In the above equation, k is the Boltzmann constant (1.38 × 10−23 J/K), T is temperature (here is the room temperature), λ is the transition wavelength, h is Planck constant and ε is the net free energy required to excite one Tm3+ ion from 3H6 to 3F4 at temperature T, and the ε value of the 3F4 → 3H6 transition is 5778 cm−1 . The calculated HTH-01-015 (3H6 → 3F4) and emission (3F4 → 3H6) cross sections for 0.95 mol% Tm3+ doped α-NaYF4 single crystal are shown in Fig. 7. The maximum value of emission cross section at 1858 nm is 0.58 × 10−20 cm2, while the absorption cross section at 1680 nm reaches its maximum value of 0.42 × 10−20 cm2. It should be mentioned that the self-absorption is involved into the emission spectrum since the sample is usually not thin enough, therefore, the emission cross section was calculated from the absorption spectrum. A relatively broad emission cross section indicates that Tm3+ doped α-NaYF4 single crystal could be a promising material for tunable laser. Some important spectral parameters of Tm3+ doped α-NaYF4 single crystal are listed in Table 5 along with a comparison with those of other well-known Tm3+ doped laser crystals. From Table 5, it is seen that Tm3+ doped α-NaYF4 single crystal has a longer fluorescence lifetime of 3F4 level than that of others, making it attractive for high energy Q-switched operation.