E. Krätzig

Composition dependence of the ultraviolet absorption edge in lithium tantalate

Comprehensive preparations of lithium tantalate crystals with compositions ranging from the congruent to stoichiometric ones have been carried out. Vapor transport equilibration treatments were used to determine the composition of the samples with an absolute accuracy of 0.05 mol %. This absolute determination of the composition can serve as the basis for convenient relative methods where an easily measurable physical property allows a simple determination of the composition. As an example, we present a study of the fundamental absorption edge in the near-ultraviolet region.

Photorefractive properties of undoped lithium tantalate crystals for various composition

Lithium tantalate crystals of compositions ranging from 48.3mol%to50.0mol% lithium oxide are fabricated by vapor transport equilibration. Light-induced refractive index changes of the crystals are investigated with holographic methods at usual cw-laser intensities (≈105W∕m2) and with a single focused laser beam at high light intensities up to 2×107W∕m2. In stoichiometric crystals the index changes are reduced by more than two orders of magnitude when compared with congruently melting ones. Simultaneously, the normalized photoconductivity σph∕I, where I is the light intensity, increases by nearly two orders of magnitude. Therefore, stoichiometric lithium tantalate is an attractive material for applications such as frequency conversion via quasi-phase matching.

Lifetime of small polarons in strontium-barium-niobate single crystals doped with cerium or chromium

Small polarons are excited by pulsed illumination (λ=532 nm) of strontium–barium–niobate single crystals doped with 0.66 mol % cerium or with 0.025 mol % chromium. The dark decay of the polarons is observed by monitoring the light-induced absorption at λ=632.8 nm and at λ=785 nm. The relaxation fulfills a stretched-exponential behavior over at least five decades. Using the Arrhenius law the temperature dependence yields the activation energy EA=(0.58±0.02) eV and a frequency factor of Z=(5±3)×1013 s−1. Delayed double-pulse technique (λ=532 nm pulse followed by a λ=1064 nm pulse) reveals a distance dependent recombination rate of the small polarons, i.e., the lifetime of a created polaron is a function of the distance to the next available deep electron trap. In SBN:Cr the R branch can be excited by pulses of λ=1064 nm with energy transfer via an unknown X center. Excitation and recombination show a single-exponential temporal evolution without any significant temperature dependence.

Holographic and electric investigations of KTiOPO/sub 4/ crystals used for second harmonic generation

Summary form only given. We present our experimental results of the investigations of KTP crystals with respect to their photorefractive behavior using holographic and electric methods. Holographic experiments were performed in the infrared spectral range using 180/spl deg/ reflection geometry to simulate the intracavity transmission of the harmonic lightwaves through the crystal. In this geometry a sinusoidal intensity modulation is produced parallel to the wave vectors. Since light illumination leads to an increase of the temperature due to absorption processes, there is a sinusoidal modulation of the temperature parallel to the wavevector of the transmitting beams and a gaussian distribution of the temperature parallel to the polar axis. Measurements of the conductivity show, that an increase of the temperature leads to an increase of the ionic conductivity of the K/sup +/-ions, which reaches values up to 10/sup -4/ /spl Omega//sup -1/ m/sup -1/ parallel to the crystallographic c-axes. Therefore, it is possible to transfer the modulation of the temperature in a modulation of the K/sup +/-ions, which changes the refractive index. We discuss the transport mechanism, which is responsible for the migration of the K/sup +/-ions in this model.