ANNUAL REPORT - MTA SzFKI

S. CHARACTERIZATION AND POINT DEFECT STUDIES
OF OPTICAL CRYSTALS
L. Kovács, I. Bányász, G. Corradi, I. Hajdara # , E. Hartmann, K. Lengyel, L. Malicskó, G.
Mandula, A. Watterich
Electron beam microanalysis on yttrium aluminum borate (YAB) crystals. ⎯ The
impurity elements playing a dominant role in the formation of as-grown imperfections of
YAB crystals growing from potassium trimolybdate melt-solutions undoped or doped with
trivalent ions, were identified by energy dispersive X-ray spectrometry in a scanning
electron microscope. These impurity elements were: Mo, K, Si, Ti and Ca.
Computer modelling of defects in LiNbO 3 . ⎯ Using a recently derived interatomic
potential, intrinsic and extrinsic defect formation energies have been calculated for
LiNbO 3 and used to calculate the energies of defect reactions giving rise to the
experimentally observed lithium deficiency in the material. Of these reactions, the
formation of antisite niobium ions compensated by lithium vacancies was found to have
the lowest energy, and this is therefore the predicted intrinsic defect model. Doping of a
wide range of divalent and trivalent cations into the material was also considered, and
predictions were made of the lowest energy sites for occupation, and the corresponding
charge compensation schemes.
Microscopic and Raman spectroscopic investigation of the domain structures in
LiNbO 3 :Y:Mg crystals. ⎯ Formation of periodic domain structures in Mg and Y doped
and Mg+Y co-doped congruent LiNbO 3 single crystals grown by an off-centred
Czochralski method in the X and 148 o Y pulling directions has been investigated by
Raman spectroscopic measurements. A band attributed to the E(TO 9 ) vibrational mode
occurred at about 610 cm -1 in the Raman spectra recorded in Z(YY)Z backscattering
geometry after chemical etching of a PPLN crystal grown in the X pulling direction. The
shift of the E(TO 8 ) band position at the domain boundaries can be explained by the growth
of the intensity of the E(TO 9 ) band. It was demonstrated by time dependent etching
measurements that the increase of the E(TO 9 ) band is a consequence of periodic step-like
forms on the surface of the crystal.
Holography in LiNbO 3 crystals and silver halide emulsions. –– We developed and
optimised a thermal treatment system and method for dehydrating and homogenising the
oxidation state of lithium niobate crystals with different compositions. The hydrogen
concentration was reduced by about two orders of magnitude in iron or manganese doped
congruent and nearly stoichiometric samples, as compared to the as-grown state. Using
this method we successfully prepared dehydrated 6 LiNbO 3 :Fe samples (at about 95% of
6 Li) for the investigation of thermal neutron dosimeter applications. Such dehydrated
samples were also used to test the holographic scattering method for the determination of
the hydrogen concentration.
We adapted the FRIINT program, developed at the Department of Physics of the Budapest
University of Technology and Economics (BME), for the evaluation of the interferograms
obtained by interference microscopic studies of holographic gratings in Fe-doped LiNbO 3
crystals.
The processing of silver halide holographic emulsions, fabricated by the Slavich Company
(Russia), has been optimised for recording holograms of the LiNbO 3 crystals. These
# Ph.D. student
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