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.

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.

Single crystals of undoped and cerium-doped yttrium aluminum garnet have been pumped with laser radiation of 220- and 266-nm wavelength. A broadband defect emission at approximately 300 nm was observed, similar to that obtained with cathodoluminescence, although decreased in magnitude. Evidence of a transfer of energy from these defect states to the Ce+3 states was inferred from the photoluminescence decay time measurements. We have associated the absorption spectra from 200–300 nm with the defect excitation spectrum, including the peaks at 225 and 270 nm.

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The electrical and optical properties of calcium and cerium-doped yttrium aluminum garnet (Ca,Ce:YAG) have been studied. Ca,Ce:YAG is a mixed ionic and electronic conductor with an ionic conductivity activation energy of 4.3 eV. Evidence of cluster formation with a consequent higher-than-expected activation energy is presented. The cerium normally enters the crystal as Ce + 4, but it may be converted to Ce + 3 under reducing atmospheres at elevated temperatures.