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UBC Theses and Dissertations

Beam coupling in holograms stored in LiNbO₃ Woods, Randall J.


The potential use of photorefractive crystals in optical data handling applications has received considerable attention during the past decade. The photorefractive effect in LiNbO₃:Fe involves the photoliberation and inhomo-geneous redistribution of electrons which are subsequently trapped in the new distribution. The resulting electric fields set up a similar distribution of variations in the refractive index allowing the formation of thick phase holograms. In the case of an elementary hologram (that written by a simple interference pattern between two equal intensity beams of monochromatic light), the holographic grating may be displaced from the fringe pattern that wrote it. This phase shift gives rise to a redistribution of energy between the two beams, called beam coupling. Measurements of the beam coupling can be used to obtain a value for the phase shift, and this in turn is useful in studying the electron transport mechanisms which give rise to the shift. Previous attempts to measure the beam coupling have not yielded reproducible results and have consequently not been very useful in determining the phase shift or studying it causes. The present set of experiments have shown that the phase shift is not constant in time during the writing process, and repeated measurements as the hologram was written showed that this variation is a linear function of time for any particular experiment. The causes of this variation can be attributed to the nature of the writing process, thermal expansion and mechanical creep of the optical components. Repeated experiments showed that the value of the shift extrapolated to t=0 of the writing process was a constant for the crystal used. Because the holograms were written under conditions which made the bulk photovoltaic effect the dominant process, it was possible to calculate the transport length due to this effect. The value arrived at was 13±3 nm.

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