Publikationen 2003
T. Granzow, Th. Woike, W. Rammensee, M. Wöhlecke, M. Imlau, R. Pankrath
Influence of Ce and Cr doping on the pyroelectric behaviour of Sr0.61Ba0.39Nb2O6
phys. stat. sol. (a) 197, No. 3, R2–R4 (2003)
Abstract
We present measurements of the temperature dependence of the pyroelectric
effect in congruently melting strontium–barium–niobate doped with cerium and
chromium in the range of the relaxor phase-transition. The influence of the
doping concentration on the transition temperature is shown and discussed with
respect to the influence of internal strain fields.
Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E.
Krätzig, M. Wöhlecke
Composition dependence of the ultraviolet absorption edge in lithium tantalate
J. Appl. Phys. 93, 3102 (2003)
Abstract
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.
Publikationen 2002
T. Volk, B. Maximov, S. Sulyanov, N. Rubinina, M. Woehlecke
Relation of the photorefracton and optical-damage resistance to the intrinsic defect structure in LiNbO3 crystals
Optical Materials xxx (2002) xxx–xxx
Abstract
The defect structure of LiNbO3 crystals
doped with optical-damage-resistant impurities Zn and In was studied by X-ray
and neutron diffraction methods. Photoinduced charge transport in these crystals
is discussed in the context of the deduced defect
models. In LiNbO3:Zn and LiNbO3:In an increase of the
photoconductivity sph and the resulting lowering
of photorefraction is related to a
variation of the intrinsic defect (NbLi and VLi)
concentration. In crystals co-doped with iron an increase of sph
is due to a change of the lattice site of the Fe3+-electron trap.
T. Granzow, Th. Woike, M. Wöhlecke, M. Imlau, and W. Kleemann
Polarization-Based Adjustable Memory Behavior in Relaxor Ferroelectrics
Phys. Rev. Lett. 89, 127601 (2002)
Abstract
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.
V. Szalay, L. Kovács, M. Wöhlecke, E. Libowitzky
Stretching potential and equilibrium length of the OH bond in solids
Chem. Phys. Letters 354, 56 - 61 (2002)
Abstract
Correlation has been observed between the experimentally determined harmonic fre-
quencies, w, and anharmonicity coefficients, X, of OH bond stretching vibrations in alkali
halide, oxide, and hydrate crystals. It has been shown that simple second order
perturbation approximation formulas of w and X can explain the
w-X correlation, and they can
be used to obtain the shape of the stretching potential and an estimate of the equilibrium
length of an OH bond in crystalline environment.
T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau ,W. Kleemann
Local electric- field- driven repoling reflected in the ferroelectric polarization of Ce-doped Sr0.61Ba0.39Nb2O6.
Appl. Phys. Letters 80, 470 - 472 (2002)
Abstract
We present pyroelectric measurements with the relaxor-ferroelectric SBN in the phase transition regime. It is
demonstrated that domains poled at high temperatures are more stable than those oriented at room temperature. This yields a
higher phase transition temperature when heating the sample and a strong repoling during the cooling process. We explain this
difference within the framework of the random field model for relaxor ferroelectrics.
T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pamkrath, M. Imlau, W. Kleemann
Evidence of random electric fields in the relaxor-ferroelectric Sr0.61Ba0.39Nb2O6
Europhysics Letters 57, 597 - 603 (2002)
Abstract
We present strong evidence of the existence of quenched random electric fields in relaxor ferroelectrics by
investigating the aging in ferroelectric hysteresis loops under light illumination. The temporal behaviour
of ferroelectric domains under external electric fields and light illumination is measured in Cerium-doped
Strontium-Barium-Niobate crystals using quasistatic surface charge detection. The aging, caused by electric
pinning centres which hinder domain wall motion, leads to a decrease in domain wall mobility. Raising the
sample conductivity by illumination eliminates the pinning centres and fully restores the polarization dynamics.
