Quenching And Luminescence Efficiency Of Nd 3+ In YAG

Abstract

ABSTRACT The effect of the concentration luminescence quenching of the F-,2 level of Nd in YAG on the relative efficiency is presented. Based on the analysis of' the decay curves in terms of the energy transfer theory, an analytical expression for the relative luminescence efficiency is obtained. In the low concentration range (up to ^1.5 at % Nd3+) , the efficien­ cy linearly decreases when Nd^+ concentration increases. It is also stressed that pairs quenching contribute about 20 % to the nonradiative energy transfer losses.Quantum efficiency of luminescence is an important parameter for the characterization of laser active media; its lowering is due to either multiphonon relaxation or energy transfer processes. The multiphonon non-radiative probability depends on the energy gap between le­ vels, on the phonon energy and temperature; usually at low activator doping it is practicab­ ly independent on concentration. On the other hand, energy transfer losses show a marked dependence on activator concentration, a fact that severely limits the range of useful con­ centration of active centers in some laser crystals.In the YAG:Nd case the minimum energy gap between the Stark components of the ^3/9the next lower level Fi5/o ^s °f about 4700 cm1 . Since in YAG the phonons most effectively coupled to the Rare Eartn Ions have an energy of ^ 700 cm~l, the probability for multiphonon relaxation from the F3/2 level, even at room temperature, is very low and therefore for low Nd3+ concentrations' the quantum efficiency is expected to be close to 1 .Several methods have been used to measure the quantum efficiency of YAG:Nd samples: basically these methods could be divided into three groups; (i) measurement of the total luminescence from the sample and comparison with the calculated pump of the 4F level; (ii) measurement of the heating, and (iii) comparison of the observed luminescence decay with the decay expected from radiative processes , i.e.where TO is the radiative lifetime and I (t) is the time evolution of luminescence. If tne decay I (t) is exponential with the lifetime T^, the relation (1) reduces tofl = L£ (2) T0a widely used relation to estimate the efficiency (sometimes even if tne decay is not expo­ nential) .The reported data on quantum efficiency for YAG:Nd show a very large spread, from 0.5 to 1. This spread of data can be due either to some physical processes in the samples, or to inaccuracies in the experimental method. Thus the measurements reported for the same sample but using different methods are 1 1 and 0.562.The value n = 1 was reported even for samoles containing as much as 1 at % Nd . In view of the recent studies on energy transfer in YAG:tfd ' this value is unreasonably lar­ ge. In this paper we discuss the limiting effect of the energy transfer processes on the quantum efficiency, at different Nd concentrations.The energy transfer processes induce departures from the exponential decay, described by a function P(t), so that the observed decay I (t) can oe written asI(t) = I0 exp (-t/t) exp(-P(t)) (3)where T is the decay time in absence of transfer. The experimental curve P(t) can be obtained by substracting from the experimental decay, I(t), the exponential part described by life­ time T, measured for very diluted samples. The parameters characterizing the transfer and their concentration dependence can be obtained by fitting the experimental curves P(t)