## 4 Conclusions  
Crystalline \(\alpha\)-Al\({}_{2}\)O\({}_{3}\):Cr spheres were easily and quickly produced using laser synthesis under air atmosphere conditions with Al\({}_{2}\)O\({}_{3}\) and Cr\({}_{2}\)O\({}_{3}\) mixture powders as precursor materials, which can be extended to a large-scale process. The spheres were seen to be monophasic and with a polycrystalline nature and average doping of 0.47 at.% and 1.19 wt.% of Cr. Photoluminescence and photoluminescence excitation revealed that the main red luminescence is due to the single Cr\({}^{3+}\) ions in trigonal distorted Al\({}^{3+}\) octahedral sites, as identified by the dominant split R-lines due to the \({}^{2}\)E \(\rightarrow\)\({}^{4}\)A\({}_{2}\) transition. Additionally,  
Figure 8: RT (**a**) TRPL spectra obtained with \({}^{4}\)A\({}_{2}\)\(\rightarrow\)\({}^{4}\)T\({}_{1}\) excitation (similar spectra were obtained with \({}^{4}\)A\({}_{2}\)\(\rightarrow\)\({}^{4}\)T\({}_{2}\) excitation) and (**b**) decay times measured at R- and N\({}_{2}\)-lines with different excitation wavelengths. Broken lines: experimental fits to a single (R-lines, emission @ 1.792 eV)- and multi-exponential (N-lines, emission @ 1.761 eV) decay.  
emission from the parity and spin-forbidden \({}^{2}\)T\({}_{1}\)\(\rightarrow\)\({}^{4}\)A\({}_{2}\) transition (R\({}^{\prime}\)-lines) was also observed. Besides the recombination from single Cr\({}^{3+}\) ions, the synthesis conditions favor the observation of the radiative recombination from chromium ion pairs (N-lines), meaning that the produced samples are heavily doped. The presence of multiple Cr\({}^{3+}\) optical centers is corroborated by the presence of a new doublet of lines (R\({}^{\prime\prime}\)-lines) at low temperature and by the FWHM of the excitation bands, which are wider than those expected for the case of single Cr\({}^{3+}\) ions. At room temperature, the R-lines exhibit a single exponential decay of about 3.2 ms, while the N-lines can be well-fitted to a multi-exponential model with a fast decay time of less than 0.5 ms, an intermediate one of about 1.5 ms, and a slow lifetime similar to that of R-lines, suggesting that energy transfer processes between the different chromium optical centers are taking place.  
Conceptualization, A.F.C.; methodology, J.R., A.F.C., A.J.S.F., F.M.C. and T.M.; formal analysis, J.R., A.F.C., A.J.S.F., F.M.C. and T.M.; investigation, J.R., A.F.C., [PERSON], A.J.S.F., F.M.C. and T.M.; data curation, [PERSON], A.F.C. and T.M.; writing--original draft preparation, [PERSON] and T.M.; writing--review and editing, [PERSON], A.F.C., J.Z., A.J.S.F., F.M.C. and T.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the FCT--Portuguese Foundation for Science and Technology, within the scope of the project i3N, UIDB/50025/2020, UIDP/50025/2020, and LA/P/0037/2020.  
Not applicable.  
The data presented in this study are available in the article.  
[PERSON] acknowledges FCT for Program Stimulus of Scientific Employment-Individual Support (grant 2022.00010.CEECIND/CP1720/CT0023, DOI: 10.54499/2022.00010. CEECIND/CP1720/CT0023).  
The authors declare no conflicts of interest.