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 Er3+ 4I13/2 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 4I13/2 emission about 2.6 times more than the 4I11/2 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.