Photon avalanche fluorescence and lasers

This paper presents an analysis of the three-level model for avalanche upconversion in the steady state case. The population of the upper level is studied with respect to the ratio between nonresonant and resonant absorptions. We deduce from this variation a limit for avalanche behaviour nearly independent of the actual compound considered. This limit corresponds to a very weak absorption from the ground state which can be estimated for multiphonon assisted absorption. Several examples from the literature are discussed on this basis and we show that a large mismatch between pump photon and electronic transition energies is necessary to observe avalanche upconversion. Application of the photon avalanche to upconversion pumped lasers is also investigated in terms of oscillation threshold and stimulated emission. Avalanche pumping suffers from two drawbacks. First, it results in a lower upconverted emission intensity than that obtained by resonant pumping, especially near avalanche threshold. In the second place, the output power of these lasers is seriously limited because of the weak absorption from the ground state.