The photon avalanche upconversion mechanism in a Ho³⁺-Yb³⁺ system under near infrared excitation is discussed. Two looping cycles in the excitation process provide the necessary population of the ⁵I₇ intermediate level of Ho³⁺ ions to achieve the avalanche regime. The first one takes place between Ho³⁺ ions, while the second one occurs between Ho³⁺ and Yb³⁺ ions. We present a simplified model of the process, which only takes into account the ground, intermediate and emitting levels of Ho³⁺ and the ground and excited levels of Yb³⁺ ions. The rate equations of the system have been solved, and the photon avalanche regime has been interpreted in terms of gain and loss of the population of the intermediate level. The model predicts a reduction of the avalanche pump power threshold when the Yb³⁺ concentration increases which is in good agreement with experimental data.
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Lahoz et al. (2005) studied this question.
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