Fluorescence quenching effects caused by multipolar energy-transfer interactions between rare-earth ions are examined. Transfer between remote ions by way of transitions that are matched in energy appears to be essentially dependent upon concentration alone (e.g., Sm3+ self-quenching) while transfer by non-resonant mechanisms can show a strong dependence upon crystal structure (e.g., Eu3+ self-quenching). Examples of thermally dependent multipolar transfer interactions are also given.

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.

We report the observation of coherent emission from three rare-earth ions in electro-optic crystals. Oscillation is observed from Nd3+ ions in Bi4Ge3O12, LiNbO3, and Ba0.75Ca0.25Nb26O at room temperature, and from Tm3+ and Ho3+ ions in LiNbO3 at 77°K. Second-harmonic radiation is obtained from LiNbO3:Tm3+ and electric-field modulation of threshold is observed for certain crystals of LiNbO3:Nd3+. The latter effect is believed to arise from crystal strain. The propagation angle for phase-matched second-harmonic generation is calculated to lie at 43° to the c axis for both LiNbO3:Tm3+ and LiNbO3:Ho3+ at 77°K. Phasematched second-harmonic generation from LiNbO3:Nd3+ is not possible, since the maser line is an extraordinary wave. However, the polarization of the maser line in LiNbO3:Nd3+ is appropriate for phasematched parametric frequency conversion in the near infrared. Attempts to observe such radiation were not successful due to the limited power available in the fundamental.

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