LiNbO3: A Paradigm for Photorefractive Materials

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Abstract
Lithium niobate (LiNbO3) is a paradigmatic photorefractive (PR) material. It provided the first experimental evidence of the PR effect and still maintains a key position in the field. This position is fostered by the fact that large congruent single crystals with good optical quality are routinely grown, enabling technological applications. A remarkable example, a PR narrow-band interference filter has recently been developed and put on the market.A relevant property of LiNbO3, the high electrooptic figure of merit,n3r, assures efficient PR performance. Another unique feature is the occurrence of a bulk photovoltaic (PV) effect, i.e., the generation of a voltage (in an open circuit) or a current (in a short circuit) as a consequence of homogeneous illumination. The bulk PV effect acts, in a way, like an internal electric field, enhancing the PR effect. On the other hand, thermal fixing or stabilization of PR gratings has been successfully accomplished through an interplay between proton and electron dynamics. LiNbO3is also the choice substrate for the commercial fabrication of waveguide devices such as modulators, wavelength filters, multiplexers, and demultiplexers. Moreover, lasing action as well as nonlinear effects have been achieved due to the marked inhibition of the PR effect caused by heavy Mg doping. This illustrates the intimate connection between PR behavior and defect structure, i.e., between optics and materials science.