Infrared reflectivity, time-domain terahertz transmission, high-frequency dielectric measurements and Raman spectroscopy of Na0.5Bi0.5TiO3 complex ferroelectric perovskite were performed in a broad temperature range. The results were analysed considering the macroscopic symmetry as well as symmetry resulting from local Na–Bi ordering in all three known phases. An overdamped infrared soft mode was revealed in the THz range which, together with central-mode type dispersion in the GHz range, contribute to the strong and broad dielectric permittivity maximum around 600 K. Anharmonic Bi and/or Na vibrations and local hopping, respectively, are suggested to be the main origins of these excitations. The broad phonon spectra and the frequency independent dielectric losses at low temperatures are compatible with the existence of nanoscopic polar regions and disorder to liquid He temperatures similar to relaxor ferroelectrics.

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Light scattering measurements are investigated as a function of temperature from 10 to900 Kin the tetragonal andcubicphasesofaPbTi0,crystal. Phononfrequencies, ionic effective charges, spontaneous polarization and contribution to the dielectric permittivity arededucedfromthesedata.Theresu1tsshowthat thesoft phononandrelaxationmode are both needed to describe the dynamics of this material.

The time behavior of bright spatial solitons in biased photorefractive media is investigated within the framework of a bidimensional band transport model. Biasing the photorefractive media requires an externally applied electric field or the presence of a photovoltaic effect. These two basically different phenomena are shown to be equivalent and additive. The mechanism of space-charge field buildup is analytically expressed, leading to a time-dependent wave propagation equation in generic photorefractive media. The temporal behavior of the soliton solutions to this equation is investigated. It shows the evolution of the soliton beam from the time the external electric field is applied to the final steady-state soliton. On the way, the so-called quasisteady soliton is retrieved, along with its properties. Furthermore, the photovoltaic soliton is described by the wave propagation equation: its behavior is the same as that of the steady-state soliton, the transient states included. Finally, low-power photorefractive bright spatial solitons are generated in a Bi12TiO20 crystal with a He-Ne laser and their temporal behavior is investigated, thus providing an experimental validation of our theoretical considerations