Comparing Pr3+ and Nd3+ for deactivating the Er3+: 4I13/2 level in lanthanum titanate glass

Abstract

Erbium lanthanum titanate glasses were prepared by levitation melting for the spectroscopic study of ways to promote the mid-infrared fluorescence. Two series of heavily erbium doped glasses (15 wt%) were prepared with the addition of either Pr³⁺ or Nd³⁺ in amounts relative to Er³⁺ of 0.05, 0.1, and 0.2. Both ions quench the lower Er³⁺ laser level with the Pr³⁺ doing so more rapidly. Although high co-dopant concentrations result in higher energy transfer, as clearly evidenced in upconversion and downconversion fluorescence measurements, the mid-infrared lifetime also suffers a reduction and, therefore, a balance must be struck in the co-dopant concentration. Lifetime and spectral measurements indicate that, at a fixed relative co-dopant amount, Pr³⁺ is more effective than Nd³⁺ at removing the bottleneck of the Er^{3+ 4}I_13/2 level. Moreover, consideration of the lifetimes alongside the absorption data of the individual ions indicates that despite the large absorption cross-section of Nd³⁺ at 808 nm, the concentration needed to yield more absorbed power than utilizing direct 976 nm excitation of Er³⁺ results in unfavorable lifetimes of the mid-infrared transition. In the end, Pr³⁺ prevails as the superior co-dopant in terms of the effects on fluorescence lifetimes as well as potential laser system design considerations. In a unique self-doping approach, a reducing melt atmosphere of Ar instead of O₂ creates a small fraction of Ti³⁺. In 5Er₂O₃-12La₂O₃-83TiO₂ glass, the presence of Ti³⁺ quenches the ⁴I_13/2 emission about 2.6 times more than the ⁴I_11/2 when lifetimes are compared to an O₂ melt environment. As an additional means of increasing the mid-infrared emission, the effect of temperature on the mid- and near- infrared lifetimes of a lightly doped lanthanum titanate composition is investigated between 77-300 K. The mid-infrared lifetime increases by ∼30% while the near-infrared lifetime increases by ∼10%, which suggests in addition to co-doping, active cooling of the gain media will further enhance performance.