Time-resolved Z-scan and thermal lens measurements in Er+3 and Nd+3 doped fluoroindate glasses

In rare earth ion doped solids, a resonant non-linear refractive index, n(2), appears when the laser pumps one of the ion excited states and the refractive index change is proportional to the excited state population. In these solids there an usually thermal and non-thermal lensing effects, where the non-thermal one is due to the polarizability difference, Delta alpha, between excited and ground states of the ions. We have used the time resolved Z-scan and a mode-mismatched thermal lens technique with Ar+ ion laser in Er+3 (20ZnF(2)-20SrF(2)-2NaF-16BaF(2)-6GaF(3)-(36 - x)InF3-xErF(3), with x = 1, 2, 3 and 4 mol%) and Nd+3 (20SrF(2)-16BaF(2)-20ZnF(2)-2GdF(3)-2NaF-(40 - x)InF3-xNdF(3), with x = 0.1, 0.25, 0.5-1 mol%) doped fluoroindate glasses, In both samples we found that the non-linear refraction is due to the thermal effect, while the non-thermal effect is negligible. This result indicates that in fluoride glasses Delta alpha is very small (less than 10(-26) cm(3)). We also measured the imaginary part of the non-linear refractive index (n(2) '') due to absorption saturation.