Publikationen 2001
V. V. Gladkii, V. A. Kirikov, E. V. Pronina, T. R. Volk, R. Pankrath, M. Wöhlecke
Anomalies in the Slow Polaritation Kinetics of a Ferroelectric relaxor in the Temperature Region of a Diffuse Phase Transition
Physics of the Solid State 43, 2140-2145 (2001)
Abstract
It is shown experimentally in the specific example of Cr-doped
barium-strontium niobate that the anomalies in the infralow-frequency dielectric
properties characteristic of a ferroelectric relaxor persist over the whole
temperature region of the diffuse phase transition and decrease gradually with
increasing temperature. Experimental data are presented on the anomalous
quasi-static dielctric hysteresis loops, the slow polarization kinetics and a
quantitative measure of the structural disorder typical of a relaxor.
T. Volk, L. Ivleva , P. Lykov, D. Isakov, V. Osiko, M. Wöhlecke
Modification of the optical and photorefractive properties of Ce-doped strontium-barium niobate by co-doping with a nonphotorefractive La impurity
Appl. Phys. Letters 79, 854 (2001)
Abstract
We demonstrate a possibility to modify photorefractive properties of a ferroelectric crystal by means of controlling the
ferroelectric phase transition. This is shown by an example of co-doping a photorefractive SBN-0.61:Ce crystal with a
nonphotorefractive La impurity, which strongly lowers the ferroelectric phase transition and in turn drastically enhances
the linear electrooptic coefficients. The observed low coercive fields enable us to build in SBN:(Ce+La) a reproducible fast
switching of the energy transfer direction. This is done by an electric field switching which changes the sign of the two-beam
coupling gain factor G.
T. Volk, L. Ivleva, P. Lykov, N. Polozkov, V. Salobutin, R. Pankrath, M. Wöhlecke
Effects of rare-earth impurity doping on the ferroelectric and photorefractive properties of strontium-barium niobate crystals
Optical Materials 18, 179 (2001)
Abstract
Doping strontium-barium niobate (SBN) crystals with rare earth (RE)
impurities strongly lowers the ferroelectric phase transition Tc
which results in significant enhancement of the dielectric permittivity and
electrooptic coefficients and in a simultaneous decrease of the coercive filed Ec.
In photorefractive SBN:Ce crystals co-doped with a non-photorefractive impurity
La we studied the hysterese dependence of the two-beam coupling gain G
on the external field.
Th. Woike, T. Granzow, U. Dörfler, Ch. Poetsch, M. Wöhlecke, R. Pankrath
Refractive Indices of congruently melting Sr0.61Ba0.39Nb2O6
phys. stat. sol. (a) 186, R13 (2001)
Abstract
Refractive indices of pure congruently melting Strontium-Barium-Niobate single crystals poled
and unpoled have been measured in the wavelength range 400 to 2330 nm. The difference in the refractive indices
of poled and unpoled crystals is interpreted in the framework of the phase transition.
OH- ions in Oxide Crystals
Critical Reviews in Solid State and Materials Sciences, 26(1):1 - 86 (2001)
Abstract
This article reviews the spectroscopic properties of hydrogen bound to a large variety of synthetic compounds like simple
oxides, perovskites, the LiNbO3 family, KTP, sillenites, eulytites, borates, garnets, spinels and some others. In almost
all compounds the OH-stretch mode is found in the range 3200-3700 cm-1, with isotope replica of deuterium
(2350-2750 cm-1) and a few examples of tritium (2050-2250 cm-1). The fundamental vibration, the isotope
replica and the corresponding overtones are described with an anharmonic oscillator model. A nonlinear relation between the
anharmonicity and the harmonic frequency is found and interpreted using recent second order perturbation
calculations. An often pronounced temperature dependence of the stretch mode and phonon
coupling is discussed. Data for the stretching vibration influenced by additional weak or strong doping are presented.
Atomistic defect models are described and tested by means of light polarization, external perturbations like pressure and electric
field, and Raman scattering analyzed with the behavior-type method. The last section briefly reviews applications profiting
from doping with hydrogen and its easy detection by means of the OH stretch mode.
T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W. Kleemann
INFLUENCE of PINNING EFFECTS on the FERROELECTRIC HYSTERESIS in CERIUM-DOPED SrxBa1-xNb2O6
Phys. Rev B 63, 174101 (2001)
Abstract
The temporal behavior of ferroelectric domains under external electric field is investigated in
cerium-doped strontium-barium-niobate crystals using quasistatic surface-charge detection. The polarization reversal depends
strongly on frequency, temperature, dark conductivity, and doping
concentration. This so-called electrical aging is attributed to electric
pinning centers which hinder the domain-wall motion.
