Pronounced temperature-dependent transfer of excitation from the Er3+ laser pump levels to lower levels of Er3+ was observed in CaF2:ErF3 single crystals at ErF3 concentrations exceeding 12.5 wt %. Transfer of excitation from Er3+ to Tm3+ occurred when TmF3 was included as a second active constituent of the crystals. Stimulated emission of the 4I11/2 → 4I13/2 transition of Er3+ at 2.69 μ at 298°K resulted independently of Tm3+ and as a consequence of concentration quenching of emission of Er3+ from levels above 4I11/2. At 100°K reduced transfer of excitation to 4I11/2 of Er3+ occurred due to radiative emission from levels above 4I11/2, and laser emission of Tm3+ was detected at 1.86 μ.
Evidence is reported of 1020 cm−3 hydroxyl impurity density affecting crystal fields and color center formations in YAG and Nd:YAG. Also reported is a high-temperature reactive atmosphere process to control H bonding and to bleach coloration. Absorption by O–H⋅⋅⋅O and O–D⋅⋅⋅O transitions in the 3–4 μm spectral region was observed in different crystals. Two independent O–H⋅⋅⋅O lines, both with weak satellites, were detected at 3340 and 3374 cm−1. H-bonding sites were assigned by (i) frequency dependence on O2−–O2− distance and (ii) Nd:YAG nonequivalent crystal field effects seen in Nd3+ fluorescence. H+ concentrations 1020 cm−3 were derived from these results. Bonding site relative densities also varied with the different crystals observed. Reactive atmosphere processing (RAP) could cause H+ or D+ to be coordinated into or out of crystals, concomitant with color center bleaching. Color center formations were dissociated by surface reaction with I2 + H2O or I2 + O2 reactive atmosphere at temperatures in the range of 1350–1880 °C. The RAP effect was to produce bulk crystal electronic transfers related to hydrogen in the crystal metal–oxide system. However, photoinduced coloration was again possible. Intense coloration also resulted from vacuum heating, causing an H2O effluent. The evidence indicated YAG and Nd:YAG contained a high concentration of largely mobile H+ affecting O− -center coloration and influencing crystal fields by bound-small polaron formations.
Efficient laser emission of Ho3+ at 2μ has been obtained from a single crystal of HoF3 at liquid-nitrogen temperatures. This is the first pure, undiluted rare-earth compound laser. Fluorescence and absorption spectra indicated a spontaneous linewidth of 1.5 cm−1 for the laser transition. The fluorescence lifetime at 77°K was 2.6 msec, or 29% of the radiative lifetime as measured in dilution in an isomorphic host. The laser threshold for a 15-mm rod was about 1 J.
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