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 continuou s-wave (cw) focused green laser beam. Surface deformations, temperature dis tributions, and changes of the refractive index of the material are investi gated by means of interferometric techniques. It turns out that light absor ption causes pronounced temperature profiles in the samples, which induce p yroelectric fields. Electronic space-charge fields that compensate these py roelectric fields remain in the crystals after the focused light is switche d off and modulate, together with bulb-photovoltaic fields, the refractive index by means of the electro-optic effect. These low-spatial-frequency eff ects 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 intensitie s. (C) 2000 Optical Society of America [S0740-3224(00)02604-7] OCIS codes: 160.3820, 210.2860, 190.4870, 090.2900, 190.2900.