We discuss the existence of pinning centers on the basis of the quenched random-field model.
T. Volk, B. Maximov, T. Chernaya, N. Rubinina, M. Wöhlecke, V. Simonov
PHOTOREFRACTIVE PROPERTIES of LiNbO3:Zn CRYSTALS RELATED to THE DEFECT STRUCTURE
Appl. Phys. B 72, 647 (2001)
Abstract
LiNbO3:Zn single crystals and powders were studied by precise X-ray diffraction methods. A structure refinement yielded new
models of the intrinsic defect structure valid for different Zn
concentration ranges. For concentrations up to 7 at.% in the crystal Zn ions incorporate onto Li-sites; at higher
concentrations Zn ions are found on both Li and Nb sites. Photorefractive properties of
LiNbO3:Zn are discussed in the context of the deduced defect models. A smooth increase of the
photoconductivity up to 7 at.%Zn is accounted for by a decrease of Nb antisites. A steeper growth of the photoconductivity at
higher Zn concentrations is related to vanishing Li-vacancies.
PHOTOREFRACTIVE PROPERTIES of Cr-doped Sr0.61Ba0.39Nb2O6
RELATED to CRYSTAL PURITY and DOPING CONCENTRATION
Appl. Phys. B 72, 661 (2001)
Abstract
The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were
determined in single crystals of chromium-doped Sr0.61Ba0.39Nb2O6 (SBN) by
instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis
(XRF). Experiments with different Cr concentrations
CCr reveal a constant Sr and Ba concentration as well as a decreasing Nb concentration
with an increasing CCr . Therefore, Cr is incorporated on Nb lattice sites. The distribution coefficient
of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two beam
coupling gain G for different CCr, we deduced response
times for the buildup of holographic gratings. The saturation value of G
depends in a nonlinear manner on CCr and reaches its maximum at about
6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping
concentrations. Thus a one-center model can be assumed for the charge transport in
SBN:Cr.
Publikationen 2000
M. Meyer, M. Wöhlecke, O. F. Schirmer
ON THE TEMPERATURE DEPENDENCE OF THE BAND EDGE OF Sr0.61Ba0.39Nb2O6
phys. stat. sol. (b) 221, R1 (2000)
Link to paper
RAMAN STUDY OF THE ORDERING IN Sr(B'0.5Nb0.5)O3 COMPOUNDS
Abstract
Strontium based complex perovskites are potential candidates for microwave integrated
circuit applications. In the present article, we report on Raman scattering studies of
cubic and non-cubic structures of Sr(B'0.5Nb0.5)O3
[B' = Ga, In, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Yb] based complex-perovskite
materials for an improved understanding of structure-property relations. The spectral
results are compared to some tantalum analogues of known crystal structure. The present
study reveals a higher degree of ordering for the tantalum compounds compared to those of
the niobium analogues. Vibrational studies show a correlation between the tolerance factor
and symmetry of these materials.
T. R. Volk, V. Yu. Salobutin,
L. I. Ivleva, N. M. Polozkov, R. Pankrath, and M. Wöhlecke
Ferroelectric Properties of Strontium Barium Niobate Crystals Doped with Rare-Earth Metals
Physics of the Solid State 42, 2129-2136 (2000)
Abstract
The
influence of doping of the SrxBa1-xNb2O6
(x = 0.61) crystals (SBN-0.61) by rare-earth element (RE) dopants has been
investigated. Taking into account the data available in the literature, it is
demonstrated that the introduction of all rare-earth metals is accompanied by a
substantial decrease in the temperature of the phase transition Tp
and an increase in its smearing. The shift in the Tp
temperature for certain rare-earth metals ranges up to 20 K per atomic
percent of dopant in the crystal. It is experimentally established that, in the
SBN-0.61:Yb, SBN-0.61:Ce, SBN-0.61:Tm, SBN-0.61:La, and SBN-0.61:(Ce + La)
crystals, a decrease in Tp brings
about a considerable increase in the permittivity and the piezoelectric and
electrooptical coefficients. The conclusion is drawn that the doping by
rare-earth metals provides a means of optimizing the properties of strontium
barium niobates. The pulse switching of the SBN-0.61 and SBN-0.61:RE crystals is
studied for the first time. It is found that the switching is characterized by a
number of features, the most important of which is a decrease in the switched
charge after the application of external fields. This effect associated with the
specific features of the switching in the relaxor ferroelectric is assumed to be
responsible for the instability of the parameters for strontium barium niobate
material.
