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Th Woike

Publications and source records attributed to Th Woike.

8 recordsLinked to original sources
PubMed2006

High-temperature-recorded index gratings in periodically poled lithium niobate.

Holographic index gratings based on a zero-electric-field photorefractive effect are recorded at high temperatures in copper-doped periodically poled lithium niobate crystals. The interplay between the domain structure and the index grating is studied: the fundamental grating with spatial frequency K is strongly suppressed. Pronounced sideband gratings with K(s)=K+sG appear, where G is the domain grating vector and s is an integer number. After development, an additional grating based on the electro-optic effect shows up. In contrast with the previously mentioned gratings, this grating allows anisotropic diffraction.

Journal Article↗
PubMed2006

Frequency mixing of photorefractive and ferroelectric gratings in lithium niobate crystals.

Holographically recorded photorefractive gratings in periodically poled lithium niobate crystals (PPLNs) are investigated. The principal spatial frequency K of the grating is strongly suppressed. Sideband gratings with grating vectors K +/- G appear. From the measurements the domain grating vector G and the duty cycle of the domain structure can be obtained. These findings allow for fast nondestructive quality inspection of PPLN and are of importance for any optical application combining holography and PPLN.

Journal Article↗
PubMed2005

Evidence for light-induced hole polarons in LiNbO3.

Transient light-induced absorption in LiNbO3 is observed in the blue-green spectral range after pulsed illumination with 532 nm. Its buildup and decay in Fe-doped LiNbO3 is satisfactorily described by a sum of two stretched exponential functions. For undoped LiNbO3, however, only one stretched exponential decay is observed. These experimental results are explained by the formation of both small Nb(Li)4+ electron polarons and O- hole polarons. The mechanism is discussed on the basis of a proposed band scheme.

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PubMed2005

Comment on "Theoretical study of the photoinduced transfer among the ground state and two metastable states in [Fe(CN)5NO]2-" [J. Chem. Phys. 122, 074314 (2005)].

We discuss the computational results of the "Theoretical study of the photoinduced transfer among the ground state and two metastable states in [Fe(CN)5NO]2-" [J. Chem. Phys. 122, 074314 (2005)] with respect to our previously reported polarized absorption study on the metastable states SI and SII in Na2[Fe(CN)5NO]2H2O [D. Schaniel, J. Schefer, B. Delley, M. Imlau, and Th. Woike, Phys. Rev. B 66, 085103 (2002)].

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PubMed2004

Change from 3D-Ising to random field-Ising-model criticality in a uniaxial relaxor ferroelectric.

Pyroelectric measurements of polarization have been used to determine the temperature dependence of the polarization in strontium barium niobate, SBN:Ce, close to its phase transition temperature T(c) approximately 317 K. A gradual increase of the critical exponent from beta approximately 0.13 to beta approximately 0.30 is observed when decreasing the initial polarization from 100% to 0.8% of the saturation value. A change from three-dimensional random-field Ising to pure Ising model behavior is conjectured and explained by a gradual compensation of quenched random electric fields by those emerging from charged fractal nanodomain walls.

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PubMed2003

New parametric scattering in photorefractive Sr0.61Ba0.39Nb2O6:Cr.

A new class of light-induced parametric scattering, not included in the conventional model, has been discovered in photorefractive Sr(0.61)Ba(0.39)Nb(2)O(6):Cr illuminated by two coherent beams. A novel model of multiwave mixing of coherent noise and transmitted light is developed to explain the new scattering phenomena. The model includes all known types and predicts a multitude of new types of parametric scattering. Generalized phase-matching conditions for parametric scattering in photorefractive crystals are proposed.

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PubMed2002

Polarization-based adjustable memory behavior in relaxor ferroelectrics.

The irreversible decay of the spontaneous polarization above the phase-transition temperature is a limiting factor in any application of ferroelectric crystals. Here we show that electric fields applied at high temperatures induce a preferred direction in the crystal which is stable even after repeated heating and cooling through the phase transition. This preference in direction leads to a reorientation of domains in the ferroelectric phase. We use pyroelectric measurements to show that the directional preference originates from internal charge carriers interacting with domain walls.

Journal Article↗