Photoinduced repolarization

Photoinduced microdomains

Photomicrodomain scattering indicatrix is well enough described within the approach of scattering by long thin cylinders (needles). What formation can play role of such needles? This could be antiparallel domains. Indeed, such microdomains can be formed in lithium niobate crystals due to the symmetry properties. However repolarization of lithium niobate crystals is known to be hampered for large values of both the Curie temperature (T c=1480 o K) and coercive field. Pure lithium niobate used to be considered to be so called frozen ferroelectric. Its coercive field was estimated to reach the value about 15 million V/cm. Thus repolarization of such crystal used to look impossible due to electric destroying the sample. Defects of the crystal structure, admixtures etc. diminish the coercive field value. But it still remains very high and reaches the value about E c ~ 300 kV/cm. So repolarization of lithium niobate is quite hardly realized but possible. Besides the process of lithium niobate crystal repolarization does start by appearance of needle-like microdomains. Such microdomain structure could be displayed by etching the crystal surfaces in acid mixture (hydrochloric acid, sulphuric acid, hydrofluoric acid). The picture demonstrates an initial stage of lithium niobate crystal repolarization under external electric field about E ext= 85 kV/cm and temperature T= 130 oC. Our experiments as well displayed the microdomain structure having appeared after photoexciting the crystal.

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Photomicrodomain light scattering

When exciting light beam propagates along optical axis of the sample, usual photoinduced light scattering is not excited because the direction of according noise hologram vector and photovoltaic current direction are perpendicular so that charge redistribution does not take place in the direction required for noise hologram record. So if we use a weak (not focused) laser beam, both optical autowaves and photoinduced light scattering are not expected. Nevertheless, we found under that conditions some light scattering does appear.

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