The influence of chemical reduction on the particular number densities of light-induced free small ${\mathrm{Nb}}_{\mathrm{Nb}}^{4+}$ electron polarons, bound small ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ electron polarons, bound small ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}:{\mathrm{Nb}}_{\mathrm{Nb}}^{4+}$ electron bipolarons, and bound small ${\mathrm{O}}^{-}$ hole polarons is investigated in nominally pure, congruently melting $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ by means of excited-state absorption spectroscopy. Characteristic changes in the sign of the light-induced absorption in the blue-green spectral range and distinctive dependencies on the pump beam intensity reflect the increasing contribution of electron polarons generated upon single intense ns-laser pulses at $\lambda =532\mathrm{nm}$ with increasing degree of reduction. The entire data set including its time dependence and spectral properties is consistently explained within a model taking into account the presence of all four types of electron and hole polarons, and residual ${\mathrm{Fe}}^{2+∕3+}$ impurities. The model includes one- and two-photon excitation processes for polaron generation, optical gating of bipolarons, and direct and two-step polaronic recombination processes. Our results indicate a mutual independence of two-photon hole polaron generation and one-photon dissociation processes of bipolarons, at least for moderate degrees of reduction.

• Received 24 April 2007

DOI:https://doi.org/10.1103/PhysRevB.76.085114