Low-spatial-frequency refractive-index changes in iron-doped lithium niobate crystals upon illumination with a focused continuous-wave laser beam

Iron-doped lithium niobate crystals are illuminated with a single continuous-wave (cw) focused green laser beam. Surface deformations, temperature distributions, and changes of the refractive index of the material are investigated by means of interferometric techniques. It turns out that light absorption causes pronounced temperature profiles in the samples, which induce pyroelectric fields. Electronic space-charge fields that compensate these pyroelectric fields remain in the crystals after the focused light is switched off and modulate, together with bulk-photovoltaic fields, the refractive index by means of the electro-optic effect. These low-spatial-frequency effects must be taken into account when focused light beams are utilized, e.g., for high-speed holographic data storage or two-beam coupling, because the effects determine an upper limit of the highest usable cw light intensities.

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