Upconversion violet emission in diluted Er:YAG crystals

In this paper we discuss the upconversion processes responsible for violet luminescence (in the domain 0.39 ÷ 0.42 μm) from the Erbium levels 2 P 3/2 (transition 2 P 3/2 → 4 I 13/2 ) and 2 H2 9/2 ( 2 H2 9/2 → 4 I 15/2 ) in diluted Er: YAG crystals, excited with pulsed (532 nm) as well with cw (at 488 nm and 784 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 (∼ 0.3 for 2 P 3/2 and ∼ 0.0004 for 2 H2 9/2 can produce, function of the pumping conditions, fluorescence spectra of comparable intensities or even with reversed intensity ratio. Thus, though for 532 nm pulse excitation (pump transition 4 I 15/2 → 4 S 3/2 , the intensity of the lines originating from 2 P 3/2 is much stronger than the intensity of the lines starting from 2 H2 9/2 , in accord with the corresponding quantum efficiencies, for cw pumping, the excitation at 488 nm (pump transition 4 I 15/2 → 4 F 7/2 ) produces luminescence spectra of comparable intensities, while for 784 nm ( 4 I 15/2 → 4 I 9/2 ) pumping, the lines originating from 2 H2 9/2 level are more intense. Various two- or three-step upconversion mechanisms (ESA at low Erbium concentrations) were proposed to explain the observed luminescence spectra. We found that a rather good description of the intensity ratio of the luminescence spectra, obtained in various experimental conditions, could be obtained in the frame of the Judd-Ofelt model, if a quantitative analysis of the resonance of the ESA transitions with the pumping quanta is performed.