Problem of a Non-Erasable Photorefractive Hologram

Summary

The chapter characterizes the methods providing a non-erasable optical readout of the photorefractive gratings. The first is the two-color holography which is based on recording a grating by a long wavelength (IR) possible due to a non-equilibrium population of shallow levels at the expense of a short-wavelength pumping-over of electrons from deeper levels. The two-step photoexcitation via short-living photoinduced centers (‘gating’) is described in LiNbO₃ and LiTaO₃ of different compositions. An emphasis is given to the involvement of the intrinsic defects, namely intermediate small polarons occurring due to the photodissociation of bipolarons in reduced crystals. A UV-gated recording in LiNbO₃ doped with optical-damage-resistant ions is described. An outline of data is given on certain combinations of impurities such as Fe and Mn in LiNbO₃ which under a short-wavelength photoexcitation form long-living metastable states resulting in slowing down the optical erasure. Another method providing a nonvolatile readout is the thermal fixation of gratings based on screening the recorded electronic space-charge by species of the opposite sign (ions or photoholes) activated when heating to certain temperatures. The thermal fixation and the current models of this process in LiNbO₃:Fe is described dwelling on the factors controlling the diffraction efficiency and storage times of a fixed grating. Particular attention is given to a discussion of protons as compensating charges in LinbO₃:Fe. A short outline of available data on thermal fixation in other photorefractive crystals closes this chapter.