R. Pankrath

XPS study of photorefractive Sr0.61Ba0.39Nb2O6:Ce crystals

Abstract X-ray photoelectron spectroscopy (XPS) measurements show that Ce occurs in photorefractive Sr0.61Ba0.39Nb2O6 (SBN) crystals mainly in the valence state 3+. We give arguments that Ce 3+ 4+ is the photorefractive center in SBN:Ce with a typical concentration ratio c Ce 3+ c Ce 4+ = 600 . Quantitative analysis of the XPS data yields a Ce distribution coefficient of about 0.6 ± 0.3.

Pyroelectrically induced photorefractive effect in Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce

Photorefractive properties of a cerium-doped strontium barium niobate single crystal (Sr(0.61)Ba(0.39)Nb(2)O(6)) are investigated with Q-switched light pulses (wavelength 532 nm). Pyroelectric fields are the dominant charge driving force and significantly enhance light-induced refractive-index changes.

Photoconductivity of photorefractive Sr0.61Ba0.39Nb2O6:Ce crystals at high light intensities

A method is presented to perform direct photoconductivity measurements of Sr0.61Ba0.39Nb2O6:Ce crystals at high intensities: A high voltage is applied to the sample and subsequent illumination yields a photocurrent through the crystal which charges a capacitor. Finally an electrometer measures the voltage of this capacitor. From voltage, capacity and exposure time the photoconductivity is calculated. Measurements with light pulses of a Q‐switched frequency‐doubled Nd:YAG laser (pulse duration 20 ns, light wavelength 532 nm) are performed. The photoconductivity σph increases sublinearly with light intensity I according to σph∝Ix with x<1. The experimental results point to Ce3+/4+ as the dominant photo‐ refractive center in Sr0.61Ba0.39Nb2O6:Ce at high intensities.

Crystal growth of K(Ta1-xNbx)O3 solid solutions

Abstract Single crystals of K(Ta1-xNbx)O3 with compositions 0.30<x<1 are grown from solutions with an excess of K2O. In order to minimize striations due to temperature fluctuations a method is applied similar to that known for the crystallization in aqueous solutions. With optimized growth conditions this kind of striations is fairly reduced. However, another type, so-called type II striations, is observed. They are formed by terraces at the growth interface. It is shown by electron microprobe measurements that the Nb/Ta ratio depends very strongly on the height of the terraces. In order to reduce type II striations Rb2O is given to the solution and forced convection is applied. It is possible to restore the excess of K2O completely by Rb2O. As a result the crystals grown from these solutions are nearly striation-free. They are suitable for optical second harmonic generation (SHG) and holographic experiments.

Influence of Fe doping on the photorefractive properties of KTa1−xNbxO3

The influence of Fe doping on the photorefractive properties of tetragonal potassium tantalate-niobate (KTN) crystals is investigated. Light-induced refractive index changes, decay time constants and gain coefficients are measured as a fucntion of external electric field and spatial frequency for KTN crystals with different Fe concentrations. The observed behaviour is interpreted in terms of a transport model which assumes one kind of photorefractive centers with both electrons and holes as free charge carriers, taking into account space charge limitations due to the available trap density.

Tilting of holograms in photorefractive Sr0.61Ba0.39Nb2O6 crystals by self-diffraction.

Holographic gratings are written in photorefractive Sr0.61Ba0.39Nb2O6 crystals by two interfering light beams. Angular-dependent readout of the holograms shows that applying an external electric field to the crystal produces a pronounced tilting of the holograms. The results are in good agreement with theoretical predictions considering self-diffraction of the recording beams.

Photorefractive and related properties of Ba0.984Sr0.016TiO3 crystals

Abstract Measurements of static dielectric constants, electrooptic coefficients, absorption, light-induced absorption, photo and dark conductivity, holographic diffraction efficiency and photorefractive two-beam coupling gain are performed with a single- crystalline Ba 0.984 Sr 0.016 TiO 3 sample. Crystals of this composition melt congruently and can be grown by the Czochralski technique. They are very attractive for photorefractive applications.

Time-resolved measurements of photoconductivity in cerium-doped photorefractive strontium–barium niobate using nanosecond light pulses

Abstract Time-resolved measurements of photoconductivity in photorefractive strontium–barium niobate crystals doped with cerium are performed. The crystals are illuminated with nanosecond light pulses of a Q-switched frequency-doubled Nd:YAG laser and an external electric field is applied to the samples. Light-induced drift current is amplified and detected with a fast oscilloscope. The relaxation time of photoconductivity yields the lifetime of electrons in the conduction band. The carrier lifetime decreases and the photoconductivity increases with increasing cerium concentration in the crystals. The observed dependences and previous measurements of integrated conductivity are explained considering the participation of shallow levels in the charge transport.

PHOTOREFRACTIVE PROPERTIES OF TETRAGONAL POTASSIUM TANTALATE-NIOBATE CRYSTALS AT HIGH LIGHT INTENSITIES

Abstract Photorefractive properties of an iron-doped tetragonal KTa0.52Nb0.48O3 single crystal are investigated with Q-switched light pulses (wavelength 532 nm). Pyro-electric fields significantly enhance light-induced refractive index changes.

Variation of doping-dependent propertiesin photorefractive Sr x Ba 1−x Nb 2 O 6 : Ce, Cr, Ce+Cr

In congruent Sr x Ba 1 m x Nb 2 O 6 (SBN, x =0.61) the photorefractive properties are significantly enhanced by doping with Ce or Cr. The visible and FIR absorption increases linearly with the dopand concentration up to about 10,000 v ppm (p.f.u.) of Ce or Cr or in double doped crystals (Ce+Cr). Simultaneously a decrease of the phase transition temperature T c from about 253 v K in pure SBN to about room temperature for doping concentrations (of Ce, Cr or both) of about 20,000 v ppm (p.f.u.) is found.