Spectroscopic excitation and quenching processes in rare-earth-ion-doped Al2O3 and their impact on amplifier and laser performance

This thesis presents in-depth spectroscopic investigations of the optical properties of Al2O3:Er3+ and Al2O3:Yb3+, materials employed for the realization of integrated optical devices such as waveguide amplifiers and lasers. The aim is to provide important spectroscopic parameters for the design and optimization of such devices. Nevertheless, some of the spectroscopic investigations presented here have a fundamental importance as well. The ion-ion process of energy-transfer upconversion (ETU) is investigated in Al2O3:Er3+. Generally this process, in combination with energy migration, can be detrimental for the amplifier or laser performance of a number of rare-earth-ion-doped compounds by depleting the population of the long-lived upper state of the corresponding luminescence transition, thereby diminishing the available optical gain. The most important energy-transfer models found in the literature from the last sixty years – Burshteîn’s and Zubenko’s microscopic treatments of ETU, as well as Grant’s macroscopic rate-equation approach – are put to the test when applied to analyze photoluminescence decay measurements under quasi-CW excitation performed on Al2O3:Er3+. Zubenko’s model provides the best agreement. A fast quenching process induced by, e.g., active ion pairs and clusters, undesired impurities, or host material defects such as voids, that is not revealed by any particular signature in the luminescence decay curves because of negligible emission by the quenched ions under quasi-CW excitation, is verified by pump-absorption experiments. Such fast quenching process is investigated in both Al2O3:Er3+ and Al2O3:Yb3+, and results are compared. A new model that takes the fast quenching into account is presented, which can be helpful in predicting and optimizing the performance of rare-earth-ion-doped devices. Finally, the impact of quenching on Al2O3:Er3+ amplifiers and on Al2O3:Yb3+ distributed-feedback lasers is discussed.