N-photon summation

In order to illustrate the difference between APTE and cooperative up-conversion, we shall discuss an example of a line-narrowing effect in n-photon summation as a means to distinguish between both processes (Auzel 1984a, 1985). Irradiating Er +-doped samples with IR photons at 1.5 pm leads to various visible emissions. Room-temperature IR F-center laser excitation between 1.4 and 1.6 pm of 10% Er -doped vitroceramic and of Er +iYPs leads to emission bands from near IR to UV. Such emissions may be ascribed to multiphoton excitation of the order of 1 to 5, either by APTE or by the cooperative type, with energy levels of single ions (APTE), or with pair levels, respectively, as illustrated by fig. 23. Successive absorptions in fig. 23 a involve a combination of several J states. The APTE effect, because of self matching by multiphonon processes, involves only and J = Y states. 

Fig. 23. n-photon summation (n= 1,. .., 5) in EP system by (a) cooperative sequential pair absorption, and (b) APTE effect (Auzel 1984a). 

Fig. 24. Excitation spectra for n-photon summation in YFstEi (Auzel 1985).

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