3. Cerium ion dopant

Figure 2 shows the energy level structure of Ce3+ doped into a fluoride host. The ground state 4f configuration has two energy levels, <sup>2</sup> F5/2 and <sup>2</sup> F7/2, which are separated by 0.2793 eV (2253 cm<sup>1</sup> ). The excited state 5d configuration also has two energy levels, <sup>2</sup> D3/2 and <sup>2</sup> D5/2, which are separated by 6.166 eV (49,733 cm<sup>1</sup> ) and 6.475 eV (52,226 cm<sup>1</sup> ) from the ground state, respectively. The exact positions of these energy levels would depend on the specific host. Lasing in the UV is based on the electric dipole-allowed interconfigurational 5d-4f transitions. In contrast, conventional trivalent lanthanide laser crystals, such as Nd:YAG, uses the intraconfigurational 4f-4f transition that results to infrared (IR) emission. As a result, UV

Figure 2. Energy level structure of Ce3+ doped into a fluoride host.

fluorescence emission from Ce3+-doped fluoride crystals have smaller radiative lifetimes of a few tens of nanoseconds compared to IR emissions that have lifetimes within hundreds of microseconds. In addition, fluorescence from the 5d-4f transitions is characterized by broad bandwidths and large Stokes shifts. The broad gain bandwidth enables tunability and ultrashort laser pulse generation from Ce3+-activated laser crystals. The large energy gap between the excited state 5d configuration and the 4f ground state configuration laser levels results to low multi-phonon related nonradiative decay. Therefore, quantum efficiencies as high as 90% are expected from Ce3+-activated laser crystals [20, 48].
