Time-resolved study electronic and thermal contributions to the nonlinear refractive index of Nd3+:SBN laser crystals

The propagation of an optical beam through dielectric media induces changes in the refractive index, Δn, which causes self-focusing or self-defocusing. In the particular case of ion-doped solids, there are thermal and non-thermal lens effects, where the latter is due to the polarizability difference, Δα, between the excited and ground states, the so-called population lens (PL) effect. PL is a pure electronic contribution to the nonlinearity, while the thermal lens (TL) effect is caused by the conversion of part of the absorbed energy into heat. In time-resolved measurements such as Z-scan and TL transient experiments, it is not easy to separate these two contributions to nonlinear refractive index because they usually have similar response times. In this work, we performed time-resolved measurements using both Z-scan and mode mismatched TL in order to discriminate thermal and electronic contributions to the laser-induced refractive index change of the Nd3+-doped Strontium Barium Niobate (SrxBa1−xNb2O6) laser crystal. Combining numerical simulations with experimental results we could successfully distinguish between the two contributions to Δn.