Publikationen 1999
T. Volk, T. Woike, U. Dörfler, H. Schmitz, R. Pankrath, M. Wöhlecke
FERROELECTRICITY DRIVEN TWO-BEAM COUPLING GAIN IN STRONTIUM-BARIUM NIOBATE CRYSTALS
Trends in Optics and Photonics (TOPS) 27, 86 (1999)
Abstract
We report on contribution from ferroelectric phenomena to the two-beam coupling gain
factor G in Sr0.6Ba0.4.Nb2O6
crystals doped with rare-earth elements. The values of G
are sensitive to the ferroelectric phase transition and may be controlled by a variation
in Tc which is achieved, in particular by doping SBN with rare-earth
impurities.
OPTICAL AND PHOTOREFRACTIVE PROPERTIES OF OPTICAL-DAMAGE RESISTANT LiNbO3:Zn CRYSTALS RELATED TO STRUCTURE PARAMETERS
Trends in Optics and Photonics (TOPS) 27, 144 (1999)
Abstract
In optical-damage resistant LiNbO3:Zn crystals a correlation is found
between dependencies of optical properties and the unit cell parameters on Zn
concentration. Non-monotonic shapes of these curves may be accounted for by different Zn
incorporation into the lattice depending on the Zn amount. Peculiarities observed at about
2-3 mol% Zn may be related to a removal of NbLi defects which is due to Zn
incorporation on Li-sites. At concentrations exceeding 6.5 mol% Zn a partial incorporation
of Zn ions on Nb-sites is detected. The wellknown extrema and anomalies in concentration
dependencies of various optical properties, including a drastic fall of photorefraction
observed formerly in the range from 6 to 7 %Zn may be accounted for in terms of these
structural data. Concentration dependencies of the electrooptical coefficients presented
here find a qualitative interpretation within the same scheme.
M. Wierschem, C. Bäumer, A. Jovanovic, M. Wöhlecke, S. Kapphan, L. Kovács
FUNDAMENTAL AND OVERTONE SPECTRA OF THE OH AND OD STRETCH MODE IN CONGRUENT AND IN STOICHIOMETRIC LiTaO3
phys. stat. sol. (b) 215, R7 (1999)
INFLUENCE OF Zn DOPING ON ELECTROOPTICAL PROPERTIES AND STRUCTURE PARAMETERS OF LITHIUM NIOBATE CRYSTALS
Appl. Phys. B 68, 795 (1999)
Abstract
We report the dependence of the unit cell parameters and the EO coefficients on Zn
doping in optical-damage-resistant LiNbO3:Zn crystals. Both properties depend
in a non-monotonic manner on the Zn content. This is accounted for by different types of
Zn ion incorporation into the lattice depending on the Zn concentration in the melt.
Extrema observed in the concentration dependence of the EO coefficients at about 2-3 and
6.4 mol.% Zn correlate with an unusual concentration dependence of the unit cell
parameters a and c. The low concentration anomalies may be accounted for
by a decrease of the Li vacancy concentration due to the Zn incorporation into Li sites.
Anomalies at high concentrations are obviously due to a partial incorporation of Zn ions
on Nb sites, which is reflected in the structure data. Anomalies in the concentration
dependence of other optical properties at about 6 -7 mol.% Zn reported recently are
obviously related to a change in the localization of the Zn ions. The combination of high
EO coefficients with a reduced optical damage for these concentrations make these crystals
attractive for applications as Q-switching or electrooptical modulation.
A HOLOGRAPHIC METHOD FOR THE DETERMINATION OF ALL ELECTROOPTIC COEFFICIENTS APPLIED TO Ce-DOPED STRONTIUM-BARIUM-NIOBATE
Appl. Phys. B 68, 843 (1999)
Abstract
Using c- and a-cut plates of Strontium-Barium-Niobate doped with 0.66 mol.% cerium
all nonzero components of the linear electrooptic tensor r13, r42, r33
including the sign are determined from angular dependence ofthe two-beam coupling
coefficient G using transmission and reflection geometry of
holographic recording. We found: r13=56 pm/V, r42=38
pm/V and r33=333 pm/V. In addition we calculated the effective trap density Neff
to be 2.2*1017 cm-3. The advantages of this method as well as the
values obtained are discussed.
