An overview of the properties of electron small polarons and bipolarons is given, which can occur in the congruently melting composition of LiNbO(3) (LN). Such polarons influence the performance of this important optical material decisively. Since coupling to the lattice strongly quenches the tunnelling of free small polarons in general, they are easily localized at one site even by weak irregularities of a crystal. The mechanism of their optical absorptions is thus shared with those of small polarons localized by binding to selected defects. It is shown that the optical properties of free electrons in LN as well as those bound to Nb(Li) antisite defects can be attributed consistently to small polarons. This is extended to electron pairs forming bipolarons bound to Nb(Li)-Nb(Nb) nearest neighbours in the LN ground state. On the basis of an elementary phenomenological approach, relying on familiar concepts of defect physics, the peak energies, lineshapes, widths of the related optical absorption bands as well as the defect binding energies induced by lattice distortion are analysed. A criterion universally identifying small polaron absorption bands in oxide materials is pointed out. For the bipolarons, the dissociation energy, 0.27 eV, derived from a corresponding study of the mass action behaviour, is shown to be consistent with the data on isolated polarons. Based on experience with simple O(-) hole small polaron systems, a mechanism is proposed which explains why the observed small polaron optical absorptions are higher above the peak energies of the bands than those predicted by the conventional theory. The parameters characterizing the optical absorptions are seen to be fully consistent with those determining the electrical conductivity, i.e. the bipolaron dissociation energy and the positions of the defect levels as well as the activation energy of mobility. A reinterpretation of previous thermopower data of reduced LN on the basis of the bipolaron model confirms that the mobility of the free polarons is activated by 0.27 eV. On the basis of the level scheme of the bipolarons as well as the bound and free polarons the temperature dependence of the electronic conductivity is explained. The polaron/bipolaron concept also allows us to account for the concentrations of the various polaron species under the combined influence of illumination and heating. The decay of free and bound polarons dissociated from bipolarons by intense short laser pulses of 532 nm light is put in the present context. A critical review of alternative models, being proposed to explain the mentioned absorption features, is given. These proposals include: single free polarons in the (diamagnetic) LN ground state, oxygen vacancies in their various conceivable charge states, quadpolarons, etc. It is shown why these models cannot explain the experimental findings consistently.

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The number densities and absorption cross sections of both optically generated and reduction-induced small electron and hole polarons in LiNbO(3) are determined by means of time-resolved pump-multiprobe spectroscopy. The data are obtained for free (Nb(Nb)(4+)) and bound (Nb(Li)(4+)) electron polarons, bound Nb(Li)(4+):Nb(Nb)(4+) electron bipolarons, and bound O(-) hole polarons. The peak absorption cross sections are in the range of σ(pol)≈(4-14) × 10(-22) m(2), comparable to that for Fe(2+). In all cases the ratio of occupied to unoccupied polaronic sites is less than 10(-2).

Transient light‐induced absorption changes αli(t), caused by optically generated small polarons, are investigated in LiNbO3:Mg below and above the optical‐damage‐resistance threshold (ODRT). The lifetime of αli(t) is reduced by three orders of magnitude above the ODRT while a significantly enhanced amplitude $ \alpha ^{\rm max} _{\rm li} $ is observed in the infrared. Our observations are in full accordance with the predictions of microscopic models for the ODRT, namely the removal of NbLi antisite defects upon incorporation of Mg ions, and an enhanced dark conductivity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hologram recording is studied in thermally reduced, nominally undoped lithium niobate in the time domain from 10 ns to 100 s by means of intense ns pump laser pulses (λ = 532 nm) and continuous-wave probe light (λ = 785 nm). It is shown that mixed absorption and phase gratings can be recorded within 8 ns that feature diffraction efficiencies up to 23 % with non-exponential relaxation and lifetimes in the ms-regime. The results are explained comprehensively in the frame of the optical generation of a spatial density modulation of Nb(Li)(4+/5+) antisites and the related optical features, i.e. absorption as well as index changes mutually related via the Kramers-Kronig-relation. Implications of our findings, such as the electrooptical properties of small bound Nb(Li)(4+) polarons, the optical features of Nb(Li)(4+):Nb(Nb)(4+) bipolarons, Nb(Nb)(4+) free polarons and O-hole-polarons, the impact of light polarization of pump and probe beams as well as of the polaron density are discussed.