Theoretical analysis of the photon avalanche dynamics in Ho3+-Yb3+ codoped systems under near-infrared excitation -: art. no. 045115

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 I-5(7) 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.