Reduction-induced
polarons and optical response of Mg-doped LiNbO 3
crystals G.K. Kitaeva1  , K.A. Kuznetsov1,
V.F. Morozova1, I.I. Naumova1,
A.N. Penin1, A.V. Shepelev3,
A.V. Viskovatich2 and
D.M. Zhigunov1
| (1) |
Department of Physics, M.V.
Lomonosov Moscow State University, 119992 Moscow,
Russia |
| (2) |
Scientific and Technological
Center of Unique Instrumentation, Russian Academy of
Science, 117342 Moscow,
Russia |
| (3) |
A.N. Kosygin Moscow State
Textile University, 117918 Moscow,
Russia |
Received:
23 July 2003 Revised:
6 January 2004 Published
online: 23 March 2004
Abstract We studied
the visible and IR dispersion of absorption coefficient and
refractive index for congruent LiNbO 3 and
Mg:LiNbO 3 crystals before and after chemical
reduction at different annealing temperatures. The
concentration of Mg in Mg:LiNbO 3 samples was just
below or above the photorefractive threshold. The
reduction-induced changes in the absorption coefficient reveal
the formation of polarons typical for doped LiNbO 3
crystals. It was shown that the polaron concentration is
maximal when the Mg concentration is just below the
photorefractive threshold and the annealing temperature is
near 500 °C. This temperature is optimal for the most
efficient polaron formation at all considered concentrations
of Mg. The fitting of the experimental absorption dispersion
curves indicates that intermediate polarons are formed in
LiNbO 3:Mg crystals preferably. The spectral
dependence of transmission for samples of lithium niobate of
various thicknesses was studied. The results indicate that
there are spatial regions with much greater absorption than
that of bulk crystals. We assume that, in general, polarons
are localized in thin near-surface regions. The spectral
dependence of the refractive index in the vicinity of the
phonon absorption edge indicates some essential changes of the
phonon subsystem taking place after reduction. The infrared
contribution into the dispersion of the dielectric function
real part increases considerably after reduction.
PACS 71.38.Ht; 71.38.-k; 78.20.Ci
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