Switchable Bragg gratings based on soft materials

Abstract

Topic of the thesis is the study of diffraction Bragg gratings holographically recorded in polymer-liquid crystal composite materials. Due to the special electro-optical properties of liquid crystals, these gratings can be electrically driven, switching between a diffraction and a transmission state. In the present work, techniques and materials used to produce such gratings are described, with emphasis on the connections between the final morphology and the preparation procedure, materials and exposure. Different types of gratings have been characterized, both in the visible and in the near infrared range, in view of a possible use as photonic devices in the optical communication systems. Grating diffraction efficiency has been measured varying the wavelength, the temperature, the angle of incidence and the applied electric field. The behavior in dynamic conditions has been analyzed by measuring switching times for electric pulse of variable amplitude and duration. These measurements have been analyzed with suitable mathematical models and related to the morphology of the studied materials, like the interface between polymer and liquid crystal, anisotropy of liquid crystal domains and the degree of phase separation. Results obtained provide a deeper insight into the structure of such materials and make possible the optimization of their properties and the design of different photonic devices. Finally, the use of variable angle ellipsometry and /or half-leaky guide mode technique to measure soft material optical properties is discussed.