Long-lived, pulse-induced transient absorption in LiNb1−xTaxO3 (0 ⩽ x ⩽ 1) solid solutions: the case of three intrinsic defect sites for electron localization with strong coupling

Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied in LiNb1−xTaxO3 (0 ⩽ x ⩽ 1, LNT) solid solutions with the aim of studying transient optical nonlinearities associated with the formation, transport and recombination of optically generated small bound electron polarons with strong coupling to the lattice. As a result, a pronounced, long-lived transient absorption is uncovered for LNT which exceeds lifetimes and starting amplitudes of LiNbO3 (LN) and LiTaO3 (LT) by a factor of up to 100 and 10, respectively. The transients reveal a stretched-exponential decay behavior and a thermally activated process which provide strong evidence for an underlying hopping transport mechanism of small bound polarons. All findings are discussed in comparison to the model systems LN and LT within the framework of appropriate band models and optical generation of polarons via two-photon excitation. To explain the significant differences, the simultaneous presence of Nb5+ Li , Ta5+ Li antisites, and Ta5+ V interstitial defects, i.e. a mixture of the intrinsic defects widely established for LN and LT, is assumed for LNT.