Publikationen 1998
OPTICAL DAMAGE RESISTANCE IN LITHIUM NIOBATE CRYSTALS
Ferroeelectrics Review 1, 195-262 (1998)
Abstract
This article reviews the current knowledge of the most important properties and
applications of damage resistant LiNbO3 crystals. Damage resistance (reduction
of optical damage or photorefraction) occurs due to doping with a few mol.% of Mg, Zn, In,
and Sc. As a result the photorefraction is reduced by two orders of magnitude. The damage
resistant impurities do not affect the transparency in the spectral range of interest.
These features allow the use of these crystals in optical setups with high stability at
strong laser intensities. The influence of damage resistant impurities on photorefraction,
the refractive index and nonlinear optical effects is discussed and the similar behavior
of all impurities explained. Photorefraction is reduced because of an enhanced
photoconductivity, which is interpreted in terms of the current model of photo-induced
charge transport. The refractive index of damage resistant LiNbO3 crystals of
different compositions is described by a generalized Sellmeier equation suitable to
calculate phase-matching conditions. Many experimental data are presented, which suggest a
common structure origin of the damage resistance. The impurity-induced removal of the
intrinsic Nb antisite defect up to a certain threshold concentration of the damage
resistant impurities is responsible for most features. The last section reviews
applications profiting from the reduced photorefraction combined with unchanged
transparency, like phase-matching configurations including quasi-phase-matching structures
and laser action.
Influence of Zn/In-codoping on the optical properties of Lithium Niobate
J. Appl. Phys. 84, 5191 (1998)
Abstract
The influence of a combined doping with Zn and In on the optical properties of
lithium niobate is studied. While each of these dopants is known to improve the material's
nonlinear optical properties, it is an open question whether further improvements
especially of the nonlinear optical features can be achieved by a suitable combination of
dopants. Our measurements yet show that lithium niobate imposes certain limits which can
not be overcome. This can be described by a simple model which is based on the defect
structure of lithium niobate.
E. Ruza, H. J. Reyher, J. Trokss, M. Wöhlecke
Optically detected magnetic resonance via the blue luminescence of Ti-doped Al2O3
J. Phys. Condens. Matter 10, 4297 (1998)
Abstract
The UV-excited blue and green luminescence bands of Ti-sapphire are characterized by
ODMR. These emission bands are attributed to two Ti3+-O--centers,
which show very similar properties and are created as a result of a charge-transfer
transition of an electron from O-- to Ti4+-ions. In both centers,
the d-electron of Ti3+ and the hole of O- are strongly coupled and
form triplet states. Doublet systems can be ruled out as source of the blue-green
luminescence. The angular dependence of the ODMR can be explained with an appropriate spin
Hamiltonian assuming orthorhombic local symmetry. The orientation of the principal axes of
the g-tensor and the crystal field tensor, found for both centers, suggest that the hole
is localized on a single oxygen ion and that the Ti3+-O- bond is
almost aligned with the Al-O bonds of the undisturbed lattice.
T. Woike, T. Volk, U. Dörfler, R. Pankrath, L. Ivleva, M. Wöhlecke
Ferroelectric and optical hysteresis in SBN doped with rare-earth elements
Ferroelectric Letters 23, 127 (1998)
Abstract
In Sr0.61Ba0.39Nb2O6 crystals doped
with Ce and Tm the dependence of the two-beam coupling gain factor G
on an external dc- field E applied after recording shows a hysteresis, which reflects the
ferroelectric P-E hysteresis. At coercive fields Ec the sign of G abruptly reverses, i.e. the energy transfer direction between the
coupled beams. The field-controlled state of G is
stable and stored after turning off the fields. The results are discussed in terms of the
concept of electrooptical effect and ferroelectricity in oxygen-octahedra ferroelectrics.
