Optically generated small electron and hole polarons in nominally undoped and Fe-doped KNbO3 investigated by transient absorption spectroscopy

Transient light-induced absorption in nominally undoped and Fe-doped KNbO₃ crystals is observed in the visible and infrared spectral ranges after single pulse illumination with λ=532nm . For nominally undoped KNbO₃ the decay of the light-induced absorption in a single step can be explained by incoherent hopping transport of optically generated small bound O^- hole and small free Nb⁴⁺ electron polarons and their mutual recombination. Iron doping causes an additional slow decay component and, remarkably, accelerates the initial decay process. A consistent model for the formation, hopping, and recombination paths of hole and electron polarons is deduced from the experimental data set for both nominally undoped and Fe-doped KNbO₃ . The decrease in the polaron hopping-transport length in Fe-doped samples is attributed to the increased number densities of optically generated hole polarons by additional one-quantum excitations.