S.A. Pollack, D.B. Chang, M. Birnbaum, and S.C. Rand
Recent experiments in Er:YLF and other crystals have demonstrated efficient laser action from energy levels populated entirely by cooperative upconversion of pump energy from pairs and trios of excited Er ions.1 This has revealed a general method for pumping solid state laser materials that is quite distinct from processes relying on the absorption of one or more photons by single ions. The new method permits excitation of high energy states by long wavelength radiation. For UV solid state lasers, it therefore offers an alternative to the use of deep ultraviolet pump sources that typically exhibit shallow penetration and can cause deleterious color center formation. Also, pair or multi-atom interactions can be exploited to achieve inversions on new transitions that may be normally self-quenching.
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