## 3 Results
### Infrared to Visible Upconversion  
Visible upconversion at room temperature was observed in both samples under continuous laser excitation in resonance with the \({}^{4}\)I\({}_{9/2}\) level, as shown in Figure 6. The observed green emissions correspond to \({}^{2}\)H\({}_{11/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) and \({}^{4}\)S\({}_{3/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) transitions of Er\({}^{3+}\) ions, which were located approximately at 525 nm and 550 nm, respectively. Nevertheless, red emission corresponding to the \({}^{4}\)F\({}_{9/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) transition was not observed in these glasses. This is due to the fact that the energy gap between levels \({}^{4}\)F\({}_{9/2}\) and \({}^{4}\)I\({}_{9/2}\) is 2264 cm\({}^{-1}\) and the maximum phonon energy is 1000 cm\({}^{-1}\) approximately [32]. Excited state absorption (ESA) and energy transfer upconversion (ETU) are the main processes associated to the upconversion emission of rare-earth ions [36]. Excitation mechanisms for populating the \({}^{2}\)H\({}_{11/2}\) and \({}^{4}\)S\({}_{3/2}\) levels under NIR excitation were investigated by studying the upconversion emission intensity \(I_{up}\) as a function of the infrared excitation power \(I_{IR}\). It is well-known that upconversion emission intensity increases proportionally to the nth power of the infrared excitation power according to the relation \(I_{up}\) k \(\propto\) (\(I_{IR}\))\({}^{n}\), where \(n\) is the number of photons involved in the pumping mechanism. Figure 7 shows the logarithmic plot of the green upconversion intensity \(I_{up}\) compared to the excitation power \(I_{IR}\) under excitation at 800 nm for both glass samples. Linear fit allowed determining the n-values, resulting in 1.47 and 1.52 for Er1 and Er4, respectively. Slope values below two indicates a saturation of the intermediate levels [37]. These results confirm that a two-photon step was involved in the upconversion process to populate the emitting levels. Figure 6: Upconversion emissions at room temperature for Er\({}^{3+}\) ions in aluminosilicate glasses under excitation at 800 nm. Figure 7: Dependence of green upconversion emission intensity on excitation power for Er\({}^{3+}\) ions under excitation at 800 nm. Possible mechanisms accounting for green upconversion emission under 800 nm excitation is presented in Figure 8. Green upconverted emission requires the population of levels with at least the energy of the \({}^{2}\)H\({}_{11/2}\) level or higher. The \({}^{4}\)I\({}_{9/2}\) level is resonantly excited by the pumping wavelength at 800 nm. Next, the thermalized levels \({}^{2}\)H\({}_{11/2}\) and \({}^{4}\)S\({}_{3/2}\) can be populated by means of two ESA processes or two ETU. On the first ESA, labelled as ESA1, a non-radiative relaxation from the \({}^{4}\)I\({}_{9/2}\) level to the lower \({}^{4}\)I\({}_{11/2}\) level is produced, from which the absorption of one photon populates the \({}^{4}\)F\({}_{3/25/2}\) level, followed by a non-radiative de-excitation to the \({}^{2}\)H\({}_{11/2}\) and \({}^{4}\)S\({}_{3/2}\) levels. On the second ESA, labelled as ESA2, an additional non-radiative relaxation from the \({}^{4}\)I\({}_{11/2}\) level to the \({}^{4}\)I\({}_{13/2}\) level is produced. Then, the absorption of one photon promotes the Er\({}^{3+}\) ions to the \({}^{2}\)H\({}_{11/2}\) and \({}^{4}\)S\({}_{3/2}\) levels. These two processes involve only one Er\({}^{3+}\) ion. Nevertheless, two upconversion mechanisms involving the interaction of two nearby Er\({}^{3+}\) ions in the \({}^{4}\)I\({}_{11/2}\) level and entailing an energy transfer are possible. In the first ETU, denoted as I, the mechanism can be described as Er\({}^{3+}\)(\({}^{4}\)I\({}_{11/2}\)) + Er\({}^{3+}\)(\({}^{4}\)I\({}_{11/2}\)) \(\rightarrow\) Er\({}^{3+}\)(\({}^{4}\)I\({}_{15/2}\)) + Er\({}^{3+}\)(\({}^{4}\)F\({}_{7/2}\)), followed by a non-radiative de-excitation to the \({}^{2}\)H\({}_{11/2}\) and \({}^{4}\)S\({}_{3/2}\) levels.