Lutetium-Induced Enhancement of Pr³⁺ Upconversion Luminescence in Yttrium Oxybromide Toward Low-Threshold, Blue-Pumped Deep-Ultraviolet Lasing

Lanthanide ion-based upconversion is a highly promising approach for achieving deep-ultraviolet (DUV) emission. In particular, the visible-to-ultraviolet upconversion process of Pr³⁺ ions, which proceeds via an intermediate energy level, serves as an efficient technical approach to realize high-efficiency DUV emission under easily accessible blue light excitation. Herein, through 5% Lu³⁺ doping, we have strengthened the lattice rigidity of the tetragonal yttrium oxybromide (YOBr) host, resulting in an approximately 40% enhancement in the DUV luminescence performance of Pr³⁺. Mechanistic studies show that the enhancement is attributed to the highly suppressed non-radiative relaxation from the 4f¹5d¹ state without significant thermal ionization. Moreover, we unambiguously demonstrate the formation of broadband DUV random lasing, Fabry-Pérot cavity lasing, and whispering-gallery-mode microcavity lasing by employing these efficient DUV-emitting microcrystals as the laser gain medium. Notably, the pumping threshold is substantially reduced compared to conventional five-photon processes utilizing Tm³⁺ or Er³⁺ ions. Our findings highlight the viability of enhancing the DUV luminescence intensity in oxyhalides through compositional engineering and realizing DUV lasing via blue-pumped Pr³⁺-based upconversion material as a gain medium.