LOOPING MECHANISM AND SEQUENTIAL 2-PHOTON ABSORPTION UP-CONVERSION - A COMPARISON IN YALO3-ER3+ CRYSTAL

Green and blue fluorescences have been obtained under continuous laser excitation in the 790-810 nm range in YAlO3:Er3+. A model is given to describe the time evolution of the green fluorescence. A two-photon a bsorption and a looping mechanism compete. Their relative efficiency i s determined theoretically by establishing the dynamics of the two sub sets of populations that they give rise to. We show that the resulting time evolutions depends on a threshold and that the condition for sel f-maintaining the looping mechanism is identical to the well-known thr eshold for photon avalanche.