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(13.2)) and (2)H2(9.2) = ((2)H2(9.2) --> I-4(15.2)), excited at room temperature in low concentrated Er : YAG crystals with pulsed (at 532 nm) as well with cw (at 488 nm) lasers. Besides the general need for new spectroscopic data concerning laser materials, the interest for this study is generated by the fact that energy levels of very different quantum efficiencies (similar 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 intensities. 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 stronger than the lines starting from H-2(9,2) in accord with the corresponding 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 sums of Lorentzian functions and the multiplet to multiplet transition probabilities were estimated with the Judd-Ofelt theory. The choice of the linewidths in our model was based on the analysis of the room temperature absorption 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 that predicted by our model was obtained. (C) 2001 Elsevier Science B.V. All rights reserved.