K. Kasemir, K. Betzler, B. Matzas, B. Tiegel, M. Wöhlecke, N. Rubinina, T. Volk
Influence of In Doping on the Refractive Indices of Lithium Niobate
physica status solidi (a) 166/1, R7 (1998)
Publikationen 1997
T. Volk, T. Woike, U. Dörfler, R. Pankrath, L. Ivleva, M. Wöhlecke
FERROELECTRIC PHENOMENA IN HOLOGRAPHIC PROPERTIES OF STRONTIUM-BARIUM NIOBATE CRYSTALS DOPED WITH RARE-EARTH ELEMENTS
Ferroelectrics 203, 457 (1997)
Abstract
We report on a relation between the two-beam coupling gain factor G
in Sr0.61Ba0.39Nb2O6 (SBN:0.6) crystals
and their ferroelectric properties. In SBN:0.6 doped with rare-earth-metal impurities Ce
and Tm we found a hysteresis dependence of G versus the
external dc-field E applied after recording, which is due to the ferroelectric P-E
hysteresis. Particularly, G-E hysteresis manyfests itself in an
alteration of the sign of G, i.e., of the direction of the
energy transfer between the coupling beams, under applying of E reversing Ps.
The value of G at room temperature essentially increases
with impurity concentrations. It is caused by an increase of the dielectric permittivity,
which in its turn is due to a significant lowering of the phase-transition temperature Tc
produced by a rare-earth doping.
T. Woike, T. Volk, U. Dörfler, R. Pankrath, L. Ivleva, M. Wöhlecke
FERROELECTRIC AND OPTICAL HYSTERESIS IN SBN DOPED WITH RARE-EARTH ELEMENTS
Ferroelectrics Letters, 23 No 5/6
Abstract
In Sr0.61Ba0.39Nb2O6 crystals doped with
Ce or Tm the dependence of the two-beam coupling gain factor G
on an external c-field E applied after recording shows a hysteresis, which reflects the
ferroelectric P-E hysteresis. At coercive fields Ec the sign of G abruptly reverses, i.e. the energy transfer direction between the
coupled beams. The field-controlled state of G is stable
and stored after turning off the fields. The results are discussed in terms of the
electrooptical effect and ferroelectricity in oxygen-octahedra ferroelectrics.
L. Kovács, G. Ruschhaupt, K. Polgár, G. Corradi, M. Wöhlecke
COMPOSITION DEPENDENCE OF THE ULTRAVIOLET ABSORPTION EDGE IN LITHIUM NIOBATE
Applied Physics Letters.70(21):2801-2803, 1997
Abstract
A convenient and fast method to determine the composition of LiNbO3 single
crystals consisting of a measurement of the position of the fundamental absorption edge in
the near UV region is refined and extended to the stoichiometric range. Using
unsophisticated apparatus and simple reflection correction, the proposed two-parameter
calibration equation provides an absolute accuracy of 0.1 mol % with the relative accuracy
reaching an unusual value of better than 0.01 mol % for near stoichiometric compositions.
M. Wöhlecke, T. Volk, H. Donnerberg
ON THE ROLE OF INTRINSIC CLUSTERS IN DAMAGE-RESISTANT LINBO3
Ferroelectrics Letters Section. 22(3-4):53-58, 1997.
Abstract
The intrinsic stoichiometry-related defect structure of LiNbO3 consists of
Nb antisites and of charge compensating cation vacancies. The electrooptic properties
of the material are mainly determined by the concentration of Nb antisites
acting as important electron traps. Recent atomistic computer simulations
suggested the agglomeration of Nb antisites and cation vacancies probably
resulting in spacious intrinsic defect clusters. Since the incorporation of
divalent impurity cations consumes stoichiometry-related defects, possible
clustering effects should be observable as peculiarities in the antisite
concentration dependence of physical properties. It is the purpose of this
letter to relate corresponding non-linearities to the existing intrinsic defect
clusters. The non-linearities, which are observed well below the threshold
concentrations of divalent impurity cations, cannot be understood on the basis
of isolated stoichiometry-related defects.
