Dynamics of refractive-index changes in a Nd:YAG laser crystal under excitation of Nd3+ ions

The mechanism for refractive-index changes that accompany changes in population during pumping and amplification of an optical beam is studied in a Nd:YAG laser crystal by both resonant four-wave mixing and interferometric techniques. It is found that these refractive-index changes (connected with the difference in polarizability of excited and unexcited Nd3+ ions) have two components: a small noninertial component that accompanies an amplified optical pulse and the major changes that are intensified with a delay time of similar to 3 mu s The latter effect is explained by population (or depopulation) of a higher-lying energy level (2)(F2)(5/2) of the 4f electron shell of Nd3+ ions, which has large polarizability in near-IR and visible light. The real part of the nonlinear optical resonant susceptibility of the invert;ed Nd:YAG caused by the refractive-index changes is also discussed. (C) 1999 Optical Society of America [S0740-3224(99)00407-5].