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 Nd<sup>3+</sup> 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 ∼3 μs. The latter effect is explained by population (or depopulation) of a higher-lying energy level <sup>2</sup>(F2)<sub>5/2</sub> of the 4f electron shell of Nd<sup>3+</sup> ions, which has large polarizability in near-IR and visible light. The real part of the nonlinear optical resonant susceptibility of the inverted Nd:YAG caused by the refractive-index changes is also discussed.
© 1999 Optical Society of America
(140.3280) Lasers and laser optics : Laser amplifiers
(140.3530) Lasers and laser optics : Lasers, neodymium
(160.3380) Materials : Laser materials
(160.5690) Materials : Rare-earth-doped materials
(190.4720) Nonlinear optics : Optical nonlinearities of condensed matter
O. L. Antipov, A. S. Kuzhelev, D. V. Chausov, and A. P. Zinov’ev, "Dynamics of refractive-index changes in a Nd:YAG laser crystal under excitation of Nd3+ ions," J. Opt. Soc. Am. B 16, 1072-1079 (1999)