Publikationen 1996
M. Wöhlecke, G. Corradi, K. Betzler
OPTICAL METHODS TO CHARACTERISE THE COMPOSITION AND HOMOGENEITY OF LITHIUM NIOBATE SINGLE CRYSTALS
Applied Physics B-Lasers & Optics.63(4):323-330, 1996
Abstract
A number of the physical properties of lithium niobate strongly depend on sample
composition. Although several procedures for the determination of the composition have
already been published, a critical survey and an introduction to standard methods
recommendable also for laboratories active in R&D of LiNbO3 devices is
still missing. Within a detailed description of a series of methods, we summarise their
capabilities and accuracy. The proposed optical characterisation methods, in particular
those based on the generation of second harmonic light and those involving the measurement
of birefringence and the UV absorption edge are found to be most convenient for an
accurate and fast standard characterisation of LiNbO3 single crystals. An
absolute accuracy of 0.1 mol% based on a comparison with the Curie temperature calibration
method and a relative accuracy of up to 0.01 mol% are available. Some of these methods are
also suited for the two or three dimensional homogeneity control of LiNbO3
single crystals.
Reyher HJ. Faust B. Kading M. Hesse H. Ruza E. Wöhlecke M.
OPTICAL ALIGNMENT OF AXIAL FE CENTERS IN KTAO3 (VOL 51, PG 6707, 1995)
Physical Review B-Condensed Matter. 54(5):3660, 1996
COMMENTS ON THE DEFECT CHEMISTRY OF MAGNESIUM-DOPED LITHIUM NIOBATE (LINBO3)
Journal of Solid State Chemistry. 123(2):208-214, 1996
Abstract
Congruently grown LiNbO3 is known to be highly defective due to its significant
Li2O deficiency. At present two stoichiometry-related defect-chemical
models are discussed which can be distinguished with respect to the occurrence
of Li or Nb vacancies, The Nb-vacancy model takes advantage of
ilmenite-structured LiNbO3 and predicts a niobium antisite
concentration being at least five times as large as in the Li-vacancy model, In
the present contribution it is argued that the magnesium-doping mechanisms are
essentially independent of these particular differences. It is argued that
extended stoichiometry-related defect clusters represent a key for a proper
understanding of the defect chemistry of impurity cations, The discussion is
mainly based on our recent shell-model simulations of perfect and defective
LiNbO3.
U. Schlarb, B. Matzas, A. Reichert, K. Betzler, M. Wöhlecke, B.
Gather, T. Volk, N. Rubinina
Refractive indices of Zn/In-Co-doped lithium niobate
Ferroelectrics 185, 269 (1996)
Abstract
We measured the refractive indices of Zn/In-co-doped lithium niobate in the
wavelength rage of 400 to 1200 nm, using samples with 5.5 mol% ZnO and various additional
concentrations of In2O3. The results can be well described by a
generalized Sellmeier equation which takes into account the defect structure of the
material. The proposed Sellmeier equation allows the treatment of nonliniear optical
effects; calculated phase-mathing conditions for second-harmonic generation are in good
agreement with experimental results.
T. Volk, M. Wöhlecke, N. Rubinina, A. Reichert, N. Razumovski
Optical-damage-resistant impurities (Mg, Zn, In, Sc) in lithium niobate
Ferroelectrics 183, 291 (1996)
Abstract
The paper reviews the results of recent studies of optical-damage-resistant LiNbO3:Me
crystals, with Me substituting for Mg, Zn, In, Sc. The effects of Me ions on LiNbO3
are very similar. This similarity manifests itself drastically in photorefractive, optical
and nonlinear optical properties. The resemblance of these effects is due to a common
influence of Me impurities on the intrinsic lattice defects NbLi.
Publikationen 1995
Schlarb U. Reichert A. Betzler K. Wöhlecke M. Gather B. Volk T. Rubinina N.
SHG PHASE MATCHING CONDITIONS FOR UNDOPED AND DOPED LITHIUM NIOBATE
Radiation Effects & Defects in Solids. 136(1-4):1029-1032, 1995.
Abstract
We measured the refractive indices of undoped and Zn- or Mg-doped lithium niobate
in the wavelength range 400 to 1200 nm. The results are described by a
generalized Sellmeier equation which takes into account the defect structure of
the material. From the generalized Sellmeier equation a simple relation between
the phase matching temperature for noncritical type I second harmonic
generation (SHG) and the internal critical phase matching angle is derived.
Good correspondence with experimental data over a wide composition and
fundamental wavelength range is observed.
