The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the ion in its excited and ground states, . The optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of in -doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its and emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative value, which is contrary to all refractive index variation (including Z-scan) measurements.
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