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We report a new Er3+:LiYF4 cryogenic upconversion laser pumped by a fourfold upconversion process. Excitation at 1.5 μm results in laser emission at 701.5 nm on a transition with an upper state at nearly four times the pump photon energy. Mechanisms and the concept of cooperative upconversion enhancement are examined.
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Xie et al. (1993) studied this question.
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