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

摘要

In ion-doped solids there is a nonlinear refractive index n_2 that is due to the polarizability difference #DELTA##alpha# 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 #lambda#=488 nm in SrAlF_5:Cr~(+3). In this crystal the PL effect is faster than that of the TL, owing to its relatively short metastable lifetime (93 #mu#s), and therefore we could temporally discriminate between these two contributions to the nonlinear refractive index. For the PL effect we measured n_2=(6.6+1.7i) X 10~(-11) cm~2/W~(-1) and calculated #DELTA##alpha#=3.1 X 10~(-26) cm~3 and #DELTA##sigma#=1.7 X 10~(-20) cm~2. From the TL measurements we obtained the thermal diffusivity D=6.5 X 10~(-3) cm~2 s~(-1) and estimated the thermal conductivity K=1.7 X 10~(-2) W cm~(-1) K~(-1) and ds/dT=-8.5 X 10~(_7) K~(-1).