Discrimination between electronic and thermal contributions to the nonlinear refractive index of SrAlF_5:Cr^+3

In ion-doped solids there is a nonlinear refractive index n2 that is due to the polarizability difference Δα between excited and ground states, the so-called population-lens (PL) effect. In addition, the thermal-lens (TL) effect is particularly important in fluoride materials, owing to their low thermal conductivity. We performed time-resolved Z-scan and mode-mismatched TL measurements at λ=488 nm in SrAlF5:Cr+3. In this crystal the PL effect is faster than that of the TL, owing to its relatively short metastable lifetime (93 µs), and therefore we could temporally discriminate between these two contributions to the nonlinear refractive index. For the PL effect we measured n2=(6.6+1.7i)×10-11 cm2/W-1 and calculated Δα=3.1×10-26 cm3 and Δσ=1.7×10-20 cm2. From the TL measurements we obtained the thermal diffusivity D=6.5×10-3 cm2 s-1 and estimated the thermal conductivity K=1.7×10-2 W cm-1 K-1 and ds/dT=-8.5×10-7 K-1.