Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
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<strong>Defects</strong> <strong>in</strong> <strong>in</strong>organic <strong>photorefractive</strong> <strong>materials</strong> <strong>and</strong> <strong>their</strong> <strong>in</strong>vestigations 9<br />
most str<strong>in</strong>gent way. It is thus ideally suited to the analysis of the optical absorption<br />
properties of defects as related to the <strong>photorefractive</strong> effect. When<br />
several broad absorption b<strong>and</strong>s are overlapp<strong>in</strong>g, as it is most often the case<br />
for defects, especially <strong>in</strong> oxides, MCD-ODMR allows such a superposition to<br />
be deconvoluted by identify<strong>in</strong>g exactly that one among the b<strong>and</strong>s, which is<br />
l<strong>in</strong>ked to a def<strong>in</strong>ite EPR-signal. An example is given at the end of Sec. 5.<br />
A necessary, but unfortunately not sufficient, precondition for the application<br />
of EPR <strong>and</strong> related methods is the paramagnetism of the defects. Such<br />
procedures therefore can be applied to only about one half of all defects,<br />
the EPR-active ones. S<strong>in</strong>ce the <strong>photorefractive</strong> effect <strong>in</strong>volves all types of defects,<br />
<strong>in</strong>dependent of whether they are EPR-active or EPR-silent, additional<br />
<strong>in</strong>formation is necessary to circumvent this problem. Actually, <strong>in</strong> the case of<br />
cubic crystals, the very absence of a MCD signal associated to a given absorption<br />
b<strong>and</strong> is a proof that the responsible defect is diamagnetic. In favorable<br />
cases (see Sec. 6), transition metal ions show <strong>in</strong>ternal transitions <strong>in</strong> the near<strong>in</strong>frared,<br />
which are characteristic of the the site symmetry <strong>and</strong> charge state of<br />
the defect. If a material conta<strong>in</strong><strong>in</strong>g the <strong>in</strong>vestigated defect is gyrotropic, such<br />
as the sillenites, then it is possible to study EPR-silent defects by <strong>their</strong> natural<br />
circular dichroism (CD) (see Section 6). The CD signals, however, do not provide<br />
any knowledge on the structure of the defects, usually derived from <strong>their</strong><br />
magnetic properties. Altogether, a comb<strong>in</strong>ation of techniques proves necessary<br />
for a reliable labell<strong>in</strong>g of defects.<br />
Light-<strong>in</strong>duced absorption changes (LIAC) <strong>and</strong> <strong>their</strong> correlation with EPRor<br />
MCD-changes have been largely exploited to label defects, both EPR-silent<br />
<strong>and</strong> EPR-active ones [20, 21, 22], <strong>and</strong> to identify between which defects charge<br />
carriers are transferred by light. This approach was <strong>in</strong>troduced recently as<br />
the basis for the quantitative prediction of the performance of <strong>photorefractive</strong><br />
<strong>materials</strong>. An example will be given <strong>in</strong> Sec. 5.<br />
Measurements of optical absorptions <strong>in</strong>duced <strong>in</strong> <strong>photorefractive</strong> crystals by<br />
specific dop<strong>in</strong>gs have sometimes been used to draw conclusions about the nature<br />
of the result<strong>in</strong>g defects. If used critically <strong>and</strong> cautiously, such results can<br />
give h<strong>in</strong>ts of the nature of the responsible defects. The absorption signals usually<br />
do not carry <strong>in</strong>formation on the charge state <strong>and</strong> the <strong>in</strong>corporation site of<br />
the defect, whether it is isolated or associated with some partner defect. Such<br />
caveats are also necessary, when <strong>in</strong>terpret<strong>in</strong>g measurements of PIXE, channel<strong>in</strong>g<br />
[23], neutron activation analysis (see for example [15]), etc., <strong>in</strong>duced by<br />
specific dop<strong>in</strong>gs.<br />
4 <strong>Defects</strong> <strong>in</strong> LiNbO 3 (LN)<br />
S<strong>in</strong>ce the discovery of the <strong>photorefractive</strong> effect, LN has played a major role<br />
<strong>in</strong> the development of this field. Correspond<strong>in</strong>gly, great efforts have gone <strong>in</strong>to<br />
the elucidation of the function of defects <strong>in</strong> this material. A considerable number<br />
among them, <strong>in</strong>tr<strong>in</strong>sic <strong>and</strong> extr<strong>in</strong>sic, could be identified by EPR or related