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.
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