No abstract

We study experimentally steady-state photorefractive screening solitons trapped in both transverse dimensionsand measure their beam profiles as they propagate throughout the crystal. The solitons are observed to be axially symmetric, and they self-bend. We characterize the soliton dependence on the optical intensity, appliedelectric-field strength, and beam diameter.

The microscopic pathway along which ions or molecules in a crystal move during a structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a "soft" phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO 3 and BaTiO 3 are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

We present an experimental study of steady-state dark photorefractive screening solitons trapped in a bulk strontium barium niobate crystal. We compare experimental measurements with theoretical calculations of the soliton properties and find good agreement between theory and experiments. We confirm the shapepreserving behavior of the dark soliton by measuring its beam profile as it propagates throughout a specially cut crystal and by guiding a beam of a different wavelength.

Femtosecond time-domain observations of soft-mode dynamics in crystals near structural phase transitions have been conducted. Impulsive stimulated Raman scattering (ISRS) experiments are reported for both the orthorhombic phase of KNb03 and the tetragonal phase of BaTi03. The data from potassium niobate clearly show a heavily damped soft mode of B2 symmetry and relaxational modes of A & symmetry but not B& symmetry. Similarly, the data from barium titanate clearly show a heavily damped Esymmetry soft mode and no relaxational modes of this symmetry. The absence of relaxational modes of the same symmetries as the soft modes is consistent with an eight-site order-disorder model of the phase transitions in this crystal class. The present results demonstrate significant advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.