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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22 B. Briat et al.<br />
Extr<strong>in</strong>sic defects:<br />
Cr 2+ [96], Cr 3+ [110], Cr 5+ [111, 112],<br />
Mn 2+ [113], Mn 4+ [114],<br />
Fe 2+ [115], Fe 3+ [116, 112], Fe 4+ [108], Fe 5+ [117, 108], Fe 3+ -V O [117], Fe 4+ -V O<br />
[112],<br />
Co 2+ [118], Co 3+ -V O [112], Co 4+ -V O [112],<br />
Ni + Ba [119],<br />
Mo 5+ [120],<br />
Rh 2+ [112], Rh 3+ [108], Rh 4+ ,Rh 5+ [108]<br />
Ir 4+ [112],<br />
Nd 3+ [121],<br />
Gd 3+ [122],<br />
Er 3+ [25],<br />
Ce 3+ [123],<br />
Na + Ba -O− [124], K + Ba -O− [124], Al 3+ -O − [112]<br />
Intr<strong>in</strong>sic defects:<br />
Ti 3+ (bound to various unidentified defects [125] or free as a conduction b<strong>and</strong><br />
polaron [10]), Ti 3+ -Nb 5+ [112]<br />
Table 3. <strong>Defects</strong> identified <strong>in</strong> BaTiO 3. The underl<strong>in</strong>ed charge states are EPR-silent.<br />
Their presence was proved <strong>in</strong>directly by comb<strong>in</strong>ed EPR/optical absorption studies.<br />
Especially <strong>in</strong> these cases the related optical absorption b<strong>and</strong>s <strong>and</strong> the charge transfer<br />
processes, <strong>in</strong> which these defects are <strong>in</strong>volved, have been determ<strong>in</strong>ed.<br />
temperatures. By dop<strong>in</strong>g with the shallow alkali acceptors, such as Na Ba ,<strong>and</strong><br />
additional oxidation [107], the Fermi-level can be lowered.<br />
Table 3 represents a large toolbox of defects whose possible <strong>in</strong>fluence on<br />
the <strong>photorefractive</strong> performance can be assessed on a microscopic basis. In<br />
this context the dop<strong>in</strong>gs Co <strong>and</strong> Ce , <strong>in</strong> addition to Rh, have received high<br />
attention. With Co-doped BaTiO 3 , hav<strong>in</strong>g a wide absorption b<strong>and</strong> at 2.25 eV,<br />
larger two- beam coupl<strong>in</strong>g ga<strong>in</strong>s were observed, us<strong>in</strong>g 515 nm light, than with<br />
Fe, Cr, or Mn dop<strong>in</strong>g [128]. The presence of Co 3+ -V O <strong>and</strong> of Co 2+ was<br />
identified <strong>in</strong> the crystals used <strong>in</strong> this <strong>in</strong>vestigation. The Ce 3+/4+ level lies near<br />
midgap <strong>and</strong> holes trapped there thus have a rather long lifetime [123, 129].<br />
Together with codoped Rh, <strong>in</strong>troduc<strong>in</strong>g a Rh 3+/4+ level more shallow than<br />
Ce 3+/4+ , the material offers potential for nonvolatile holographic storage by<br />
us<strong>in</strong>g gated two-wave illum<strong>in</strong>ation [123].<br />
5.2 Ba 1−x Ca x TiO 3 (BCT)<br />
The major drawback of BaTiO 3 for its application as a <strong>photorefractive</strong> material<br />
is the fact, that its phase transition from the tetragonal to the orthorhombic<br />
phase near 8 ◦ C tends to deteriorate the optical quality of the crystals.<br />
The replacement of part of Ba by Ca forms the isostructural Ba 1−x Ca x TiO 3