Red-Emitting Cr³⁺ on α-Al₂O₃:Cr Spheres Obtained in Seconds Using Laser Processing

Cr-doped Al2O3 spheres with strong red emission were produced using a simple laser processing approach with a 50 W continuous CO2 laser. Structural characterization revealed that the produced spheres were monophasic, comprising the α-Al2O3 phase. Photoluminescence (PL) studies indicated that the observed red emission originates from multiple Cr3+ optical centers being dominated by the 2E → 4A2 transition (R-lines), with a further contribution from the parity and spin forbidden 2T1 → 4A2 transition (R′-lines). The identification of additional radiative recombination from chromium ion pairs (N-lines) evidences that the produced samples are heavily doped. As such, energy transfer processes between the different chromium optical centers are seen to take place, as suggested by the lifetime decay analysis. PL excitation revealed that the room temperature luminescence is preferentially populated via the spin-allowed 4A2 → 4T2,4T1 transitions and by the parity and spin-forbidden 4A2 → 2T2 (B-lines), 2T1 (R′-lines), 2E (R-lines). Such results demonstrate that the present synthesis method is able to deliver high-optical-quality Al2O3:Cr crystals in a fast and simple way, with potential interest for optical, sensing, or lasing applications.