Laser Operating Schemes with Strong Depopulation of the Ground States of Generating Ln3+ Ions

Among the many crystalline lasers based on insulating compounds doped with Ln³⁺ ions, ¹ only one laser is known to generate according to a classical three-level operating scheme, this being the “blue-green” lasing of LiYF₄:Pr³⁺ (its stimulated emission [SE] transition terminates at the lower [zero] Stark level of the ground ³H₄ state of Pr³⁺ ions). ² For all others the terminal laser level usually has an energy E_term ≫ kT. In these cases the number of excited Ln³⁺ ions distributed over the working levels, mainly, the initial laser ones, is small and amounts to a few percent of the total number of generating ions in a crystal. This indicates that SE in those multilevel Ln³⁺ systems proceeds with practically fully populated levels of the ground state (their usual population is N₀ = 10¹⁹ to 10²¹ cm⁻³, which corresponds to Ln³⁺ ion concentration amounting to C_Ln = 0.1 to 10 at.%).