The polarization self-modulation effect was applied for effective measurement of the characteristic response time of nominally pure Bi12SiO20 (BSO) at wavelengths of 810 and 980 nm. Owing to oxygen deficiency in the crystal lattice, the BSO crystals showed unusual photorefractive sensitivity and remarkable operation speed in the near-infrared spectral region. A response time of 130 ms was measured at 810 nm, and a response time of 540 ms was measured at 980 nm, with incident intensities of 110 and 200 mW/cm2, respectively. To our knowledge, this is the first experimental evidence of a subsecond response in the infrared for a nonsemiconductor photorefractive material.

Fast photorefractive response in B(12)SiO(20) in the near infrared.

Analysis of two-wave coupling in cubic crystal classes 23 and 4¯3m is presented for arbitrary orientation of the grating vector and the applied electric field. We show that the most efficient two-wave coupling is achieved when both the grating vector and the external electric field are parallel to the 〈111〉 axis in the (110) crystal cut. However, beam coupling is asymmetric in this geometry: The enhancement coefficient may differ by orders of magnitude whether interacting beams propagate in the 〈110〉 direction or backwards. Because of this asymmetry, the far-field fanning patterns of photorefractive Bi12TiO20 crystals strongly depend on the direction of the pump-beam propagation. A theoretical model is able to explain main features of the observed fanning patterns if the piezoelectric and photoelastic effects are incorporated into the system of vectorial coupled-wave equations.

Asymmetry of two-wave coupling in cubic photorefractive crystals

Optical activity measurements in the photorefractive Bi12TiO20 single crystal fibers

Volume holographic recording of complex images with a double phase-conjugation mirror in a fiberlike Bi12TiO20 single crystal, mutually pumped by two separate He–Ne lasers, is experimentally demonstrated. Complex images can be stored and reconstructed by use of even a partial input image. The advantages of the small-cross-section, light-confining photorefractive sample in comparison with bulk crystal are a wider spatial bandwidth of recorded images and the possibility of recording holograms by application of smaller ac voltages.

Holographic recording of complex images by double phase conjugation in Bi(12)TiO(20) fiberlike crystal.

The influence of temperature gradients on structural perfection of single-crystal sillenite fibers grown by the LHPG method

Double phase conjugation in Bi12TiO20 photorefractive fiber-like crystal

Numerical calculation of the two-wave mixing gain factor for arbitrary orientation of the holographic grating vector using the coupled wave theory developed for optically active electro-optic crystal enabling pump-beam depletion is presented. Two-dimensional angular dependencies of the gain factor are measured in Bi 12 TiO 20 photorefractive crystal under an alternating external electric field using the fanning effect. It is found experimentally that the angular ditribution of fanning light is independent of the polarization state of the pump beam. This fact could not be explained by the existing theory.

Risto Ravattinen

Papers

Double phase conjugation in Bi12TiO20 photorefractive fiber-like crystal

Two-wave mixing and fanning effect in Bi12TiO20 under an alternating electric field