Spectroscopy and gain measurements of Nd^3+ in SrF_2 and other fluorite-structure hosts

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Abstract
We investigate the optical properties of Nd3+ in CaF2, SrF2, and BaF2 with the intent of determining whether any of these materials might usefully serve as a laser-pumped-amplifier medium. The Nd3+ impurities tend to cluster at low levels of doping in CaF2, leading to the formation of nonluminescent centers. The addition of La or Y buffer ions to CaF2:Nd serves to increase the luminescent yield, but it also renders the system spectrally inhomogeneous. Although single-ion centers predominate in BaF2, the interstitial fluoride compensator occurs at the next-nearest-neighbor position relative to Nd3+, leading to unsuitably low transition strengths. The interstitial fluoride occurs at the nearest-neighbor site of Nd3+ in SrF2 and thereby induces significant oscillator strength into the 4f–4f transitions by breaking the inversion symmetry. The radiative lifetime of SrF2:Nd is found to be 1470 μsec by measuring the emission lifetime and quantum efficiency; this value was confirmed by Judd–Ofelt analysis of the absorption features. The peak-emission cross section at room temperature was determined to be 1.7 × 10−20 cm at 1036.5 nm. A maximum of 0.20 at. % Nd3+ may be doped into SrF2 without the occurrence of significant Nd clustering. Direct measurements of the gain spectrum in SrF2:Nd reveal the presence of the 4F3/2 → 2G9/2 excited-state absorption, although its effect on the emission cross section is only minor.