Ultraviolet ͑UV͒-light-induced optical absorption in the near infrared ͑NIR͒ region was observed in diluted KTa 1−x Nb x O 3 single crystals ͑x = 0 , 0.004, 0.007, 0.012, 0.07͒ at low temperatures. Illumination by wideband light ͑3.10-4.13 eV, 300-400 nm͒ is accompanied by the appearance of a broad NIR absorption band with the position of the maxima varying in the 0.69-0.8 eV ͑1.54-1.79, T = 1.3 K͒ region for different Nb concentrations. This UV-light-induced absorption is absent in nominally pure KTaO 3 , as well as in all Nb diluted specimens at elevated temperatures. The centers responsible for the photochromic NIR absorption bands are tied to interband optical transitions of pair Nb 4+ electronic polarons. The photochromic experimental data, supplemented by luminescence studies in the visible range, evidence the strong localization of the photocharge carriers by pair Nb 4+ polarons at low temperatures. It is suggested that namely the strong localization of the photocarriers plays a crucial role in photoinduced gigantic dielectric effects and possible phase transitions, which have been recognized recently in incipient ferroelectrics at low temperatures.

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Pure KTaO 3 with cubic symmetry down to lowest temperatures is a model incipient ferroelectric exhibiting the existence of intrinsic inversion symmetry breaking defects in a wide range of experiments. At low temperature a strong second harmonic generation (SHG) of Nd:YAG laser light is observed, despite the overall cubic symmetry of the crystal. Under excitation with UV-light in the band gap region (about 330 nm (3.7 eV)) a strong green luminescence (GL, at 510 nm (2.43 eV)) appears with excitation maximum near 330 nm (3.7 eV). Both, SHG and GL depend strongly on previous oxidation/reduction treatments, increasing with the reduction. In absorption at low temperature broad bands peaking at 1.2 eV and at 1.5 eV are observed in reduced KTaO 3 samples. Additional UV-illumination in the band gap region at low temperature gives rise to broad light-induced absorption bands in the same 1-2 eV region. These observations are being discussed on the basis of polaronic holes, of charge transfer vibronic exitons (CTVEs) and of CTVE clusters interacting with oxygen vacancies.

The promising photorefractive crystal Sr x Ba 1-x Nb 2 O 6 (SBN, congruent composition x=0.61) can be doped with polyvalent ions like Ce to enhance photorefractive properties. Illumination with blue Ar + -laser light at low temperature creates light-induced NIR and VIS absorption bands. The NIR absorption has been identified as being due to Nb 4+ electron polarons and their properties have been investigated in some detail. The VIS-Centers are less well understood and their nature is still under discussion. It has been shown by our previous measurements that illumination with red Kr + -laser light (647 nm) or red laser diodes (673 nm) leads to a dissociation of the VIS-Centers and a simultaneous build-up of a transient NIR-polaron absorption. We report here some new facts important for the simplified charge transfer model needed to describe quantitatively the equilibrium values and the dynamics of the NIR-polaron and VIS-center absorption. These features give new insight into the dynamic nature of these VIS-centers, which are currently being discussed as either bipolarons (in analogy to LiNbO 3 ) or polarons trapped at charged centers, or charge transfer vibronic exitons (CTVEs) being trapped at charged centers.

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