Electronic refractive index changes and measurement of saturation intensity in Cr3+-doped YAG crystal

The electronic refractive index variation is associated with the difference in the polarizabilities (Delta alpha(p)) of the Cr3+ ion in its ground and excited states. In order to further address the physical origin of Amp in a Cr3+-doped YAG crystal, time-resolved Z-scan measurements were performed and analyzed at lambda = 457 nm by using a chopped Ar+ ion laser. It is found a nonlinear refractive index with the real and imaginary parts n(2)' = 2.2 x 10(-8) cm(2)/W and n(2)" = 2.8 x 10(-1)0 cm(2)/W, respectively. The real part is associated with a polarizability difference Delta alpha(p) = 2.2 x 10(-25) cm(3). The imaginary part indicates that excited state absorption (ESA) occurs and that Cr:YAG behaves as a saturable absorber. The transient response of the Z-scan signal decreases with the laser intensity as tau(-1)= tau(-1)(0)(1+I/Is), where To is the excited state lifetime and I-s the saturation intensity. By measuring this transient response at different laser intensities, it was possible to confirm the tau(0) value which can be derived from fluorescence measurements and to determine a Is value of 8.3 kW/cm(2). (C) 2018 Elsevier B.V. All rights reserved.