Photon avalanche upconversion in Tm³⁺-doped fluoroindogallate glasses

In this work, a study of photon avalanche at 476 nm (¹G₄→³H₆) in Tm-doped fluoroindogallate glasses is reported. Transient Tm³⁺ emissions at 476, 450 nm (¹D₂→³F₄) and transmission at the pumping wavelength (645 nm) were monitored as a function of excitation power and Tm³⁺ concentration. We propose a theoretical treatment based on a set of five non-linear coupled rate equations, which include sequential two-photon absorption (TPA), two cross-relaxations among Tm³⁺ ions, and energy migration at the ³F₄ metastable state. It is demonstrated that energy migration in highly doped samples plays a fundamental role in the photon avalanche process, and may even suppress it. Additionally, sequential TPA must be considered in order to explain the transient behaviour of the 450 nm emission in the photon avalanche regime.