Fluorescence dynamics in Tm,Ho:YLF following 800 nm pulsed laser excitation

Abstract

We have performed spectroscopic studies that ex- amined the time evolution of the fluorescence at wavelengths between 400 and 2500 nm, emitted from an optically excited Tm,Ho:YLF crystal in response to pulsed laser excitation at 780 nm. The fluorescence intensity decay patterns have been analyzed in terms of their time and amplitude characteris- tics, paying attention to the dependence of these characteris- tics on excitation laser fluence. With the aid of a simplified model for numerical simulations, we have been able to quan- tify and evaluate the effects of a number of processes whose relative importance varies with excitation fluence, such as up- conversion losses and ground-state depletion. provided by the analysis of the temporal evolution of the fluo- rescence emitted in the visible and in the near infrared, in re- sponse to pulsed excitation. Unfortunately, the data reported in the literature are rather sparse and incomplete, particular- ly for co-doped YLF crystals. Reported measurements of the transient behavior of the fluorescence from these crystals are restricted to emissions from a very few multiplets, namely the Tm 3 H4 and 3 F4 ,a nd theHo 5 I7 (6-12). In this work, the results of a comprehensive experimental investigation of the fluorescence emitted from a Tm,Ho:YLF crystal following 800 nm pulsed laser excitation are present- ed. In particular, the results of measurements of the time evolution of all the emissions we were able to detect in the spectral region between 400 and 2500 nm are reported. The measurements were carried out at room temperature as a function of pumping laser fluence under conditions ranging from weak to intense excitation. This has allowed the de- termination of the laser fluence dependence of the emission intensities, rise-times and lifetimes. These data complement data already available in the literature and are important for the understanding of the relationships among the processes taking place inside these co-doped crystals, and consequent- ly for optimizing Tm,Ho lasers. Moreover, these data can be exploited for developing reliable models for numerical simu- lations of the optical pumping dynamics. Validation of these models requires comparison of their predictions with a com- plete and consistent set of experimental spectroscopic data. As a final part of this work, we have verified the experimen- tal data against the predictions of a simplified rate equation model of the co-doped system. This has enabled the deter- mination of some of the rate parameters governing the most relevant energy transfer processes in a Tm,Ho:YLF crystal.