Electrical fixing of photorefractive holograms in Sro . 7 5 Bao . 25 Nb 2 O 6

Volume holograms recorded in photorefractive materials can find important applications in optical memories and optical computing systems. One problem with a photorefractive hologram is that it gets erased by the readout light. Nondestructive readout can be achieved by hologram fixing, and several fixing methods have been reported. Thermal fixing of holograms was demonstrated in LiNbO3,' Bi1 2SiO2 0 , 2 KNbO3 , 3 and BaTiO3 , 4 where a compensating ionic charge grating (which cannot be erased optically) is formed at an elevated crystal temperature. Micheron and Bismuth demonstrated hologram fixing in Sr0 .75 Ba0 .2rNb 2O6 (Ref. 5; SBN:75) and BaTiO3 (Ref. 6) through the creation of a ferroelectric domain pattern by applying an external field at room temperature. Hologram fixing in SBN:75 was also achieved by cooling the exposed crystal through the ferroelectric phase transition. Leyva et al. demonstrated hologram fixing in KTal-,Nb.O3 by cooling the exposed crystal under an applied field through the ferroelectric phase transition.8 In general, electrical fixing is preferable from a practical point of view because of its relative simplicity. In this Letter we report the results of our investigation on electrical fixing of photorefractive holograms recorded in SBN:75. We were able to reproduce some of the effects that Micheron and Bismuth reported in Ref. 5, but our observations were different in several important respects. In addition, we report two novel ways of electrically fixing holograms in SBN:75 that give improved performance and demonstrated that holograms of images can be fixed and faithfully reproduced. The crystal sample used in the experiment was grown and poled at Rockwell International Science Center. It has dimensions of 6 mm X 6 mm x 6 mm, with its c axis parallel to the edges. An eternal electric field can be applied along the c axis, and it is called positive (negative) if its direction is same as (opposite) that of the initial poling filed. In our experimental setup (Fig. 1), an ordinary-polarized plane wave from an argon laser (A = 488 nm) is split into three beams, two of which are used for recording a grating in the crystal, with the third used as a nonBragg-matched erasing beam. The grating vectors are approximately parallel to the c axis and the total recording intensity is -10 mW/cm2. The diffraction efficiency 9 is monitored with a low-intensity, extraordinary-polarized He-Ne laser beam incident at the Bragg angle. The diffraction efficiency is calculated by subtracting the background noise level from the measured diffracted light and dividing the difference by the transmitted light power. In the first experiment, a holographic grating with a grating spacing A = 11.6 1ttm was recorded in the completely poled crystal without any applied field. After the diffraction efficiency Y7 reached its saturation value (-q = 11%), the recording beams were blocked, and a negative-voltage pulse with amplitude V = -1 kV and duration t = 0.5 s was applied to the crystal, which caused 77 to fall quickly. After the voltage pulse was removed, 77 recovered a portion of its initial value before the pulse. Then the crystal was illuminated with the non-Bragg-matched erasing beam, and q, decreased further until it reached a steady-state value of 'i 0.06%. This fixed grating could not be erased by the erasing beam. Then the erasing beam was blocked, and a positive-voltage pulse, with amplitude V = +2 kV and duration ofa