Defects in inorganic photorefractive materials and their investigations

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28 B. Briat et al. orbitally degenerate, spin-orbit coupling causing the occurrence of two oppositely signed MCD contributions for band ’2’. Bands ’2’ and ’3’ are thus likely to be attributed to optical excitations of a small hole polaron bound to the antisite defect Bi 3+ M .AlsotheAl3+ M defect, similarly having the structure Al4+ M -O − , has optical features supporting the assignment to optical excitation of a hole polaron next to Al 3+ M [150]. Newer neutron [159] and X-ray diffraction [160] studies of the crystal structure of the sillenites indicate that the large Bi 3+ M ion, consisting of a xenon core and a (6s 2 ) lone electron pair, can hardly fit into the space of a tetrahedrally coordinated M 4+ ion. There is evidence that Bi 3+ M is in fact neighbored by an oxygen vacancy, V O , accommodating the 6s 2 lone pair. Also in this situation optical absorption and MCD spectra of the type indicated above are expected. The trigonal symmetry is now induced by the vacancy, the hole captured near the antisite defect replacing one of the 6s 2 electrons since these are the least tightly bound. 6.2 Additional observations In the case of BTO, it was found experimentally that a complete dark erasure of light-induced absorption requires months at room temperature [161]. Quite consistently, the MCD spectra of as-received BMO samples invariably demonstrate that Bi 4+ M is present [147, 148]) to an amount varying from sample to sample. So usually one operates with specimens far from equilibrium, unless they are carefully thermally bleached. There are several additional observations which are based on this fact. From thermal bleaching studies [162] it was concluded that three relatively shallow electron traps (0.65 - 0.97 eV) play a major role in the case of BGO. These are certainly correlated with the absorption and MCD bands observed in the low energy region of Fig. 11. Studies of the absorption kinetics with changes of temperature [163] or illumination [161] indicate that two step processes among the donor levels occur. Also, the time dependence of the relaxation of photorefractive gratings depends on the charge density stored in the shallow levels [164, 165]. The wavelength dependence of the photoionization processes can be estimated grossly from a deconvolution of the CD, absorption and MCD spectra. Quite clearly, however, the strength of the bands (the concentration of defects) depends upon the thermal and optical history of the sample. Only electron excitations are possible for bleached crystals, starting from Bi 3+ M (Fig. 12. After an illumination with a wavelength near or below 514.5 nm, Bi 4+ M and D0 defects are created. Electron and hole transitions can then occur simultaneously at certain wavelengths. Furthermore, electron excitations from occupied shallow levels become possible in the near IR. Complementary gratings were observed in BSO at 785 nm [166], a spectral region where bands 3 and 4 (Fig. 11) overlap at room temperature.
Defects in inorganic photorefractive materials and their investigations 29 Photorefractivity in the sillenites was usually studied with green or blue light, but there is a need to improve their properties in the red and near-IR in order benefit from laser diodes in this spectral region. The spectroscopic results reported above suggest that the shallow defects leading to bands ’4’ and ’5’ in Fig. 6 can be used for that purpose. Their electron population can be raised substantially via a preliminary blue homogeneous illumination. For example, an enhancement of the beam coupling at 1.06 µm was observed [167] in undoped BTO by preexposure to visible light. Consistently with the possible assignment of bands ’4’ and ’5’ to oxygen vacancies, crystals grown under argon show an unusual photorefractive sensitivity and a remarkable operation speed in this spectral region [168]; the photoconductivity in red light is two orders of magnitude higher than in crystals grown in the presence of oxygen [169]. 6.3 Extrinsic defects In principle, the introduction of extrinsic defects offers more choices for tailoring the photorefractive properties of the sillenites. A few encouraging results were obtained by doping with transition metal ions. Cr improves the diffraction efficiency and increases the response speed of BSO at 633 nm [170]. Holographic gratings were recently recorded in the near IR in in rutheniumdoped BTO [171] and BSO [172] samples. In the case of Bi 12 Ti 0.76 V 0.24 O 20 , homogeneous illumination and the application of an external electric field after writing enhance the diffraction efficiency by a factor of almost 40 [173] at 514 nm. The mechanisms underlying these experimental observations are far from being understood. In spite of intensive research with absorption and CD spectroscopies [174], most progress with respect to the definite assignment of absorption bands has been reached by MCD and ODMR studies [175]. Many defects have been identified in this way and their optical excitations were greatly clarified: Cr 4+ M and Cr 5+ M [146], Mn5+ M and Mn4+ M [146, 176, 177], Cu2+ M [146] and Cu2+ Bi [178, 175], Fe 3+ M [145, 146], V4+ M [177], Co2+ M and Ni2+ M [146], and Run+ Bi (n =3− 5) [172, 179]. Until now, V M was thought to be always in the 5+ diamagnetic state since no EPR signal could be found by conventional means [180]. As has been demonstrated above with the intrinsic defects, most detailed information is obtained when experiments are carried out in the bleached and colored states, but also in the near-IR (likely to be connected with internal transitions) as well as in the visible (charge transfer or intervalence transitions). Fig. 14 illustrates such information related to Cr-doping [146, 175]. In the initial state, MCD-active Bi 4+ M is absent and the broad features observed in the visible are thus associated with chromium. Very characteristic MCD (and CD) sharp features are detected for Cr 4+ Ge (3 A 2 → 1 E transition) and Cr 5+ Ge ( 2 E → 2 T 2 ) in the near-IR. The coexistence of both charge states indicates that the Fermi-level is pinned at the Cr 4+/5+ Ge level in thermal equilibrium. Illumination with E
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