influence of interionic upconversion, for the dependence of ground-state bleaching, excitation density, and storage time on pump parameters and dopant concentration.