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Fluoride crystals are among the most important hosts for laser materials because of their special optical properties. Of these, LiYF 4 (YLF) is one of the most common rare-earth-doped laser materials, with a variety of efficient mid-IR laser lines from the UV (Ce 3+ :YLF) to mid-IR range. Generally, YLF has good optical properties with high transparency throughout the emission spectrum of the conventional sources used for pumping solid state lasers. YLF does not show UV damage, and it has lower nonradiative decay rates for processes occurring between electronic levels participating in the pumping and lasing process. YLF also has a low, two-photon absorption coefficient. Because of its low nonradiative rates, the material can be used for cascade emission between intermediate levels as well as an up-converter, as will be discussed later.
YLF is also a good medium for mode locking at 1047 or 1053 nm and 1.313 mm as a result of its natural birefringence and low thermal lensing. Mode-locked pulses from YLF are shorter thanks to its broader linewidth, both for the 1047/1053-nm and 1.313-μm emission peaks. The crystallographic structure of LiYF 4 (or YLF) is the same as CaWO 4 , which was developed years ago as a potential laser material. However, when a trivalent rare-earth material substitutes for the Ca 2+ ion, charge compensation is necessary. However, the process of charge compensation may result in inhomogeneities in the crystal and is a source of disordered crystal structure. No charge compensation is necessary with YLF throughout the doping process, since the trivalent rare-earth-ion substitutes for the Y 3+ ion. As a result, a single undisturbed site exists. The crystal has tetragonal symmetry; the important optical and physical properties are shown in Tables 4.1 and 4.2. Figure 5.1 shows a schematic energy-level diagram of those levels participating in the lasing process in Nd:YLF .
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