The photorefractive effect in a nominally undoped 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 single crystal is measured. We report what is, to our knowledge, the first observation of the photorefractive effect in Pb-based relaxor ferroelectric crystals. The crystal is grown by the flux solution method. Then it is cut into a 2 mm x 4 mm x 8 mm piece and electrically poled along the [111] direction. The coupling constant of the two-wave mixing is 17 cm(-1), and the normalized time constant under 1-W/cm(2) illumination is 12 s at a wave-length of 476 nm. The effective trap density is calculated as 5 x 10(16) cm(-3) from the Debye screening length under the assumption of Kukhtarev's band-transport model. The dominant carrier is identified to be holes from the direction of two-wave mixing energy transfer.
Photorefractive gratings recorded in LiNbO, are usually volatile: the grating buildup rate is equal to the grating erasure rate if the recording intensity is equal to the erasing intensity.' On the other hand, high asymmetry in the grating buildup and erasure rates is desired in some applications. For example, holographic memory applications require nonvolatility of recorded gratings during readout while they require the recording sensitivity as high as possible.2 Here we report on the near-infrared (one-color) photorefractivity and the observation of high asymmetry in the grating buildup and erasure rates in LiNbO,:In, Fe.We studied the dynamics of grating buildup and readout processes in slightly reduced congruent LiNbO,:In,Fe (In: 1.0 mol.%, Fe: 0.03 wt.%). The grating was recorded by two extraordinarily polarized 780-nm beams of equal intensities at a 1.1-pm grating spacing. The grating wave vector was set to be along the crystalline c-axis. After recording the grating was read out by one of the recording beams with the other recording beam synchronously off. Figure 1 shows a typical temporal trace of the grating buildup and readout processes at a total recording intensity I, of 4 W/cmZ and a readout intensity Iread of 2 W/cmZ. It is seen that the recorded grating is highly resistant against readout erasure. For more than 6.5hour continuous reading the diffraction efficiency q decreases only by a few percent from its initial value of about 60%. Such high asymmetry in the grating buildup and readout erasure rates implies that the grating recorded in LiNbO,:In,Fe is quasi-nonvolatile.We also observed that when the recording process was interrupted before the saturation of 1, the diffracted signal first decreased to take a minimum value followed by a monotonic inn -I I 8 I 8 I I I readout 0.0 0 100 200 300 400 Time (min) CTuR4 Fig. 1. Dynamics of grating buildup and readout processes for LiNbO,:In,Fe at I, = 4 W/cm2 and Iread = 2 W/cm2.crease during readout. Such behaviors of q during readout were independent of the orientation of the crystalline c-axis but the rate of the monotonic increase in q was dependent on the orientation, as shown in Fig. 2. It is evident that the initial decrease in q is attributed to partial erasure of the recorded grating during readout. On the other hand, the monotonic increase in q may be due to the transient beam coupling (both intensity and phase) between the undiffracted and diffracted readout beams through the complex coupling ~oefficient"~ that is found to have comparable real and imaginary components for our LiNbO,:In,Fe. It is clear that the coupling effects slow down the readout erasure. However, the observed high resistance of the grating against readout erasure was confirmed not to solely originate from the coupling effects, as seen from Fig. 3, where the high asymmetry in the grating buildup and non-Bragg-matched erasure rates is clearly shown. We therefore attribute the observed high asymmetry to a noticeable decrease in the near-infrared photorefractive sensitivity aft...
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