Energy-Transfer-Upconversion Models, Their Applicability and Breakdown in the Presence of Spectroscopically Distinct Ion Classes: A Case Study in Amorphous Al₂O₃:Er³⁺

Abstract

The influence of energy migration and energy-transfer upconversion (ETU) among neighboring Er3+ ions on luminescence decay and steady-state population densities in Al2O3:Er3+ thin films is investigated by means of photoluminescence decay measurements under quasi-CW excitation. The experimental results are analyzed by several models. As expected from the basic physical assumptions made by these models, only Zubenko’s microscopic model provides good agreement with the experimental data, while other donor–acceptor treatments found in the literature are unsuccessful and the macroscopic rate-equation approach provides meaningful results only when misinterpreting the intrinsic lifetime as a free fit parameter. Furthermore, 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. This quenching process strongly affects device performance as an amplifier. Since Zubenko’s microscopic model treats all ions equally, it is unable to describe a second, spectroscopically distinct class of ions involving a fast quenching process. The model is extended to take into account the fraction of quenched ions. This approach finally leads to excellent agreement between the luminescence-decay, pump-absorption, and small-signal-gain experiments within the frame of a single theoretical description.

High pressure polymorphs and amorphization of upconversion host material NaY(WO4)2

“…Amorphous materials have also been used as the host materials for upconversion and solid state lasers, such as amorphous silicon, Al 2 O 3 , and Y 2 O 3 . [40][41][42][43] It is known that local crystal field affects the upconversion efficiency and it is very important to understand its effect on the luminescent properties. One solution is to synthesize both the bulk and amorphous host materials and make a comparative study.…”

Potassium double tungstate waveguides with high ytterbium concentration for optical amplification

“…This is because the clustering of active ions may lead to undesirable energy transfer between the ions, hence degrading the gain and power efficiency. Apart from that, the increase of active ion concentration could increase the magnitude of interionic processes such as energy transfer upconversion and cross-relaxation, which in turn impose a limit to the gain achievable in the material [22,28].…”

“…Thanks to the long excited state lifetime up to ~7.5 ms [28], high bit rate amplification at 170 Gbps had been demonstrated in erbium-doped waveguide amplifier without significant gain modulation [34]. On the other hand, high bit rate signal amplification (e.g.…”

“…2.25-2.27 can be simplified by considering a low beam intensity during the absorption loss measurement. Hence, the approximation N0 ≈ Nd applies, leading to , , 28) where the absorption coefficient α can be determined from the measured loss and the sample's thickness. Hence, based on the measured α and Nd, the effective absorption cross-section σabs can be calculated with , ,…”

“…The model is modified from [28] and it takes into account two distinctive classes of Yb 3+ ions, namely active ions and quenched ions. The active ions participate in usual absorption and emission processes.…”