Tm3+ doped α-NaYF4 single crystal for 2 μm laser application

High quality α-NaYF4 single crystals with various Tm3+ concentrations ranging from 0.95 to 7.80 mol% were grown by an improved Bridgman method in which KF was used as flux and a high temperature gradient (70–90 °C/cm) for solid–liquid interface was employed. The differential thermal analysis, thermogravimetry analysis, X-ray powder diffraction, absorption spectra, emission spectra measured upon 800 nm excitation and fluorescence decay at 1.8 μm were measured to investigate the structural and luminescent properties of the crystals. The emission intensity at 1.8 μm increases with the increase of Tm3+ concentration, and reaches a maximum value when the Tm3+ concentration is about 1.90 mol%, but it decreases rapidly when the Tm3+ concentration further increases from 2.88 mol% to 7.80 mol%. The fluorescence lifetime of Tm3+: 3F4 decreases with the increase of Tm3+ doping level. The cross relaxation energy transfer (3H6 + 3H4 → 3F4 + 3F4) process and the concentration quenching effect of Tm3+ are mainly responsible for the change of emission intensity. The stimulated emission cross section for the 0.95 mol% doped sample was calculated, and the maximum value reaches 0.58 × 10−20 cm2 at 1858 nm. The optical gain spectra were also calculated based on the absorption and emission cross section spectra.