Yellow light is widely used in many fields, but the existing techniques for obtaining yellow lasers (such as dyes laser, semiconductors laser, and nonlinear frequency conversion technologies) have the disadvantages of complex systems, high cost, difficult operation, and poor beam quality. The emerging approach of using high-power blue laser diodes (LDs) to directly excite rare earth ions (especially Dy³?) for yellow light generation provides a more economical and efficient way to obtain yellow light. Dy³? ions are ideal yellow emission centers due to their specific energy level transitions. The maturity of GaN-based blue LD technology has promoted the development of Dy³⁺-doped solid-state yellow lasers. Co-doping with the deactivator ion Tb³? can optimize the luminescence properties of Dy³⁺. At the same time, the introduction of Y³⁺ (an inactive center ion) complicates the crystal structure and further improves the luminescence intensity of rare earth ions. In this paper, Na₅Lu₉F₃₂ (NLF) fluoride single crystals were selected as the matrix, as their excellent physical and chemical properties and low phonon energy can reduce non-radiative relaxation. Dy³⁺/Tb³⁺/Y³⁺ co-doped NLF single crystals were grown by the Bridgman method, and the effect of Y³⁺ doping concentration on yellow light enhancement was studied to obtain a strong yellow-light-emitting material as an effective gain medium for high-power blue LD-directly pumped yellow lasers.

High-quality NLF single crystals doped with Dy³⁺, Tb³⁺, and Y³⁺ were grown by the Bridgman method. The raw materials used for crystal growth were fluorides: NaF (99.999%), LuF₃ (99.999%), DyF₃ (99.999%), TbF₃ (99.999%), and YF₃ (99.999%). The doping concentration (atomic fraction) of Dy³⁺ and Tb³⁺ ions was fixed at 0.5%, and the doping concentration of Y³⁺ ions was set to 0, 1.0%, 2.0%, and 4.0%, respectively, according to the compositional ratio of 50NaF-(49-γ)LuF₃-0.5DyF₃-0.5TbF₃-γYF₃. The single crystals were characterized by an X-ray diffractometer (XRD; Bruker D8 Advance, Germany). Rietveld refinement analyses were performed using XRD data and the FullProf program. The absorption spectra were measured by a Cary5000 UV/VIS/NIR spectrophotometer. An FLSP920 spectrometer (Edinburgh, UK) was used to detect the emission spectrum, fluorescence spectrum, and fluorescence decay curve at 573 nm.

The doped ions (Dy³⁺, Tb³⁺, and Y³⁺) successfully replaced the lattice sites of Lu³⁺ ions in NLF crystals (Table 1). With the doping of Y³⁺ ions, the Ω₂ value of the crystal increased, which means that the symmetry of the local crystal field around Dy³⁺ ions gradually decreases (Table 2). Dy³⁺ ions in Dy³⁺/Tb³⁺/Y³⁺ tri-doped NLF single crystals had a high probability of radiative transition, indicating that the crystal can release energy in the form of light more effectively, thus exhibiting higher luminous efficiency. At the same time, the radiative lifetime of the Dy³⁺:⁴F_9/2 energy level was longer than that of other Dy³⁺-doped materials, indicating that the crystal has good energy storage capacity (Table 3). Under excitation with 453 nm blue light, a prominent yellow emission at 573 nm was observed. When the doping concentration of Y³⁺ reached 1.0%, the yellow emission reached its maximum intensity; thereafter, the fluorescence intensity gradually decreased as the Y³⁺ doping concentration further increased to 4.0%. The maximum emission and absorption cross sections of Y³⁺-doped Dy³⁺/Tb³⁺∶NLF single crystals at 573 nm were calculated to be 4.67×10?²¹ cm² and 7.24×10?²¹ cm², respectively.

The Bridgman method is a suitable process for the preparation of highly transparent Dy³⁺/Tb³⁺/Y³⁺ tri-doped NLF single crystals. XRD and Rietveld refinement analyses confirmed that Y³⁺ ions were effectively incorporated into the Lu³⁺ lattice sites of NLF single crystals. The incorporation of inactive rare earth ions (Y³⁺) can effectively enhance the 573 nm yellow luminescence intensity of Dy³⁺/Tb³⁺-doped NLF single crystals, which is mainly attributed to the substitution of Lu³⁺ sites by Y³⁺ ions upon their doping into NLF single crystals, resulting in lattice distortion. However, high Y³⁺ doping concentrations caused a decrease in yellow luminescence, which is due to the dilution of the relative concentration of Dy³⁺ and the increased distance between Dy³⁺ and Tb³⁺ ions. Under excitation at 453 nm, the maximum emission cross section and maximum absorption cross section for the 573 nm emission are 4.67×10?²¹ cm² and 7.24×10?²¹ cm², respectively. Y³⁺-modified Dy³⁺/Tb³⁺∶NLF single crystals are potential yellow optical materials for all-solid-state yellow lasers and related yellow light-emitting devices.