Direct microscopic observation of hologram build-up in photorefractive crystals

Abstract

A method, based on phase-contrast and interference microscopy, was developed for direct microscopic observation of temporal evolution of phase holograms in photorefractive crystals. Interference microscopy was adapted to the study of photorefractive holograms. First a hologram was recorded in the sample, and diffraction efficiency was monitored during hologram build-up using inactinic laser light. Thus kinetics of hologram build-up could be determined. The initial hologram was erased using white light. Then a series of write-erase cycles were performed with increasing exposure times up to an exposure corresponding to saturation of the grating. Holograms were observed by interference microscope after each exposure. The time elapsed between the exposure and the microscopic observation was negligible compared to the relaxation time of the hologram. The obtained temporal evolution of grating profile gives a deeper insight into the physical mechanism of hologram formation in photorefractive materials than diffraction efficiency measurements. Congruently grown samples of LiNbO3: Fe, with Fe concentrations in melting of 10-3 were studied by the above method. Sample thickness was set to 300 μm to allow correct microscopic observation. Plane-wave holograms were recorded in the samples using an Ar-ion laser at λ=514 nm of grating constants of 3.3 and 7.0 μm.