<title>Information storage with a barium-calcium titanate crystal</title>

Abstract

Barium-calcium titanate is a novel photorefractive crystal for optical storage applications. It is grown from a congruently melting composition from BaTiO3 and CaTiO3. In contrast to pure BaTiO3 the melting temperature is with I 592 °C nearly 30 ° deeper, the crystal is cubic at the growth temperature and shows a phase transition to the tetragonal structure at 98°C. It means that as in BaTiO3 a spontaneous poling appears. The lattice constants are smaller than in BaTiO3, a =b = 3.962 nm and c =3.999 nm and decreace with increasing Ca content. Therefore, the crystal is an optical uniaxial one. Asecond phase transition like in BaTiO3 is not found /1/. The refractive indices are slightly smaller than those of BaTiO3 but the electrooptic coefficients r13 =36 pm/V and r33 = 140 pm/V are larger (2 = 514 nm) /2/. Therefore, the BaiCaTiO3-crystal is a very promising material for optical information storage. In our experiments we used a 1 cm 1 cm 1 cm large crystal, poled in an external electric field, therfore obtaining a single domain sample. All sides are polished to optical quality. The sample shows in ordinarily polarized light extremly high fanning effects. We have studied the two wave mixing (TWM) for ordinary and extraordinary polarized light (A =514 nm). As expected, with extraordinary polarized light the buildup time is much longer (about 10 s) than with ordinarily polarized light (5 s), but the reached gain for a given intensity ratio between pump wave and signal wave is 2 orders higher. Varying the intensity ratio the logaritmic dependence of the gain on the intensity ratio is nearly linear. (The maximum value reached in our experiments is 4.6, it means the signal intensity behind the crystal is 40 000 times higher than the incident signal intensity). Very interesting is the behavior that the amplification will allways reach a maximum absolute value - nearly 3% of the pump wave is coupled to the signal wave. Reading the refractive index grating in the crystal after switching off the signal wave, a typical temporal behavior ofthe reconstructed signal wave is observed. This behavior can be explained with a multi level model ofthe photorefractive process inside the crystal /3, 4, 5/ The information will be recorded over a very long time interval, and multiple storage (for example via angular coding) is possible.