Material Parameters Study in Barium Titanate Using a Laser-induced Grating Spectroscopic Probe Technique

Material parameters, including refractive index modulation depth, space charge field, and trap charge density, in photorefractive barium titanate are determined using a novel laser-induced grating spectroscopic probe technique. In this method, shown in Fig. 1, coherent laser beams (grating writing beams) intersect inside the crystal and induce (write) a refractive index grating via the photorefractive effect. A laser probe beam is applied to the crystal parallel to the grating planes (i.e. at normal incidence to the plane of the grating writing beams, in a Raman-Nath type configuration) and is diffracted into higher order modes (|m|≥1) located symetrically at angles ±θDm about the undiffracted (m=0) mode. The crystal is in air and is supported by an optical flat attached to a rotating platform (not shown in Fig. 1) which allows us to rotate the crystal about the z-axis, thus varying the intersection angle of the grating writing beams in the crystal (for fixed writing beams in air), thereby varying the grating spacing, Λg, in the crystal. Additionally, the platform provides for tilt adjustment of the x-y plane in order to precisely align the induced phase grating planes and the probe beam to obtain symmetry in the intensities of the positive and negative modes (orders) of diffraction.