## 2 Materials and Methods  
Material synthesis was carried out using a commercial CO\({}_{2}\) 50 W laser marking system (Redsail M500) with air-assist off and the XY motion disabled. Al\({}_{2}\)O\({}_{3}\) (Sigma-Aldrich, St. Louis, MO, USA, activated, basic, Brockmann I, powder \(\geq\) 99.5%) and Cr\({}_{2}\)O\({}_{3}\) (Sigma-Aldrich, St. Louis, MO, USA, powder \(\geq\) 98%) powders were mixed in an agate mortar in a 97:3 (\(w/w\)) proportion, according to reference [42], to lead to the formation of Cr-doped Al\({}_{2}\)O\({}_{3}\). Then, the powders were inserted in a \(\O\) 6 mm \(\times\) 6 mm graphite crucible. The crucible was then placed into the laser system with the top of the powder set at 4 cm below the focal point of the system. This process is performed out-of-focus to minimize thermal gradients and allow the production of larger spheres. Also, if the laser was focused on the powder surface, a violent ejection of material would occur at the \(\sim\)100 \(\upmu\)m irradiated region. After five 1-second bursts at 5 W to sinter the surface of the powder and prevent the ejection of powder during temperature ramp-up, the laser was set at 50 W for 20 s. The sample was then left to cool, and the produced sphere was removed from the crucible. One sphere was produced each time. Figure 1 displays a schematic representation of the experimental setup used to synthesize the Al\({}_{2}\)O\({}_{3}\):Cr spheres. Sample morphology was evaluated using secondary-electron scanning electron microscopy (SEM) on a TESCAN VEGA3 microscope with a working distance of 15 mm and a 10 keV electron beam. A carbon coating was applied to reduce the charge effect. Additionally, energy dispersive X-ray (EDS) spectra were acquired with a Bruker XFlash Detector 410-M to assess the doping levels. X-ray diffraction (XRD) analysis was employed to identify the crystalline structure of the prepared samples using a PANalytical X'Pert PRO diffractometer operating in the Bragg-Brentano configuration and with CuK\(\alpha\) radiation (\(\lambda\) = 1.54056 A). Raman spectroscopy measurements were performed at room temperature (RT) in a backscattering configuration on a Horiba HR800 micro-Raman spectrometer equipped with a 600 grooves mm\({}^{-1}\) grating and using the 441.6 nm laser line from a HeCd laser (Kimmon IK Series, Fukushima, Japan) and a 50\(\times\) objective (spot size \(\sim\)2 \(\upmu\)m, NA = 0.7, Olympus). A neutral density filter OD = 1 was used to attenuate the laser power to prevent the luminescence from saturating the detector. Steady-state PL was carried out by exciting the samples with the 325 nm (\(\sim\)3.81 eV) line from a cw He-Cd laser (Kimmon IK Series, excitation power density \(\sim\)0.6 W cm\({}^{-2}\)). The samples were placed in a cold finger of a closed-cycle helium cryostat and the temperature was controlled in the range between 14 K and RT. A dispersive system SPEX 1704 monochromator (equipped with a 1200 grooves mm\({}^{-1}\) diffraction grating) coupled to a cooled Hamamatsu R928 photomultiplier tube was used to measure the luminescence of the produced spheres. Further RT energy excitation-dependent PL and PLE experiments  
Figure 1: Schematic representation of the experimental setup used to synthesize the spheres. were performed in a Fluorolog-3 Horiba Scientific modular system, equipped with a double additive grating Gemini 180 scanning monochromator (with 1200 grooves mm\({}^{-1}\) diffraction grating) for excitation, and an iHR8550 spectrometer (1200 grooves mm\({}^{-1}\) diffraction grating blazed at 500 nm) for the emission. This system is also fitted with an R928P photomultiplier detector. A 450 W Xe arc lamp was used as the excitation source. The PLE measurements were conducted by setting the emission monochromator at the maxima of the Cr\({}^{3+}\) emission lines and the excitation wavelengths being scanned to higher energies.