# Spectral Separation of Up-Conversion Luminescence Processes
## 3 Experiment  
We have performed measurements of UCL on a sample of the fluorophosphate glass, doped with rare-earth Yb\({}^{3+}\) and Tm\({}^{3+}\) ions at concentrations of 10% and 0.1%, respectively. The chemical composition of the glass samples was 5 Ba(PO\({}_{3}\))\({}_{2}\)-84.9(AlF\({}_{3}\)-CaF\({}_{2}\)-MgF\({}_{2}\)-BaF\({}_{2}\)-SrF\({}_{2}\))-10 YbF\({}_{3}\)-0.1 TmF\({}_{3}\). The sample of the fluorophosphate glass was fabricated at the ITMO University, St. Petersburg, Russia [19]. This sample was also used in our recent work for line verification in luminescence spectra of up-conversion processes [18]. The measurement scheme is depicted in Figure 2. UCL was excited by the CW diode laser ML151-980 (\"Milon Laser\"Co Ltd., St. Petersburg, Russia) with a power of up to 2 W at the near-infrared region \(\lambda_{pump}\approx 975\) nm. The UCL radiation was recorded with the spectrometer S-100 (\"Solar LS\", Minsk, Belarus). UCL was collected from the lateral face of the sample to simplify filtering of the pump. Each spectral point was determined as a result of averaging over 10 measurements; each measurement was carried on during 7 ms. In our scheme, the UCL was sensitized. The laser excited ions Yb\({}^{3+}\). Then, one has the upconverting energy transferred from the sensitizer Yb\({}^{3+}\) to the acceptor ion Tm\({}^{3+}\). Such an excitation scheme is typical for this sensitizer-acceptor ion pair, and allows for an efficient cascaded energy transfer through a sequence of Tm\({}^{3+}\) levels (the scheme of the participating levels is shown in Figure 1) [18; 20; 54]. For our consideration, we used the following observed UCL processes excited by the laser at 975 nm in the wavelength range of 300-900 nm (for the state notations see Figure 1). Figure 2: The scheme of the experimental set-up for measuring up-conversion luminescence spectra of a fluorophosphate glass sample. In the region of 335-380 nm with a sufficiently strong excitation power (700-2000 mW), one detects UCL from two different levels, \({}^{1}\!I_{6}\to{}^{3}\!F_{4}\) at 350 nm and \({}^{1}\!D_{2}\to{}^{3}\!H_{6}\) at 365 nm, corresponding to the processes with different ENs. The third UCL process \({}^{3}\!P_{0}\to{}^{3}\!F_{4}\) is quite weak, and left only negligible trace at 337 nm. Also notice that the processes \({}^{1}\!I_{6}\to{}^{3}\!F_{4}\) and \({}^{3}\!P_{0}\to{}^{3}\!F_{4}\) have a very close EN, and could hardly be separated by our method in (6) and (7) for the considered range of pump powers. These processes can still be separated using a stronger pump and larger Tm\({}^{3+}\) ions concentrations such that cross-relaxation processes strongly impact the observed ENs. However, for such pump powers, the validity of the factorization (2) is questionable. So, we are not considering this possibility in the current work. Here, we considering only UCL from two different levels \({}^{1}\!I_{6}\to{}^{3}\!F_{4}\) at 350 nm and \({}^{1}\!D_{2}\to{}^{3}\!H_{6}\) at 365 nm, corresponding to the processes with different ENs. 2. In the region of 440-500 nm, three processes with different ENs are possible as well: very weak UCL at 450 nm \({}^{1}\!I_{6}\to{}^{3}\!H_{4}\), relatively small UCL at 450 nm \({}^{1}\!D_{2}\to{}^{3}\!F_{4}\), and almost an order of magnitude more UCL at 475-480 nm \({}^{1}\!G_{4}\to{}^{3}\!H_{6}\). 3. In the third region near 620-680 nm, two significantly overlapping lines are observed \({}^{1}\!D_{2}\to{}^{3}\!H_{4}\) at 643 nm; and \({}^{1}\!G_{4}\to{}^{3}\!F_{4}\) at 648 nm. 4. In the IR region at 720-840 nm, two processes of UCL with comparable efficiency are observed \({}^{1}\!G_{4}\to{}^{3}\!H_{5}\) at 775 nm, and \({}^{3}\!H_{4}\to{}^{3}\!H_{6}\) at 795 nm. No traces of UCL through processes \({}^{1}\!D_{2}\to{}^{3}\!F_{2,3}\) were observed, apparently because the share in the branching of these processes from level \({}^{1}\!D_{2}\) is quite small (4% and 6%) [17], and the excitation process is not sufficiently effective to excite them.