Judd–Ofelt Analysis and Laser Optical Properties of Nd3+-Doped S-FAP Nanocrystals as Precursors for Transparent Laser Ceramics

Nd:S-FAP (Nd3+-doped Sr5(PO4)3F) is a high-performance laser material recognized for its large stimulated emission cross-section and broad absorption bands, which are highly desirable for achieving efficient optical gain at the nanoscale. In this work, 5% Nd-doped Nd:S-FAP nanocrystals with an average grain size of approximately 9.2 nm were successfully synthesized via an improved hot-injection method. The results show that the fluorescence lifetime of the nanocrystals is 66 μs (166.56 μs for the ceramic), the quantum yield is 19.7% (69.6% for the ceramic), and the stimulated emission cross-section is 2.29 × 10−20 cm2 (5.74 × 10−20 cm2 for the ceramic). These discrepancies are primarily governed by surface-to-volume ratio variation-related surface effects, lattice distortions, and the weakening of non-radiative f–f transition processes due to quantum confinement at the nanoscale. This study reports for the first time the laser optical parameters of Nd:S-FAP nanocrystals, providing an experimental basis for the optimization of precursors for transparent laser ceramics and holding significant importance for the design of laser materials.