1995) Fine structure of the absorption and emission spectra of NIThe optical properties of Niz+-ion impurities in a new crystal, the inverted fluoroperovskite BaLiF3, have been studied. In particular we have investigated the fine structure of the transition by means of absorption and emission spectra. By analogy to previous results we have assigned the structured absorption band around 1200 nm to the 3A2 +3 TI transition. Four zero-phonon lines are clearly visible in the absorption spectrum. The emission in the region 1250-1600 nm, due to the same 3A2 -+ 3T1 transition, consists of two sharp lines with their vibrational sidebands.

The luminescent-channel efficiency for Ho + ions in HoLiF4 crystals was measured and compared with the case of Ho + as a low-concentration dopant (1.71%) in crystals of YLiF4. The 'S2~'I7, I8 luminescent transitions are strongly quenched by two possible cross-relaxation processes: one with almost zero mismatch energy, favoring the population of the 'I5, 'I6, and 'I7 levels, and the other, a laser transition 'I7~'I8 in the mid-infrared region. Also seen was a strong decrease of the total Ho'+ luminescence by approximately 6.6 times in HoLiF4 crystals from 10 to 300 K. This quenching effect was attributed to an energy transfer from the Ho' excited 'I6 and 'I7 levels to traps present in the crystal. I. INTR(ODUCTION II. EXPERIMENT Laser emissions in the near-infrared region of various crystal hosts doped with rare-earth ions have attracted considerable attention in recent years as a result of a search for an eye-safe laser source with low atmospheric attenuation. In particular, the 2-IMm laser line of Ho + turned out to be a potential source for tunable operational amplifiers, ' range finders, and target illuminators. The 2-LMm laser line is also applicable in medicine' and in industrial processes. A significant advantage of the Ho + ion for laser operation at 2 pm is its high gain cross section and the long lifetime of the I7 upper laser level, which results in high energy-storage capability and efficient Q-switched operation. ' As a result, several studies of cw and pulsed holmium laser operation as well as spectroscopic studies of some Ho +-doped crystals, have been conducted and reported, " with emphasis on YLiF4 and yttrium aluminum garnet (YAG) doped with Ho +. It is well known that trivalent rare-earth ions can transfer energy from visible and near-visible transitions to nearand midinfrared transitions by thermally assisted cross-relaxation processes in highly concentrated samples, changing the laser properties of the host crystals. In this paper we present a quantitative investigation of the e%ciency of each holmium luminescent-channel in HoLiF4 crystals when the 66 state, well above the Auorescent S2 level, is excited. This was done by measuring the luminescence with respect to the analog luminescent signals also measured for the diluted system Ho +: YLiF4 at 1.71at. % concentration. This study presents a very important approach to investigating the behavior of trivalent rare-earth ions as a light source in laser materials that are susceptible to contamination during synthesis or in the crystal growth process. Thus this investigation provides information for further development of laser crystals. The samples used in this investigation were single crystals of pure HoLiF4 and doped YLiF4 containing 1.71 at. % of Ho + ions. The starting materials for the crystal growth were synthesized from ultrapure rare-earth oxides by a conventional hydroAuorination procedure. The Ho +: YLiF4 thus synthesized was grown by the conventional Czochralski method under argon atmosphere. The Ho +: YLiF4 and HoLiF4 boules were subm...