Multiphonon absorption and photon avalanche criterion in erbium doped materials

The proton avalanche effect has been recently observed at room temperature in erbium doped fluoride glasses and crystals, in a particularly clear way, displaying simultaneously all the typical features of this effect: (i) existence of a threshold for transmission at the excitation wavelength, (ii) up-conversion excitation threshold, (iii) long delay for threshold establishment. Analysing the first step of the avalanche effect as an anti-Stokes multiphonon absorption, it can be shown why this effect is so clearly observed at room temperature ill our erbium studies. Our results are compared with other cases of avalanche, with cases which have been called ''looping effects'' and ''quasi-avalanche'' ones.