Infrared-pumped stimulated emission near 6700 Å in BaY2F8 : Yb3+–Er3+ and at 5515 Å in BaY2F8 : Yb3+–Ho3+ is reported. In BaY2F8 : Yb3+–Er3+, oscillation takes place at 77 °K in Er3+ transitions from 4F9/2 to the highest level of the ground state at 410 cm−1. Pumping of Er3+ results predominantly from up-conversion produced by successive energy transfers from Yb3+; direct pumping of the metastable state and higher-lying states of Er3+ is substantially less effective than up-conversion. Oscillation in Ho3+ occurs in a transition from 5S2 to the highest level of the ground manifold at 385 cm−1. Up-conversion and direct pumping contribute about equally to the inversion.
An analysis of infrared conversion to visible and ultraviolet radiation by Yb3+–Er3+, Yb3+–Ho3+, and Yb3+–Tm3+ ions in crystals is presented. The expression for the visible power output in the presence of back transfer from the active ion to the energy-transferring ion (Yb3+) is given and the relationship between the intermediate state transfer and back transfer coefficients for maximum output is found. If this relationship is satisfied the visible output in the presence of back transfer is equal to the maximum output with no back transfer. The behavior of the power output when the transfer and back transfer coefficients depart significantly from this optimum condition is examined and the parameters which govern the power output under these conditions are determined. The analysis is applied to Yb3+–Er3+, Yb3+–Ho3+, and Yb3+–Tm3+ ions in BaYF5 and BaY2F8. Measurements on the brightest composition of Yb3+–Er3+ ions in BaYF5 indicate that the advantage of a long-lived intermediate state is diminished by back transfer. A power-conversion efficiency of 0.1% has been obtained for the green emission from BaYF5:Yb3+–Er3+ when pumped by 0.93-μ radiation from a 17% efficient Si–GaAs diode. Conversion efficiencies of 0.03% are obtained for the green emission from BaY2F8:Yb3+–Ho3+ and for the blue emission at 4800 Å from BaYF5:Yb3+–Tm3+. Efficient conversion of 1.5-μ radiation to the visible by BaYF5:Er3+ is also described.
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