Excited-state-absorption in low concentrated Er : YAG crystals for pulsed and cw pumping

in this paper we discuss the upconversion processes responsible for violet luminescence from the erbium levels P-2(3.2) (transition P-2(3.2) --> I-4(1 3.2)) and (2)H2(9.2) = ((2)H2(9.2) --> I-4(15.2)), excited at room temperat ure in low concentrated Er : YAG crystals with pulsed (at 532 nm) as well w ith cw (at 488 nm) lasers. Besides the general need for new spectroscopic d ata concerning laser materials, the interest for this study is generated by the fact that energy levels of very different quantum efficiencies (simila r to0.25 for P-2(3.2) and similar to0.0004 for (2)H2(9.2)) can produce, in favorable pumping conditions, fluorescence spectra of comparable intensitie s. Thus, though for 532 nn pulse excitation (pump transition I-4(15.2) --> S-4(3.2)), the intensity of the lines originating from P-2(3.2) is much str onger than the lines starting from H-2(9,2) in accord with the correspondin g quantum efficiencies, cw pumping at 488 nm (pump transition I-4(5.2) --> F-4(7.2)) produces luminescence spectra of comparable intensities. In order to explain the relative intensities of the observed luminescence spectra, various two-step upconversion mechanisms (excited state absorption (ESA) at low erbium concentrations) were proposed. We found that for cw excitation, an analysis of the energetic resonance between ESA transitions and pumping quanta was necessary. For this, we used a simple model to simulate the ESA spectra, in which both initial and final states were approximated with sum s of Lorentzian functions and the multiplet to multiplet transition probabi lities were estimated with the Judd-Ofelt theory. The choice of the linewid ths in our model was based on the analysis of the room temperature absorpti on and fluorescence spectra and on the possible resonances with the phonons . A fairly good agreement between the observed intensity ratio of the P-2(3 .2) --> I-4(13.2) and (2)H2(9.2) --> I-4(15.2) luminescence spectra and tha t predicted by our model was obtained. (C) 2001 Elsevier Science B.V. All r ights reserved.