The most popular photorefractive material BaTiO 3 suffers from a crippling defect when applications are considered : its phase transition around room temperature that can destroy its photorefractive characteristics, if not the crystal itself. Some years ago, University of Osnabrück We will present here some theoretical and experimental results that allow us to answer this question and that show that despite some differences in the photorefractive properties between BaTiO 3 and BCT, the new crystal is an extremely promising materials for photorefractive applications, such as phase conjugation or dynamic holographic intracavity laser mode selection.
I. The photorefractive gainThe photorefractive performances of a photorefractive crystals are usually determined through a two beam coupling experiment and characterized by a photorefractive gain that writes as ( )(ê S .ê P )where λ o is the wavelength in vacuum, n the mean refractive index and (ê s .ê p ) is the scalar product of the polarization vectors of the beams. ( ) is the space charge field that depends on the grating spacing k r and on I the incident illumination on the crystal. In a new crystal such as BCT both quantities are a priori unknown and will have to be determined through photorefractive experiments.The main idea of this characterization is linked to the nature of the two quantities we want to determine. First, the effective electro-optic coefficient will be linked to the crystalline structure of the material, it will be dopant independent. Crystals grown with different doping conditions (as soon as they present photorefractive effect) will have similar orientation dependence of the gain.Second, for a given orientation, the grating spacing and intensity dependence of the photorefractive gain will be only dependent on the deep defects responsible for the photorefractive effect. Both problems are thus separable and, in a first approximation, we can deduce some information on the charge transport model valid for BCT without knowing the exact value of the electo-optic coefficients, and inversely, we can measure the effective electro-optic coefficient without knowing the nature of the defect involved in the space charge field. This nevertheless requires to perform some specific experiments that will be presented in the following.
II. Two beam coupling energy transfer measurementThe basic and mostly used experiment for photorefractive crystal characterization is the two beam coupling energy transfer gain. The two beam coupling experimental set-up is presented in The parameters of the experiment (grating spacing, illumination, crystal orientation, polarization,…) are changed and the variation of the gain as a function of these parameters is measured.
III. Orientation dependence of the photorefractive gain : determination of the electro-optic coefficientsTwo symmetrical beams forming an angle 2θ inside the material enter symmetrically the crystal, which then turns round the normal to the crystal (y-axis) (Figure 3) [4]. The angle of rotation is α. We start w...