Showing papers by "Hans P. Jenssen published in 1994"

Femtosecond time-resolved spectroscopy of soft modes in structural phase transitions of perovskites

Abstract: 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 ${\mathrm{KNbO}}_{3}$ and the tetragonal phase of ${\mathrm{BaTiO}}_{3}$. The data from potassium niobate clearly show a heavily damped soft mode of ${\mathit{B}}_{2}$ symmetry and relaxational modes of ${\mathit{A}}_{1}$ symmetry but not ${\mathit{B}}_{2}$ symmetry. Similarly, the data from barium titanate clearly show a heavily damped E-symmetry 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.

Intensity‐dependent photorefractive properties in an n‐type BaTiO3

Abstract: The beam coupling and response time of reduced BaTiO3 are reported. The major carrier changes from hole to electron after reduction. The electro‐optic gain and effective trap density increase with the illumination intensity. Although the electro‐optic gain varies little, the response time τ decreases by a factor of 2.8 after reduction. Experimental results indicate that at least three energy levels including shallow level(s) exist in this BaTiO3.

Fixing and IR response of doped barium titanate

Abstract: Two key obstacles to the development of high-capacity three-dimensional information mass storage systems using photorefractive crystals are: (1) readout of holograms stored in the conventional manner is destructive, and (2) undoped photorefractive crystals are typically quite insensitive to red and infrared write light from diode lasers (the preferred source). As a consequence, bulky, expensive ion lasers have been necessary for hologram writing. Thus hologram fixing and high infrared sensitivity in these materials are both important if practical systems are to be built. We present our own high-temperature (80-110°C) photorefractive grating writing results for a variety of pure and doped barium titanate (BaTiO 3 ) crystals. We found strong fixable secondary gratings that correlated with increasing levels of Fe and Ni in doped crystals. Fixing was not observed in our pure crystals or those doped with V, Rh, or Co. Fixing was enhanced in an iron-doped crystal reduced by high-temperature annealing at low oxygen partial pressures. We also found that the readout diffraction efficiency of a fixed grating in BaTiO 3 is a rapidly increasing function of readout temperature above a threshold temperature, which depends upon the crystal orientation. Lower temperatures lead to longer storage times, as would be expected if the carriers forming the fixed grating have a thermally activated mobility. We also report preliminary experimental results on blue Rh-doped BaTiO 3 crystals with fast and high-gain infrared (840 nm) response.