T. Volk, M. Wöhlecke, N. Rubinina, N. V. Razumovski, F. Jermann, C. Fischer, R. Böwer
LiNbO3 with the damage-resistant impurity indium
Appl. Phys. A 60, 217 (1995)
Abstract
We report on a detailed investigation (EPR, SHG, optical absorption, luminescence
and Raman scattering) on the new damage-resistant impurity indium in LiNbO3,
where the increased photoconductivity strongly reduces the photorefractive
effect. EPR and optical absorption measurements point to a complete
disappearance of the Nb antisite in LiNbO3:In for all In concentrations.
We believe that the very effective driving out of Nb antisites by In is due to
the trivalent charge state of In and the possibility of charge
self-compensation. Similarities in the properties of Mg-, Zn- or In-doped
samples are discussed. Simultaneous doping with In and Zn leads to an addition
of both contributions, in particular for optical frequency doubling and
luminescence. Raman studies prove that In does not improve the stoichiometry of
the crystals. Indium doping provides the possibility to control simultaneously
phase-matching conditions and to reduce drastically photorefraction. Therefore,
In co-doped LiNbO3 compositions are promising materials for applications after
solving contemporary growth problems.
T. Volk, M. Wöhlecke, A. Reichert, F. Jermann, N. Rubinina
The peculiar impurity concentration ranges in damage resistant LiNbO3 crystals doped with Mg, Zn, In and Sc
Ferroelectrics Lett. 20, 97 (1995)
Abstract
We report on a comparative study of the optical damage-resistant impurities Mg and
Zn, as well as In and Sc in LiNbO3. In LiNbO3 doped with these ions
a certain behavior of physical properties (e.g., the shape of OH stretch mode bands,
phase-matching temperatures of optical frequency doubling, etc.) on the impurity
concentration is observed. The corresponding ('threshold') concentrations are about 5-7
mol.% for the divalent cations Mg and Zn and only 1 mol.% for the trivalent In and Sc. The
results are discussed assuming that these ions incorporate on Li-sites thus removing the
intrinsic defect NbLi from the lattice.
O. F. Schirmer, H. J. Reyher, M. Wöhlecke
Characterisation of point defects in photorefractive oxide crystals by paramagnetic resonance methods
Abstract
not available
H.-J. Reyher, B. Faust, M. Käding, H. Hesse, E. Ruza, M. Wöhlecke
OPTICAL ALIGNMENT OF AXIAL FE CENTERS IN KTAO3
Physical Review B-Condensed Matter. 51(10):6707-6710, 1995
Abstract
Axial Fe3+-OI centers in KTaO3 can be aligned using
linearly polarized Ar+-ion laser light with l = 456
nm. The alignment is observed directly via the change of the characteristic line pattern
of the optically detected magnetic resonance (ODMR) of these centers. The ODMR is
monitored by the magnetic circular dichroism (MCD) of absorption and is unambiguously
attributed to Fe3+-OI, because the angular dependence of the ODMR
lines agrees well with known EPR results. By observing the strength of the ODMR as a
function of the photon energy, the MCD spectrum and, hence, the absorption bands of Fe3+-OI
centers in KTaO3 are identified. These bands are also found in the spectrum of
the linear dichroism (LD), which is produced by photo-alignment of the axial centers. The
results lead to the interpretation, that the orientation of Fe3+-OI
has a local, molecular character and is similar to the case of FA-centers in
alkali halides.
U. Schlarb, A. Reichert, K. Betzler, M. Wöhlecke, B. Gather, T. Volk, N. Rubinina
SHG phase matching conditions for undoped and doped lithium niobate
Radiation Effects and Defects in Solids 136, 1029 (1995)
Abstract
U. Schlarb, M. Wöhlecke, B. Gather, A. Reichert, K. Betzler, T. Volk, N. Rubinina
Refractive indices of Zn-doped lithium niobate
Optical Materials 4, 791 (1995)
Abstract
We measured the index of refraction n(l) of Zn-doped LiNbO3
grown from a congruent melt as a function of the Zn concentration in the crystal. The Zn
concentration cZn in a sample was determined by electron microprobe technique.
The refractive index n(l,cZn)
is described by a generalized Sellmeir equation for 400 < l
1200 nm and cZn up to 8 mol% ZnO. Calculated phase-matching conditions
of nonlinear effects like second-harmonic generation are in excellent agreement with
experimental results obtained with the same set of crystals.