Due to the co-existence of optical activity and linear birefringence in a Bi 12 SiO 20 crystal, a phase gratting induced in this medium has many interesting and unusual properties. We study the ellipticity and orientation of the directly transmitted and diffracted beams as a function of the read-beam vector orientation. In image processing experiments the signal to noise ratio can be increased significantly by appropriately choosing the read-beam orientation such that the strong direct beam is linearly polarized and can be eliminated with the help of a polarizer.
We demonstrate experimentally two regimes of an enhanced beam amplification in photorefractive waveguide. Enhancement is achieved by matching the optical activity and the reflections of the beams in waveguide. Beam amplification is assisted by stimulated photorefractive scattering.
We prove that groups acting geometrically on delta-quasiconvex spaces contain no essential Baumslag-Solitar quotients as subgroups. This implies that they are translation discrete, meaning that the translation numbers of their nontorsion elements are bounded away from zero.