Energy flow in manganese-neodymium-doped fluoride glasses

Measurement of the time-response of the excited-state concentration of manganese and neodymium has confirmed our previous hypothesis of fast energy transfer betwen donors and acceptors in fluoride glasses. I. lNTRODUCTION The absorption and fluorescence properties of solid-state materials can often be enhanced by codoping with two ions: one ion (referred to as the donor) which absorbs the energy from the pump and transfers the energy to a second ion (referred to as the acceptor) which subsequently lases or fluoresces. The basic models for such non-radiative energy transfer have been developed extensively by Forsterl, Ilexter2, and Inokuti et al.3 In a recent paper,4 we developed a multiple-mechanism model, in which we proposed that within a certain donor-acceptor separation distance, the energy transfer occurs in a qualitatively faster manner than when the donor-acceptor separation distance is greater than this distance. This effect appears to be particularly prominent in glasses.5-7 In this paper, we will present data from manganese (donor) and neodymium (acceptor) doped fluoride glasses. We find an anomalously fast manganese-neodymium energy transfer, achieved by pumping at 409 nm and observing neodymium emission at 870 nm. We demonstrate how the relevant parameters of the transfer can be systematically derived from the experimental data. II. EXPERIMENTAL PROCEDURE We measured the temporal response of two fluoride glasses to optical pumping, Crystal A (36 PbF2, 24 MnF2, 35 GaF3, 2 AlF3, Y F3, 3.8 LaF3.0.2 Nd F3) and Crystal B (same as Crystal A but with 2 LaF3 and 2 NdF3). Thus, Crystal B is considerably more heavily doped than Crystal A. The measurements were conducted with an experimental setup with a 100 ns impulse response (determined by measuring the detector response to a sub-nanosecond dye-laser pulse train). The 409 nm excitation pulses (corresponding to a sharp Mn+2 Enrique Berman Professor of Solar Energy Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19944128 JOURNAL DE PHYSIQUE IV absorption band) were obtained from a PRA-LNlO2 dye laser pumped at 337 nm by PRA -LN103 nitrogen laser. The fluorescent output was filtered through a band pass filter centered at 600 nm (passing Mn+2 emission) or alternately a high pass filter at 780 nm (passing only the Nd+3 870 nmemission). The fluorescence was measured by a PM55 Hamamatsu photo-multiplier and recorded on a Tektronix 2430 digital scope. 111. RESULTS AND DISCUSSION The theory and parameters in our model have been previously described in Ref. 7. Figs. 1 4 show the donor excited concentration 605 nm decay obtained from Crystals A and B (pumping at 409 nm). (Two time scales are shown.) The decay shows a definite initial fast decay followed by a slower decay. The analysis described in Ref. 7 was performed and the results obtained are shown in Table 1. The fit obtained to the donor data is shown in these figures. The parameters of Table 1 were then used to calculate the predicted acceptor decay. An excellent fit is shown in Figs. 5 and 6. The actual results obtained for rl and the fact that it changes with crystal composition are very interesting. Future work should be directed towards analyzing phenomena present in glasses on the order of tens of nanometers which could contribute to the energy transfer. IV. SUMMARY We have confiied that our previously hypothesized fast transfer between Mn+z and Nd+3 is indeed fact. The inadequacy of the standard model for the initial decay in these systems has been shown. The efficacy of studying the donor and acceptor excited-state concentration for determining the coefficients of the ionic interactions has been validated. Acknowledgement This work was partially funded by the Israel National Council forResearch and Development. References[I] T. Forster, Ann. Physik. 2, 55 (1948).[21 D. L. Dexter, J. Chem. Phys. 21, 836 (1953).[3] M. Inokuti and F. Hirayama, J . Chem. Phys. 43, 1978 (1965).[4] S. R. Rotman, Chem. Phys. Lett. 173 (4), 349 (1990).[5] R. Reisfeld and M. Eyal, Acta Phys. Pol. A 72,799 (1987).[6] O.Maoz, S. R. Rotman, A. M. Weiss, R. Reisfeld and M. Eyal, J. Lum. 48-49, 213(1991).[7] S. R. Rotman, A. Hadad, P. Koczelnik, 0. Maoz, A. M. Weiss, R. Reisfeld and M.Eyal,Chem. Phys. Lett. 191 (1,2), 65, 1992. Crvstal A5.8 nm10.4 nm8.6 nm580 psec630 msecCrystal B3.6 nm8.4 nm5.4 nm580 psec380 psec Table 1: Results of the analysis of the lightly and heavily doped crystals (A and Brespectively).