Fluorescent Emissions (1800 nm) of LiLuF<sub>4</sub> Single Crystals Doped with Various Tm<sup>3+</sup> Concentrations

LiLuF4 single crystals doped with molar fractions of 0.45%, 0.90%, 1.63%, and 3.25% (x, molar fraction) Tm3+ ions were fabricated by an improved Bridgman method. Absorption spectra in the 400-2000 nm region of the crystals were measured. The emissions from 1400 to 2000 nm under excitation of an 808-nm laser diode (LD) were carried out and compared. Two emission bands at 1470 and 1800 nm were observed. First, the emission intensity at 1800 nm increased with the increase in Tm3+ concentration, reaching a maximum value when the Tm3+ concentration was ca 0.90%. Thereafter, it decreased considerably as the Tm3+ doping levels further increased to 3.25%. However, the emission intensity at 1470 nm showed the contrary tendency to that at 1800 nm. It was found that the 1800-nm emission lifetime of the Tm3+:F-3(4) manifold systematically decreased with an increase in Tm3+ concentration. The trend in the fluorescent intensity change can be explained by the cross-relaxation (H-3(6), H-3(4)-> F-3(4), F-3(4)) between the Tm3+ ions and the concentration quenching effect of Tm3+. Meanwhile, the emission cross-section was calculated, providing a maximum of 0.392x10(-20) cm(2) at 1890 nm for the 0.90% doped sample. Based on the measured lifetime and calculated radiative lifetime, the largest quantum efficiency between Tm3+ ions reached similar to 120%.