Lasing scheme

The versatility and wide applicability of lanthanide ions for lasers arises from several desirable spectroscopic features. The electronic states of the ground 4fn configurations provide complex and varied optical energy level structures, thus many different lasing schemes are possible. 

Figure 1. Representative energy-level diagram and transitions for four-level and cascade lasing schemes vP and vL are the pump and laser frequencies wavy lines denote nonradiative transitions.

S2- 515 lasing was-obtained within 0.5 ys. As a result, sufficient population buildup occurred in 5 to achieve 515- 516 oscillation before significant spontaneous decay from the 5l5 state. Cascade lasing schemes of Ho3+ in Gd3Ga50]2 crystals including 16 18 transitions were reported recently (Z8). 

An examination of the trivalent actinide energy level schemes reveals several possibilities for laser action. These are discussed in light of the general properties cited above. Only conventional broadband optical pump sources are considered. Obviously with selective laser excitation and cascade lasing schemes, stimulated emission from many more states should be possible, but these special situations are too numerous to be considered in detail here. 

In view of the ease and success of lasing lanthanide ions, only some compelling reason such as the requirement of a specific wavelength would warrant development of some of the actinide lasing schemes discussed. Perhaps additional spectroscopy will reveal advantages of using actinide ions in other valence states and hosts for efficient laser action. 

No well-verified Gd " laser action has been reported. Fluorescence from the lowest excited level, P7/2, occurs at 0.31 /im and terminates on the 87/2 ground state to form a three-level lasing scheme. The high threshold for three-level operation and the requirements of good host transparency and an ultraviolet source (A <0.31 /xm) for optical pumping are obstacles to obtaining stimulated emission. 

Figure 4 Main schemes of nonlinear stimulated processes for frequency conversion. (A) Stimulated Raman and (B) up-conversion lasing schemes.

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