On the Mechanisms of Light-induced Optical Anisotropy in Organic and Bioorganic Systems
Andrzej Miniewicz, Anna Sobolewska, Anna Kochalska, Ewa Schab-Balcerzak, Antoni C. Mitus, Grzegorz Pawlik, Bouchta Sahraoui and Francois Kajzar
Light-induced optical anisotropy is a fundamental phenomenon occurring in many organic optical materials, like photochromic and photorefractive polymers, low-molecular mass liquid crystals and polymer liquid crystals, as well as, in bioorganic systems like bacteriorhodopsin (BR). Bistable photochromic molecules embedded in various matrices are responsible for photoinduced anisotropy in photochromic materials but also in bacteriorhodopsin. However, mechanisms leading to the appearance of substantial light induced birefringence or dichroism are different. In liquid crystals molecular reorientations originate from interaction of molecules with light induced electric field changes, in photorefractive polymers are partially due to linear electrooptic effect and in photochromic polymers molecules undergo conformational changes due to photoisomerisation. In any case, we deal with near resonance optical linear or nonlinear processes mediated frequently by environment. The characteristic features underlying these phenomena, studied by optical wave-mixing techniques, will be presented and discussed. Comparative studies of diffraction grating inscription using continuos and pulsed laser light illumination and employing polarisation and moving gratings experiments in different types of materials allowed us to observe differences and similarities between various processes.