Optically pumped lanthanide lasers

Table IV. Ions used as sensitizers for optically-pumped lanthanide lasers.

Because of the optical quality and chemical stability of crystals, the low probability of non radiative processes and the wide transmission range, fluorides are the most appropriate materials for solid state lasers with specific wavelengths. Thus the 4f - 4f line emissions of lanthanide ions have been used in order to obtain infrared laser radiation up to 4.34 fxm and blue or green radiation by up-conversion pumping. Tunable laser operation in the ultraviolet has been demonstrated using the broad 5d 4f emission of Ce3+. Tunable lasers in the UV or IR ranges have also been experimented using Ag+, Pb+, 3d ions. 

Both f-f and f-d transitions have been used for lanthanide and actinide lasers. The spectroscopic properties of these transitions are compared in Table I. Since the d states have shorter lifetimes, faster pumping as well as higher energies are required for excitation. Possible pumping sources include ultrafast flashlamps, other lasers, electron beams, or synchrotron radiation, with one exception, all lanthanide and actinide lasers have been optically pumped. 

Alkaline-earth fluorides have been the principal hosts for divalent lanthanide lasers. These are relatively soft, optically isotropic materials. Lanthanides enter the alkaline earth sites substitutionally without charge compensation. Because these sites have inversion symmetry, only magnetic-dipole or vibronic transitions are allowed between 4f states. These are weak and the resulting radiative lifetimes are long. In comparison, the radiative lifetimes of 5d->4f transitions, which are parity allowed, are-short. The 4f->5d transitions are broad and thus provide good absorption bands for optical pumping. 

Trivalent lanthanide ions are used extensively for optically-pumped solid-state lasers because they possess suitable absorption bands and numerous fluorescence lines of high quantum efficiency in the visible and near-infrared. Figure 35.11 summarizes the energy levels, transitions, and approximate wavelengths of trivalent lanthanide ion lasers. In cases where the transitions are to Stark levels of the ground /-state manifold, operation at low temperatures is usually required. The number of different crystalline hosts in which each ion has lased is indicated in fig. 35.1. For several ions, stimulated emission has been observed between more than one pair of / states. A frequency-selective element (e.g., prism, grating, filter) is usually added to the resonator cavity to accomplish this. 

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