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Phone: þ420 224 358 605, Fax: þ420 224 358 601.We report the results of a multifaceted study of optical and dielectric properties of erbium-doped (500 ppm) potassium tantalate incipient ferroelectric single crystals. Studies of optical absorption and photoluminescence spectra allowed us to determine the system of energy levels that control the main internal optical transitions in the 4f electronic shell of Er 3þ in KTaO 3 . It was found that at least two types of Er 3þ centres, major and minor ones, are present. The major centres are noncubic, formed by Er 3þ substituting for K þ sites; these are responsible for the n-type conductivity of KTaO 3 :Er crystals.Wideband optical absorption with a maximum at 1.13 eV (l ¼ 1097 nm) was observed in the near-IR range and was attributed to polaron formation. Low-temperature far-infrared reflectivity studies revealed an increase in the frequency of the lowest transverse optical mode and a decrease in the dielectric permittivity in comparison with undoped KTaO 3 crystals. This stiffening means the suppression of the ferroelectric instability in the system. 1 Introduction Functional ABO 3 ferroelectric oxides and related materials are the materials of choice in various applications from actuators to memory and spintronic devices. A key advantage is that their properties can be tailored by adding suitable impurities. However, the improvement and optimization of the functional properties of these oxides are hampered by our lack of understanding of the impurity centre microstructure and impurity-lattice interactions in these highly polarizable materials with TO 1 soft phonon modes. In this regard, studies of optical spectra of rare earth (RE) ion-doped KTaO 3 (KTO), one of the top models of the ABO 3 perovskite-like oxides, is of special interest. We report on the optical absorption and luminescence spectra of KTO crystals doped with Er ions, one of the most promising RE activators, attracting a great deal of interest in impurity centre optics in dielectrics. Such research is especially interesting for a variety of reasons. On the one hand, it deepens our knowledge of impurity centre optics and photo-induced charge transport in nonlinear materials with soft phonon modes, pronounced photorefractive properties, and a tendency to structural and ferroelectric phase transitions. On the other hand, experiments with model probe Er centres can bring new insight into the properties and

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If light beam propagates through matter containing point impurity centers, the amount of energy absorbed by the media is expected to be either independent of the impurity concentration N or proportional to N, corresponding to the intrinsic absorption or impurity absorption, respectively. Comparative studies of the resonant transmission of light in the vicinity of exciton resonances measured for 15 few-micron GaAs crystal slabs with different values of N, reveal a surprising tendency. While N spans almost five decimal orders of magnitude, the normalized spectrally-integrated absorption of light scales with the impurity concentration as N1/6. We show analytically that this dependence is a signature of the diffusive mechanism of propagation of exciton-polaritons in a semiconductor.

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