Excited-state dynamics of the Tm3+ ions and Tm3+ → Ho3+ energy transfers in LiYF4

Effects of co-dopant concentrations and excitation conditions on the 2 ?m fluorescence dynamics in Tm, Ho:YLF crystals

“…The efficiency of process (i) is enhanced by an increase of the Tm concentration (due to very efficient T m~T m cross-relaxation). On the other hand the efficiency of the Tm 3H4~Ho 515 energy transfer process increases with the relative Ho concentration: Brenier et al [9] have shown that the efficiency of this process becomes comparable to that of T m~T m cross-relaxation for a suitably high value of the ratio nuo/nTm. Samples B and C are characterized by concentration ratios high enough to reasonably assume that the two processes are strongly competing.…”

“…Two additional processes have been recognized as likely occurring [9]. The first is up-conversion to Ho 5I 5 involving the metestable Tm 3F~ and Ho 5/7 states, shown as process (iii) in Fig.…”

“…A better understanding may be gained by monitoring the concentration dependence of selected processes taking place in the excited crystal. One of the most important among these processes is the time evolution of the Ho 3 + ion lowest excited level population, i.e., of the upper laser level population for lasers operating at 2 p.m.Measurements of the transient behavior of the fluorescence from this level have been reported in the literature for a large variety of co-dopant concentrations [9,11,12]. These data, together with those relative to fluorescence from the other excited levels in the Tm,Ho:YLF system, have permitted to draw a qualitative picture of the energy distribution mechanisms and to put forth hypotheses on those which predominate.…”

“…In order to operate the most appropriate choice one has to understand the effects of the concentrations on the relevant spectroscopic and lasing properties, and how these effects vary with the operating conditions. At present, this understanding is not complete, even though the Tm,Ho:YLF system has been extensively studied in recent years [5][6][7][8], and a significant amount of work has been done on measuring the fundamental system properties [9][10][11][12]. A better understanding may be gained by monitoring the concentration dependence of selected processes taking place in the excited crystal.…”

“…Just few data are available under intense pumping, and in this case the parameter which is considered as characterizing the upper laser level transient behavior is an "effective energy-storage time" derived from Q-switched pulse energy versus repetition rate measurements for cw pumping [12]. Where codopant-concentration effects are investigated [9,11,12], little or no mention is made of the pumping fluence effects on the fluorescence build-up and decay.The purpose of the work described here is to provide some original data that could help in completing those available in the literature. Consequently, we report results of measurements of the time evolution of the fluorescence at 2 gm, emitted from the 517 manifold of the Ho 3+ ion, carried out as a function of pumping laser fluence under conditions ranging from weak to very intense pulsed excitation.…”